Author: Kevin B. Kenny <email@example.com> State: Final Type: Project Vote: Done Created: 22-Sep-2008 Post-History: Tcl-Version: 8.6
This TIP proposes to modify the lset command to allow it to extend lists.
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)
lset list end+1 0 foo
lset list end+1 end+1 foo
is equivalent to
lappend list [list foo]
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.
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 © 2008 by Kevin B. Kenny.
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