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<div style="text-align: center;"><big><big><span
style="font-weight: bold;">Writing Optimal Code with TJC<br>
</span></big></big><big><big><span style="font-weight: bold;"></span></big></big></div>
<div style="margin-left: 40px;"><span style="font-family: monospace;"><br>
<br>
</span></div>
Writing optimal Tcl code is a not a black art. A developer need only
understand a few Tcl evaluation basics in order to avoid usage problems
that lead to non-optimal code. The TJC compiler is able to generate
optimized code for most common Tcl command usage. The page describes
some of the usage problems that would keep the TJC compiler from
generating optimized code.<br>
<h4>Braced Expressions</h4>
By far, the most important thing a developer can do is check that each
math expression in a Tcl script is brace quoted. The <span
style="font-family: monospace;">expr</span> command accepts math
expression arguments. A math expression can also be found in an <span
style="font-family: monospace;">if</span>, <span
style="font-family: monospace;">for</span>, or <span
style="font-family: monospace;">while</span> command. An unbraced
expression cannot be compiled and will execute very slowly
since the entire expression will be reparsed each time the command is
executed. A brace quoted expression argument can be compiled and
optimized. The following example shows an unbraced and a braced
expression.<br>
<br>
<ol style="font-family: monospace;">
<li>expr $a < 3<br>
</li>
<li>expr {$a < 3}</li>
</ol>
<br>
The unbraced <span style="font-family: monospace;">expr (1)</span>
passes three arguments to the expr command at runtime. In this example,
assume the variable <span style="font-family: monospace;">a</span> is
set to 1. The unbraced expr command would be called with the argument
strings "1", "<", and "3". These arguments would then be
concatenated into the string "1 < 3" and parsed into an operator
tree structure like the following:<br>
<br>
<span style="font-family: monospace;"> <</span><br
style="font-family: monospace;">
<span style="font-family: monospace;"> / \</span><br
style="font-family: monospace;">
<span style="font-family: monospace;"> 1 3</span><br>
<br>
The operator tree is then evaluated at runtime and the logical value 1
is returned since the left operand is smaller than the right operand.<br>
<br>
The braced <span style="font-family: monospace;">expr (2)</span> has a
single brace quoted argument. The braced expr can be compiled and
inlined by TJC, and will not invoke the expr command at runtime. This
braced expr is parsed into an operator tree structure like the
following:<br>
<br>
<span style="font-family: monospace;"> <</span><br
style="font-family: monospace;">
<span style="font-family: monospace;"> / \</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">$a 3</span><br>
<br>
The important difference to note here is that the variable <span
style="font-family: monospace;">a</span> is not replaced with the
string "1" in this case. This is important because the compiler is able
to tell that there is only one operator and than the left operand is a
value contained in the variable <span style="font-family: monospace;">a</span>.
