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'\"
'\" Copyright (c) 2001 by Kevin B. Kenny <kennykb@acm.org>. All rights reserved.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
'\" RCS: @(#) $Id: lset.n,v 1.18 2008/10/06 04:25:24 kennykb Exp $
'\"
.so man.macros
.TH lset n 8.4 Tcl "Tcl Built-In Commands"
.BS
'\" Note: do not modify the .SH NAME line immediately below!
.SH NAME
lset \- Change an element in a list
.SH SYNOPSIS
\fBlset \fIvarName ?index ...? newValue\fR
.BE
.SH DESCRIPTION
.PP
The \fBlset\fR command accepts a parameter, \fIvarName\fR, which
it interprets as the name of a variable containing a Tcl list.
It also accepts zero or more \fIindices\fR into
the list. The indices may be presented either consecutively on the
command line, or grouped in a
Tcl list and presented as a single argument.
Finally, it accepts a new value for an element of \fIvarName\fR.
.PP
If no indices are presented, the command takes the form:
.CS
lset varName newValue
.CE
or
.CS
lset varName {} newValue
.CE
In this case, \fInewValue\fR replaces the old value of the variable
\fIvarName\fR.
.PP
When presented with a single index, the \fBlset\fR command
treats the content of the \fIvarName\fR variable as a Tcl list.
It addresses the \fIindex\fR'th element in it
(0 refers to the first element of the list).
When interpreting the list, \fBlset\fR observes the same rules
concerning braces and quotes and backslashes as the Tcl command
interpreter; however, variable
substitution and command substitution do not occur.
The command constructs a new list in which the designated element is
replaced with \fInewValue\fR. This new list is stored in the
variable \fIvarName\fR, and is also the return value from the \fBlset\fR
command.
.PP
If \fIindex\fR is negative or greater than the number
of elements in \fI$varName\fR, then an error occurs.
.PP
If \fIindex\fR is equal to the numnber of elements in \fI$varName\fR,
then the given element is appended to the list.
.PP
The interpretation of each simple \fIindex\fR value is the same as
for the command \fBstring index\fR, supporting simple index
arithmetic and indices relative to the end of the list.
.PP
If additional \fIindex\fR arguments are supplied, then each argument is
used in turn to address an element within a sublist designated
by the previous indexing operation,
allowing the script to alter elements in sublists (or append elements
to sublists). The command,
.CS
lset a 1 2 newValue
.CE
or
.CS
lset a {1 2} newValue
.CE
replaces element 2 of sublist 1 with \fInewValue\fR.
.PP
The integer appearing in each \fIindex\fR argument must be greater
than or equal to zero. The integer appearing in each \fIindex\fR
argument must be less than or equal to the length of the corresponding
list. In other words, the \fBlset\fR command can change the size
of a list only by appending an element (setting the one after the current
end). If an index is outside the permitted range, an error is reported.
.SH EXAMPLES
.PP
In each of these examples, the initial value of \fIx\fR is:
.CS
set x [list [list a b c] [list d e f] [list g h i]]
\fI\(-> {a b c} {d e f} {g h i}\fR
.CE
The indicated return value also becomes the new value of \fIx\fR
(except in the last case, which is an error which leaves the value of
\fIx\fR unchanged.)
.CS
\fBlset\fR x {j k l}
\fI\(-> j k l\fR
\fBlset\fR x {} {j k l}
\fI\(-> j k l\fR
\fBlset\fR x 0 j
\fI\(-> j {d e f} {g h i}\fR
\fBlset\fR x 2 j
\fI\(-> {a b c} {d e f} j\fR
\fBlset\fR x end j
\fI\(-> {a b c} {d e f} j\fR
\fBlset\fR x end-1 j
\fI\(-> {a b c} j {g h i}\fR
\fBlset\fR x 2 1 j
\fI\(-> {a b c} {d e f} {g j i}\fR
\fBlset\fR x {2 1} j
\fI\(-> {a b c} {d e f} {g j i}\fR
\fBlset\fR x {2 3} j
\fI\(-> list index out of range\fR
.CE
In the following examples, the initial value of \fIx\fR is:
.CS
set x [list [list [list a b] [list c d]] \e
[list [list e f] [list g h]]]
\fI\(-> {{a b} {c d}} {{e f} {g h}}\fR
.CE
The indicated return value also becomes the new value of \fIx\fR.
.CS
\fBlset\fR x 1 1 0 j
\fI\(-> {{a b} {c d}} {{e f} {j h}}\fR
\fBlset\fR x {1 1 0} j
\fI\(-> {{a b} {c d}} {{e f} {j h}}\fR
.CE
.SH "SEE ALSO"
list(n), lappend(n), lindex(n), linsert(n), llength(n), lsearch(n),
lsort(n), lrange(n), lreplace(n),
string(n)
.SH KEYWORDS
element, index, list, replace, set
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