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'\" Copyright (c) 2025 Donal K. Fellows
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.TH lfilter n "" Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
lfilter \- Select elements from a list based on an expression
.SH SYNOPSIS
\fBlfilter \fIvarname list expression\fR
.br
\fBlfilter \fIvarlist1 list1\fR ?\fIvarlist2 list2 ...\fR? \fIbody\fR
.BE
.SH DESCRIPTION
.PP
The \fBlfilter\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 values from the
(first) list where the \fIbody\fR script evaluates to a true value.
.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), \fBlfilter\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 script completes normally and produces a true value,
that element of \fIlist\fR 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.
For each iteration, the value that is appended when \fIbody\fR yields
a true value (i.e., anything accepted by \fBTcl_GetBooleanFromObj\fR as a
boolean) is the value that was assigned to the first variable of the first
list of variables; all other values are not collected.
.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 lead value
is not appended to the accumulator list.
.SH EXAMPLES
.PP
Select the elements of the list that are odd:
.PP
.CS
\fBlfilter\fR x {0 2 4 1 3 5 10 8 6 11 9 7} {
    expr {($x % 2) == 1}
}
# The result is "1 3 5 11 9 7"
.CE
.PP
Select elements of a list according to a condition in another list:
.PP
.CS
\fBlfilter\fR x [lseq 10] y [lseq 0 .. 1 by 0.1] {
    expr {$y > 0.3 && $y < 0.7}
}
# The result is "4 5 6"
.CE
.PP
Select the elements containing at least five characters:
.PP
.CS
set sentence "The quick brown fox jumps over the lazy dog."
\fBlfilter\fR word [split $sentence] {
    expr {[string length $word] >= 5}
}
# The result is "quick brown jumps"
.CE
.PP
Select the elements that are true palindromes:
.PP
.CS
set usernames {tom bob steve dood}
\fBlfilter\fR word $usernames {
    string equal $word [string reverse $word]
}
# The result is "bob dood"
.CE
.SH "SEE ALSO"
dict(n), foreach(n), lmap(n), lsearch(n)
.SH KEYWORDS
foreach, iteration, list, loop, filter
'\" Local Variables:
'\" mode: nroff
'\" End:
