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 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,
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 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}}