lset - Change an element in a list
lset varName ?index ...? newValue
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:
or
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 the number of elements in
$varName, then an error occurs.
If
index is equal to the number of elements in
$varName, then the
given element is appended to the list.
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 (or
append elements to sublists). The command,
or
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
less than or equal to the length of the corresponding list. In other words,
the
lset 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.
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}}
list(3tcl),
lappend(3tcl),
lindex(3tcl),
linsert(3tcl),
llength(3tcl),
lsearch(3tcl),
lsort(3tcl),
lrange(3tcl),
lreplace(3tcl),
string(3tcl)
element, index, list, replace, set