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lset(3tcl) Tcl Built-In Commands lset(3tcl) 


NAME

lset - Change an element in a list

SYNOPSIS

lset varName ?index...? newValue



 


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.


If index has the value end, it refers to the last element in the
  list, and end-integer refers to the last element in the list
  minus the specified integer offset.


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)

 


KEYWORDS

element, index, list, replace, set



  

    8.4
    Tcl