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lset(n)                      Tcl Built-In Commands                     lset(n)

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NAME
       lset - Change an element in a list

SYNOPSIS
       lset varName ?index ...? newValue
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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 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.

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(n), lappend(n), lindex(n), linsert(n), llength(n), lsearch(n),
       lsort(n), lrange(n), lreplace(n), string(n)

KEYWORDS
       element, index, list, replace, set

Tcl                                   8.4                              lset(n)