The compiler is able to inline the variable access, the operator logic,
and the constant integer operand 3. The braced expr executes more
quickly because the slow process of parsing the expression into a tree
is avoided at runtime.<br>
<br>
The unbraced <span style="font-family: monospace;">expr (1)</span>
can't be parsed into a tree at compile time because it is impossible to
know what the variable might evaluate to. For example, the variable <span
style="font-family: monospace;">a</span> could be set to a string that
evaluates to an additional operator.<br>
<br style="font-family: monospace;">
<span style="font-family: monospace;">% set a "2 + 2"</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">2 + 2</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">% expr $a < 3</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">0</span><br>
<br>
While both <span style="font-family: monospace;">expr</span> command
usages described above will produce the same results, the compiled
version will execute much more quickly. The rule of thumb to remember
is that every expression should be enclosed in brace quote characters <span
style="font-family: monospace;">{}</span>.<br>
<br>
The same logic applies to expression arguments to the <span
style="font-family: monospace;">if</span>, <span
style="font-family: monospace;">for</span>, or <span
style="font-family: monospace;">while</span> commands.<br>
<br>
<ol style="font-family: monospace;">
<li>if "$a < 3" {puts "a is less than 3"}<br>
</li>
<li>if {$a < 3} {puts "a is less than 3"}</li>
</ol>
The unbraced <span style="font-family: monospace;">if (1)</span>
command would not be compiled while the braced <span
style="font-family: monospace;">if (2)</span> command would be
compiled. The unbraced <span style="font-family: monospace;">if (1)</span>
command would execute significantly more slowly when compare to the
braced <span style="font-family: monospace;">if (2)</span> command.<br>
<br>
The only exception to this rule is an expression that is a constant
boolean value (like 0, 1, true, false) need not be braced in order to
be compiled. This is supported so that either of the following usages
will eliminate dead code at compile time.<br>
<br>
<span style="font-family: monospace;">if 0 {<br>
never_call_1<br>
}<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;">if {0} {<br>
never_call_2<br>
}</span><br>
</span>
<h4>Braced Script Arguments<br>
</h4>
In general, commands that accept scripts as arguments (<span
style="font-family: monospace;">catch, expr, for, foreach, if,
switch, and while</span>) can only be compiled when each of the script
arguments is brace quoted. For example, the following for loop cannot
be compiled.<br>
<br>
<span style="font-family: monospace;">set script "incr i"<br>
for {set i 0} {$i < 10} $script {<br>
puts "i is $i"<br>
}<br>
<br>
</span>The type is usage is valid Tcl code, but it is not common usage
and cannot be compiled.<br>
<h4>The foreach Command<br>
</h4>
Use of the <span style="font-family: monospace;">foreach</span>
command is significantly optimized by the TJC compiler. The most common
usage of foreach is to loop over a single list using a single variable:<br>
<br>
<span style="font-family: monospace;">set l {1 2 3 4}<br>
foreach v $l {<br>
puts "v is $v"<br>
}<br>
<br>
</span>The <span style="font-family: monospace;">foreach</span>
command can also be used to loop over multiple elements in a list. The
following example shows how one might implement looping over multiple
elements with a <span style="font-family: monospace;">for</span>
command and how the same thing can be accomplished with a <span
style="font-family: monospace;">foreach</span> command.<span
style="font-family: monospace;"><br>
</span><br>
<span style="font-family: monospace;">set l {1 2 3 4}<br>
<br>
set len [</span><span style="font-family: monospace;"><span
style="font-family: monospace;">llength $l]</span><br>
</span><span style="font-family: monospace;">for {set i 0} {$i <
$len} {incr i 2} {<br>
set v1 [lindex $l $i]<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> set v2 [lindex $l
[expr {$i + 1}]]</span><br>
</span><span style="font-family: monospace;"> puts
"v1 is $v1"<br>
puts "v2 is $v2"<br>
}<br>
<br>
foreach {v1 v2} $l {<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> puts "v1 is $v1"<br>
puts "v2 is $v2"</span><br>
</span><span style="font-family: monospace;">}<br>
</span><br>
The <span style="font-family: monospace;">foreach</span> command can
also loop over multiple lists with a single loop. The following example
shows how one might implement looping over multiple lists with a <span
style="font-family: monospace;">for</span> command and how and how the
same thing can be accomplished with a <span
style="font-family: monospace;">foreach</span> command.<br>
<br>
<span style="font-family: monospace;">set l1 {1 2 3 4}<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;">set l2 {-1 -2 -3 -4}</span><br>
</span><span style="font-family: monospace;"><br>
set len [</span><span style="font-family: monospace;"><span
style="font-family: monospace;">llength $l1]</span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
</span></span></span></span><span style="font-family: monospace;">for
{set i 0} {$i < $len} {incr i} {<br>
set v1 [lindex $l1 $i]<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> set v2 [lindex $l2
$i]</span><br>
</span><span style="font-family: monospace;"> puts
"v1 is $v1"<br>
puts "v2 is $v2"<br>
}<br>
<br>
foreach v1 $l1 v2 $l2 {<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> puts "v1 is $v1"<br>
puts "v2 is $v2"</span><br>
</span><span style="font-family: monospace;">
}<br>
</span><br>
Using a <span style="font-family: monospace;">foreach</span> command
in situations like those shown above is always going to execute more
quickly than a <span style="font-family: monospace;">for</span>
command because the TJC compiler contains specific optimizations to
cover these usages.<br>
<br>
Be aware that in order to compile a <span
style="font-family: monospace;">foreach</span> command, the variable
name argument(s) must be either a constant string or a constant list.
