lset(3tcl)

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 or equal to the number of elements
   in $varName, then an error occurs.

   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.  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 ALSO

   list(3tcl), lappend(3tcl), lindex(3tcl), linsert(3tcl),  llength(3tcl),
   lsearch(3tcl), lsort(3tcl), lrange(3tcl), lreplace(3tcl), string(3tcl)  

KEYWORDS

   element, index, list, replace, set

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