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man pages section 1: User Commands

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Updated: Wednesday, February 9, 2022

lset (1t)


lset - Change an element in a list


lset varName ?index ...? newValue


lset(1t)                     Tcl Built-In Commands                    lset(1t)


       lset - Change an element in a list

       lset varName ?index ...? newValue

       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 con-
       secutively 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


              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 ele-
       ment in it (0 refers to the first element of the  list).   When  inter-
       preting  the  list,  lset observes the same rules concerning braces and
       quotes and backslashes as the Tcl command interpreter;  however,  vari-
       able  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  $var-
       Name, 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 previ-
       ous indexing operation, allowing the script to alter elements  in  sub-
       lists (or append elements to sublists).  The command,

              lset a 1 2 newValue


              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

              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 attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

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

       element, index, list, replace, set

       Source  code  for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   8.4                             lset(1t)