The following usages are <span style="font-weight: bold;">NOT</span>
recommended. The variable name(s) are not known at compile time, so the
loop body would not get compiled and the resulting code would execute
very slowly.<br>
<br>
<span style="font-family: monospace;">foreach $varname {1 2 3 4 5} {<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> # Very slow!</span><br>
</span><span style="font-family: monospace;">
}<br>
</span><span style="font-family: monospace;">
foreach arr($key) {1 2 3 4 5} {<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> # Very slow!</span><br>
</span><span style="font-family: monospace;">
}<br>
</span>
<h4>The incr Command<br>
</h4>
Use of the <span style="font-family: monospace;">incr</span>
command is significantly optimized by the TJC compiler. The <span
style="font-family: monospace;">incr</span> command supports both
array and scalar variables. Be aware that <span
style="font-family: monospace;">incr</span> executes significantly
faster when used with scalar variables. For example:<br>
<br>
<span style="font-family: monospace;">for {set i 0} {$i < 1000}
{incr i} {<br>
# Do something<br>
}<br>
<br>
</span>The loop above would not execute as quickly if an array variable
was used as the loop var:<span style="font-family: monospace;"><br>
<br>
<span style="font-family: monospace;"></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;">for
{set arr(i) 0} {$arr(i) < 1000} {incr </span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">arr(i)</span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;">}
{<br>
# Do something<br>
}<br>
</span></span>
<h4>Constant Values In Loops<br>
</h4>
It is generally a good idea to initialize a local variable to the value
of an expression that will remain constant during a loop instead of
evaluating the expression each time the loop is executed. In the
following example, the <span style="font-family: monospace;">llength</span>
command is called each time the loop is executed.<br>
<br>
<span style="font-family: monospace;">set l {1 2 3 4 5 6}</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">for {set i 2} {$i < [llength
$l]} {incr i} {<br>
puts "value is [lindex $l $i]"<br>
}<br>
</span><br>
The value returned by <span style="font-family: monospace;">llength</span>
will always be 6, so it is more efficient to invoke this command just
once before the loop begins.<br>
<span style="font-family: monospace;"><br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;">set l {1 2 3 4 5 6}<br>
set len [llength $l]<br style="font-family: monospace;">
</span><span style="font-family: monospace;">for {set i 2} {$i <
$len} {incr i} {<br>
puts "value is [lindex $l $i]"<br>
}</span></span><span style="font-family: monospace;"><br>
</span>
<h4>The switch Command<br>
</h4>
The <span style="font-family: monospace;">switch</span> command
supports two usages that are legal in Tcl code.<br>
<br>
<span style="font-family: monospace;">switch $string {<br>
"Foo" {puts "matched Foo"}<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> "Bar" {puts
"matched Bar"}</span><br>
</span><span style="font-family: monospace;">}<br>
<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;">switch $str \<br>
"Foo" </span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">{puts "matched Foo"} \<br>
</span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">
"Bar" </span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">{puts
"matched Bar"}</span></span></span><br>
</span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"></span></span><br>
The first usage supports a static list of pattern/scripts
elements and is the most common way switch is used. In the example
above, the second usage is identical. The second usage exists because
the developer might want to match against patterns that are not
constant strings. For example:<br>
<br>
<span style="font-family: monospace;">set pat1 "Foo"</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">set pat2 "Bar"</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">switch $str \<br>
$pat1 {puts "matched Foo"} \<br>
$pat2 {puts "matched Bar"}<br>
</span><br>
Both of these switch command usages are compiled by TJC. The only thing
to be aware of is that in either case the matched string cannot start
with the '-' character. This is because the switch command also accepts
option arguments like <span style="font-family: monospace;">-glob,
-regexp, -exact, and --</span>. Typically, the user would not want an
error to be generated if the string being matched just happened to
start with a '-' character. This error condition can be avoided by
adding -- just after the switch command.<br>
<br>
<span style="font-family: monospace;">switch -- $string {<br>
"Foo" {puts "matched Foo"}<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> "Bar" {puts
"matched Bar"}</span><br>
</span><span style="font-family: monospace;">}<br>
</span><br>
Adding -- as the second argument is the safest way to use the switch
command. TJC will also generate slightly better code when the --
argument is used with a switch command.<br>
<span style="font-family: monospace;"><br>
</span>A TJC compiled <span style="font-family: monospace;">switch</span>
