TIP:		331
Title:		Allow [lset] to Extend Lists
Version:	$Revision: 1.5 $
Author:		Kevin B. Kenny <kennykb@acm.org>
State:		Final
Type:		Project
Vote:		Done
Created:	22-Sep-2008
Post-History:
Tcl-Version:	8.6

~ Abstract

This TIP proposes to modify the '''lset''' command to allow it to extend
lists.

~ Proposal

The '''lset''' command shall be modified to allow the index ''end+1'' (or any
equivalent index designating the element one beyond the last element of the
list). For the simple usage:

|    lset list end+1 foo

the effect is the same as if the script had evaluated the command:

|    lappend list foo

For the usage:

|    lset list $n end+1 foo

the effect is much the same as if the script had evaluated:

|    set temp [lindex $list $n]
|    lappend temp foo
|    lset list $n $temp

(except, of course that no ''temp'' variable is created)

The usage:

|    lset list end+1 0 foo

or, equivalently,

|    lset list end+1 end+1 foo

is equivalent to

|    lappend list [list foo]

~ Rationale

Clearly, there are equivalent constructs to all of these usages. Nevertheless,
it is convenient not to have to worry about whether a given index is in
bounds, particularly when iterating through a vector or matrix. With the
proposed change, many of the standard algorithms in linear algebra will just
work, without either having to preinitialize a list to a given size or to have
a test to determine whether to use '''lappend''' or '''lset''' to store a
given element.

~ Alternatives

Explicitly out of scope is the treatment of indices greater than or equal to
''end+2''. Setting non-contiguous elements would raise false expectations of
sparse lists, '''NULL''' elements, and so on.

~ Copyright

Copyright © 2008 by Kevin B. Kenny.

This document may be distributed subject to the terms and conditions set forth
in the Open Publication License, version 1.0
[http://www.opencontent.org/openpub/].