TIP:		303
Title:		Enhance 'llength' Command to Support Nested Lists
Version:	$Revision: 1.2 $
Author:		Wolf-Dieter Busch <wolf-dieter.busch@gmx.de>
State:		Draft
Type:		Project
Tcl-Version:	8.6
Vote:		Pending
Created:	29-Jan-2007
Post-History:	
Keywords:	Tcl, lindex

~ Abstract

The command '''llength''' currently returns the length of a list. Sometimes it
is desirable to know the length of a nested list. This TIP proposes an
improvement to '''llength''' to do that.

~ Description

Currently, finding the length of a nested list requires the combination of
'''llength''' and '''lindex'''. This works, but is not very clean for a
comparatively common operation when you compare with straight '''lindex'''
usage. This TIP proposes to enhance '''llength''' so that it also does the
indexing, making it's usage in such situations cleaner and less subject to
programming errors.

~~ Proposed Change

The '''llength''' command shall be updated to have syntax like this:

 > '''llength''' ''list'' ?''indexList'' ...?

When no ''indexList'' is supplied, the current behavior is used. When one or
more ''indexList'' arguments are supplied, they are used to restrict which
part of ''list'' is taken the length of, just as if '''lindex''' had been used
to index into ''list''.

Thus, [['''llength {a {b c} d}''']] shall return 3 as nowadays, [['''llength
{a {b c} d} 1''']]> shall return the length of the nested list on index 1,
here {b c} => 2, and [['''llength {a {b c} d} 1 0''']] shall return the length
of the nested list on index 0 of nested list on index 1, i.e., 1 in this case.

~~ Compatibility

As this only changes a way of use of the '''llength''' that currently returns
an error, there are no compatibility problems.

~ Example Implementation

The procedure '''idxllength''' (below) does what is described above, but
inefficiently:

|proc idxllength args {
|    llength [lindex {*}$args]
|}

~ Copyright

This document has been placed in the public domain.