command is optimized for constant string patterns. Using a <span
style="font-family: monospace;">switch</span> command to compare a
string to a number of constant string values is always going to be
faster than using an <span style="font-family: monospace;">if/elseif</span>
command.<br>
<br>
<span style="font-family: monospace;">switch -- $string {<br>
"Foo" {puts "matched Foo"}<br>
"Bar" {puts "matched Bar"}<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"> "Baz" -<br>
</span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"> "Zaz" {puts
"matched Baz or Zaz"}</span></span><br>
</span></span><span style="font-family: monospace;">}<br>
</span><br style="font-family: monospace;">
<span style="font-family: monospace;">if {$</span><span
style="font-family: monospace;"><span style="font-family: monospace;">string</span></span><span
style="font-family: monospace;"> == "Foo"} {<br>
puts "matched Foo"}<br>
} elseif {$</span><span style="font-family: monospace;"><span
style="font-family: monospace;">string</span></span><span
style="font-family: monospace;"> == "Bar"} {<br>
puts "matched bar"}<br>
} elseif {$string == "Baz" || $string == "Zaz"} {<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">puts
"matched Baz or Zaz"</span></span></span></span><br>
</span><span style="font-family: monospace;">}<br>
</span>
<h4>The lappend Command<br>
</h4>
The <span style="font-family: monospace;">lappend</span>
command is significantly optimized by the TJC compiler, but some usage
is better than others. For example, the following code is significantly
optimized.<br>
<br>
<span style="font-family: monospace;">catch {unset </span><span
style="font-family: monospace;"><span style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;">}</span><br>
<span style="font-family: monospace;">set mylist [list]<br>
lappend </span><span style="font-family: monospace;"><span
style="font-family: monospace;">mylist </span></span><span
style="font-family: monospace;"> A B C<br>
</span><span style="font-family: monospace;"><br>
</span>The<span style="font-family: monospace;"> <span
style="font-family: monospace;">lappend</span>
</span>command supports appending to a variable that is not yet set,
but this usage is not optimized. For example, the following lappend
usage would not execute as quickly as the example above.<span
style="font-family: monospace;"><br>
<br>
catch {unset </span><span style="font-family: monospace;"><span
style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;">}</span><span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
lappend </span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span><span
style="font-family: monospace;"><span style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;"></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;">
A B C</span><span style="font-family: monospace;"><br>
</span><br>
</span>The<span style="font-family: monospace;"><span
style="font-family: monospace;"> <span style="font-family: monospace;">lappend</span>
</span></span>command also supports initializing a variable value to an
empty list, but this usage is not optimized. The following code would
also execute less quickly that the first example given above.<span
style="font-family: monospace;"><br>
<span style="font-family: monospace;"><br>
catch {unset </span><span style="font-family: monospace;"><span
style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;">}</span><span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
lappend </span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span><span
style="font-family: monospace;"><span style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;"></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
</span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">lappend </span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span><span
style="font-family: monospace;"><span style="font-family: monospace;">mylist
</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">A B C</span><span
style="font-family: monospace;"><br>
</span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><br>
</span></span>The <span style="font-family: monospace;">lappend</span>
command supports passing multiple values to be appended. It is more
efficient to pass multiple items to be appended to a single <span
style="font-family: monospace;">lappend</span> command as opposed to
using multiple <span style="font-family: monospace;">lappend</span>
commands. For example, the following code:<br>
<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;">catch {unset </span><span
style="font-family: monospace;"><span style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;">}<br>
set mylist [list]<br>
lappend mylist A B \<br>
REAL_LONG_ELEMENT<br>
<br>
</span><span style="font-family: monospace;"><span
style="font-family: monospace;"></span></span></span>Will produce more
efficient code than the following:<span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><br>
</span></span><br>
</span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">catch {unset </span><span
style="font-family: monospace;"><span style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;">}<br>
set mylist [list]<br>
lappend mylist A<br>
