lset(3tcl)
NAME
lset - Change an element in a list
SYNOPSIS
lset varName ?index...? newValue ______________________________________________________________________________
DESCRIPTION
The lset command accepts a parameter, varName, which it interprets as
the name of a variable containing a Tcl list. It also accepts zero or
more indices 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 varName.
If no indices are presented, the command takes the form:
lset varName newValue
or
lset varName {} newValue
In this case, newValue replaces the old value of the variable varName.
When presented with a single index, the lset command treats the content
of the varName variable as a Tcl list. It addresses the index'th
element in it (0 refers to the first element of the list). When
interpreting the list, lset 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 newValue. This new list is stored in the variable
varName, and is also the return value from the lset command.
If index is negative or greater than or equal to the number of elements
in $varName, then an error occurs.
The interpretation of each simple index value is the same as for the
command string index, supporting simple index arithmetic and indices
relative to the end of the list.
If additional index 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. The command,
lset a 1 2 newValue
or
lset a {1 2} newValue
replaces element 2 of sublist 1 with newValue.
The integer appearing in each index argument must be greater than or
equal to zero. The integer appearing in each index argument must be
strictly less than the length of the corresponding list. In other
words, the lset command cannot change the size of a list. If an index
is outside the permitted range, an error is reported.
EXAMPLES
In each of these examples, the initial value of x is:
set x [list [list a b c] [list d e f] [list g h i]]
{a b c} {d e f} {g h i}
The indicated return value also becomes the new value of x (except in
the last case, which is an error which leaves the value of x
unchanged.)
lset x {j k l}
j k l
lset x {} {j k l}
j k l
lset x 0 j
j {d e f} {g h i}
lset x 2 j
{a b c} {d e f} j
lset x end j
{a b c} {d e f} j
lset x end-1 j
{a b c} j {g h i}
lset x 2 1 j
{a b c} {d e f} {g j i}
lset x {2 1} j
{a b c} {d e f} {g j i}
lset x {2 3} j
list index out of range
In the following examples, the initial value of x is:
set x [list [list [list a b] [list c d]] \
[list [list e f] [list g h]]]
{{a b} {c d}} {{e f} {g h}}
The indicated return value also becomes the new value of x.
lset x 1 1 0 j
{{a b} {c d}} {{e f} {j h}}
lset x {1 1 0} j
{{a b} {c d}} {{e f} {j h}}
SEE ALSO
list(3tcl), lappend(3tcl), lindex(3tcl), linsert(3tcl), llength(3tcl), lsearch(3tcl), lsort(3tcl), lrange(3tcl), lreplace(3tcl), string(3tcl)
KEYWORDS
element, index, list, replace, set
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