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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 mapeach n "" Tcl "Tcl Built-In Commands"
.BS
'\" Note: do not modify the .SH NAME line immediately below!
.SH NAME
mapeach \- Iterate over all elements in one or more lists and collect results
.SH SYNOPSIS
\fBmapeach \fIvarname list body\fR
.br
\fBmapeach \fIvarlist1 list1\fR ?\fIvarlist2 list2 ...\fR? \fIbody\fR
.BE
.SH DESCRIPTION
.PP
The \fBmapeach\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), \fBmapeach\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. \fBmapeach\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
'\" Maintainers: notice the tab hacking below!
.ta 3i
set list1 {a b c d}
set list2 {1 2 3 4}
set zipped [\fBmapeach\fR a $list1 b $list2 {list $a $b}]
# The value of zipped is "{a 1} {b 2} {c 3} {d 4}"
.CE
.PP
Filter a list:
.PP
.CS
set values {1 2 3 4 5 6 7 8}
proc isGood {n} { expr { ($n % 2) == 0 } }
set goodOnes [\fBmapeach\fR x $values {expr {[isGood $x] ? $x : [continue]}}]
# The value of goodOnes is "2 4 6 8"
.CE
.PP
Take a prefix from a list:
.PP
.CS
set values {8 7 6 5 4 3 2 1}
proc isGood {n} { expr { $n > 3 } }
set prefix [\fBmapeach\fR x $values {expr {[isGood $x] ? $x : [break]}}]
# The value of prefix is "8 7 6 5 4"
.CE
.SH "SEE ALSO"
for(n), while(n), break(n), continue(n), foreach(n)
.SH KEYWORDS
foreach, iteration, list, loop, map
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