</span></span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">lappend mylist B<br>
</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">lappend
mylist </span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">REAL_LONG_ELEMENT</span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span></span></span><br>
</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
<br>
</span></span></span></span>The <span style="font-family: monospace;">lappend</span>
command supports appending to an array variable, as in the following
example:<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span><span
style="font-family: monospace;"><br>
set arr(elems) [list]<br>
lappend </span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">arr(elems)</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">
A B C<br>
</span></span><br>
</span></span>While using <span style="font-family: monospace;">lappend</span>
with array variables is optimized by TJC, it is much faster to use the <span
style="font-family: monospace;">lappend</span> command with scalar
variables. Using a scalar will not make much difference with just one <span
style="font-family: monospace;">lappend</span> command, but a number
of <span style="font-family: monospace;">lappend</span> command in a
loop would execute more quickly if a scalar local variable is used
instead of an array variable. For example, the loop:<span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span><span
style="font-family: monospace;"><br>
set arr(elems) [list]<br>
foreach var {1 2 3 4 5 6 7 8 9 10} {<br>
lappend </span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">arr(elems)</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">
$var<br>
}<br>
</span></span></span></span><br>
</span></span>Would execute more quickly when rewritten as:<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span><span
style="font-family: monospace;"><br>
set l [list]<br>
foreach var {1 2 3 4 5 6 7 8 9 10} {<br>
lappend </span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">l</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">
$var<br>
}<br>
</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">
set arr(elems) $l</span></span></span></span></span></span></span></span><br>
</span></span>
<h4>The append Command<br>
</h4>
The <span style="font-family: monospace;">append</span>
command is significantly optimized by the TJC compiler, but some usage
is better than others. For example, the following code is
significantlyoptimized.<br>
<br>
<span style="font-family: monospace;">catch {unset </span><span
style="font-family: monospace;"><span style="font-family: monospace;">myvar</span></span><span
style="font-family: monospace;">}</span><br>
<span style="font-family: monospace;">set myvar ""<br>
append </span><span style="font-family: monospace;"><span
style="font-family: monospace;">myvar </span></span><span
style="font-family: monospace;">STR1 STR2 STR3<br>
</span><span style="font-family: monospace;"><br>
</span>The <span style="font-family: monospace;">append</span>
command supports appending to a variable that is not yet set,
but this usage is not optimized. For example, the following <span
style="font-family: monospace;">append</span>
usage would not execute as quickly as the example above.<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
catch {unset </span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">myvar}</span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span><span
style="font-family: monospace;"></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
append </span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">myvar</span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;">
</span></span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">STR1
STR2 STR3</span></span></span></span></span><br>
<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span></span></span></span>The <span
style="font-family: monospace;">append</span>
command supports passing multiple values to be appended. It is more
efficient to pass multiple items to be appended to a single <span
style="font-family: monospace;">append</span> command as opposed to
using multiple <span style="font-family: monospace;">append</span>
commands. For example, the following code:<span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;">catch {unset </span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">myvar}</span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span><span
style="font-family: monospace;"><br>
set </span></span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">myvar </span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">""<br>
</span></span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">append </span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">myvar</span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;">
</span></span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">STR1
STR2 \<br>
REAL_LONG_STRING</span></span></span></span></span><br>
</span></span></span></span><br>
Will produce more efficient code than the following:<span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><br>
</span></span><br>
</span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">catch {unset </span><span
