lset(n) Tcl Built-In Commands lset(n)______________________________________________________________________________NAMElset - Change an element in a list
SYNOPSISlset 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 con-
secutively 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 ele-
ment in it (0 refers to the first element of the list). When inter-
preting the list, lset observes the same rules concerning braces and
quotes and backslashes as the Tcl command interpreter; however, vari-
able 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.
If index has the value end, it refers to the last element in the list,
and end-integer refers to the last element in the list minus the speci-
fied integer offset.
If additional index arguments are supplied, then each argument is used
in turn to address an element within a sublist designated by the previ-
ous indexing operation, allowing the script to alter elements in sub-
lists. 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 ALSOlist(n), lappend(n), lindex(n), linsert(n), llength(n), lsearch(n),
lsort(n), lrange(n), lreplace(n)KEYWORDS
element, index, list, replace, set
Tcl 8.4 lset(n)
