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'\"
'\" Copyright (c) 2012 Trevor Davel
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
.so man.macros
.TH lmap n "" Tcl "Tcl Built-In Commands"
.BS
'\" Note: do not modify the .SH NAME line immediately below!
.SH NAME
lmap \- Iterate over all elements in one or more lists and collect results
.SH SYNOPSIS
\fBlmap \fIvarname list body\fR
.br
\fBlmap \fIvarlist1 list1\fR ?\fIvarlist2 list2 ...\fR? \fIbody\fR
.BE
.SH DESCRIPTION
.PP
The \fBlmap\fR command implements a loop where the loop variable(s) take on
values from one or more lists, and the loop returns a list of results
collected from each iteration.
.PP
In the simplest case there is one loop variable, \fIvarname\fR, and one list,
\fIlist\fR, that is a list of values to assign to \fIvarname\fR. The
\fIbody\fR argument is a Tcl script. For each element of \fIlist\fR (in order
from first to last), \fBlmap\fR assigns the contents of the element to
\fIvarname\fR as if the \fBlindex\fR command had been used to extract the
element, then calls the Tcl interpreter to execute \fIbody\fR. If execution of
the body completes normally then the result of the body is appended to an
accumulator list. \fBlmap\fR returns the accumulator list.
.PP
In the general case there can be more than one value list (e.g., \fIlist1\fR
and \fIlist2\fR), and each value list can be associated with a list of loop
variables (e.g., \fIvarlist1\fR and \fIvarlist2\fR). During each iteration of
the loop the variables of each \fIvarlist\fR are assigned consecutive values
from the corresponding \fIlist\fR. Values in each \fIlist\fR are used in order
from first to last, and each value is used exactly once. The total number of
loop iterations is large enough to use up all the values from all the value
lists. If a value list does not contain enough elements for each of its loop
variables in each iteration, empty values are used for the missing elements.
.PP
The \fBbreak\fR and \fBcontinue\fR statements may be invoked inside
\fIbody\fR, with the same effect as in the \fBfor\fR and \fBforeach\fR
commands. In these cases the body does not complete normally and the result is
not appended to the accumulator list.
.SH EXAMPLES
.PP
Zip lists together:
.PP
.CS
set list1 {a b c d}
set list2 {1 2 3 4}
set zipped [\fBlmap\fR a $list1 b $list2 {list $a $b}]
# The value of zipped is "{a 1} {b 2} {c 3} {d 4}"
.CE
.PP
Filter a list to remove odd values:
.PP
.CS
set values {1 2 3 4 5 6 7 8}
proc isEven {n} {expr {($n % 2) == 0}}
set goodOnes [\fBlmap\fR x $values {expr {
[isEven $x] ? $x : [continue]
}}]
# The value of goodOnes is "2 4 6 8"
.CE
.PP
Take a prefix from a list based on the contents of the list:
.PP
.CS
set values {8 7 6 5 4 3 2 1}
proc isGood {counter} {expr {$n > 3}}
set prefix [\fBlmap\fR x $values {expr {
[isGood $x] ? $x : [break]
}}]
# The value of prefix is "8 7 6 5 4"
.CE
.SH "SEE ALSO"
break(n), continue(n), for(n), foreach(n), while(n)
.SH KEYWORDS
foreach, iteration, list, loop, map
'\" Local Variables:
'\" mode: nroff
'\" End:
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