style="font-family: monospace;"><span style="font-family: monospace;">myvar</span></span><span
style="font-family: monospace;">}<br>
set myvar ""<br>
append myvar STR1<br>
</span></span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">append myvar STR2<br>
</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">append
myvar </span></span></span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">REAL_LONG_STRING</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span></span><br>
</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span><br>
<br>
</span></span>The <span style="font-family: monospace;">append</span>
command supports appending to an array variable, as in the following
example:<span style="font-family: monospace;"><span
style="font-family: monospace;"><br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"></span><span
style="font-family: monospace;"><br>
set arr(str) ""<br>
append </span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">arr(str)</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">
HELLO<br>
</span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">append
</span></span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">arr(str)</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">
" "</span></span></span></span></span></span><br>
</span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">append
</span></span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">arr(str)</span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">
THERE</span></span></span></span></span></span><br>
</span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span><br>
</span></span></span></span>While supported, <span
style="font-family: monospace;">append</span> operations on array
variables can't be optimized and will execute very slowly. Developers
should use <span style="font-family: monospace;">append</span>
exclusively with scalar variables. The example given above should be
rewritten as:<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><br>
set str ""<br>
append str </span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">HELLO<br>
</span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">append
str </span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">"
"<br>
</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">append
str </span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">THERE<br>
set arr(str) $str</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span></span></span></span></span></span></span></span></span></span><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"></span></span></span></span></span></span><br>
<h4>The list Command<br>
</h4>
The <span style="font-family: monospace;">list</span>
command is optimized for use with many common list operation. The
following example shows the optimal way to initialize a variable for
use with <span style="font-family: monospace;">lappend</span> or
any other list operation.<br>
<br>
<span style="font-family: monospace;">catch {unset </span><span
style="font-family: monospace;"><span style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;">}</span><br>
<span style="font-family: monospace;">set mylist [list]<br>
lappend </span><span style="font-family: monospace;"><span
style="font-family: monospace;">mylist </span></span><span
style="font-family: monospace;"> A B C<br>
</span><br>
The example code above is more efficient than the following two
examples. Both of the examples below will initialize the variable to an
empty string instead of an empty list.<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><br>
</span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;">catch
{unset </span><span style="font-family: monospace;"><span
style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;">}</span><br>
<span style="font-family: monospace;">set mylist ""<br>
lappend </span><span style="font-family: monospace;"><span
style="font-family: monospace;">mylist </span></span><span
style="font-family: monospace;"> A B C<br>
<br>
</span></span></span><span style="font-family: monospace;"><span
style="font-family: monospace;"><span style="font-family: monospace;"><span
style="font-family: monospace;">catch {unset </span><span
style="font-family: monospace;"><span style="font-family: monospace;">mylist</span></span><span
style="font-family: monospace;">}</span><br>
<span style="font-family: monospace;">set mylist {}<br>
lappend </span><span style="font-family: monospace;"><span
style="font-family: monospace;">mylist </span></span><span
style="font-family: monospace;"> A B C<br>
</span><br>
</span></span></span>All three of the examples above will produce the
exact same results, but the first example that makes explicit use of
the <span style="font-family: monospace;">list</span> command to
initialize the variable will execute more quickly.<br>
<span style="font-family: monospace;"><span
style="font-family: monospace;"><br>
<br>
</span></span><span style="font-family: monospace;"></span> <span
style="font-family: monospace;"></span>
</body>
</html>
distrib
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Mageia
>
5
>
i586
>
media
>
core-release
>
by-pkgid
>
3728ff00d2930b6add4240a6383264ca
>
files
>
38
