lset

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)