TIP #331: Allow [lset] to Extend Lists
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TIP:	331
Title:	Allow [lset] to Extend Lists
Version:	$Revision: 1.5 $
Authors:	Kevin B. Kenny <kennykb@acm.org>
State:	Final
Type:	Project
Tcl-Version:	8.6
Vote:	Done
Created:	Mon Sep 22 00:00:00 UTC 2008
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/].
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