TIP:            136
Title:          Large List Initialisation
Version:        $Revision: 1.5 $
Author:         Simon Geard <simon.geard@ntlworld.com>
State:          Final
Type:           Project
Vote:           Done
Created:        25-May-2003
Post-History:   
Tcl-Version:    8.5

~ Abstract

This TIP proposes the addition of a list initialisation command so
that large lists can be easily and efficiently initialised.

~Rationale

With the advent of the ''lset'' command in Tcl 8.4 it seems to me that
we need a method of efficiently initialising large lists that can then
be used as areas of preallocated memory. From a users point of view it
can be much easier to preallocate say a 1000 element array and then
use ''lset'' and ''lindex'' to manipulate it than using ''lappend'' to
build it up. Having posted the idea to the tcl-core mailing list
various alternatives were suggested to create a list of 1000 x
characters:

 1.	''lappend'' in a loop

| unset -nocomplain s
| for {set i 0} {$i < 1000} {incr i} {lappend s {x}}

 2.	''split''ting of a string

| set s [split [string repeat x 1000] ""]

 3.	Direct construction of the string form

| set s x[string repeat " x" 999]

None of these is particularly satisfactory.  (1) seems inefficient
since there are 1000 lappend operations, (2) is not general enough
since it doesn't generalise to more than one character and (3) doesn't
actually create a list.  (2) and (3) also suffer from the problem that
they are not at all obvious to new users and do nothing to dispel the
notion that "everything is a string" in Tcl.

~Implementation

I propose the introduction of a new command, ''lrepeat'':

| lrepeat <number> <element1> ?<element2>? ?<element3>? ...

which returns a list of length <number> * (number of elements). The new list is the given element sequence repeated <number> times.
<number> must be a positive integer and each <element.> a list or
string.

Examples:

| lrepeat 100 0                 - returns a list of 100 zeros
| lrepeat 100 [lrepeat 100 0]   - returns a 100x100 matrix (list of lists) of zeros
| lrepeat 3 a b c               - returns a nine-element list {a b c a b c a b c}
| lrepeat 1 a b c               - identical to [list a b c]

~Reference Implementation

I have implemented this command.  A patch against the Tcl 8.4.3
source, which implements the command and provides both tests and
documentation, is available from
http://homepage.ntlworld.com/whiteowl/tcl/tcl843-lrepeat.patch
The directory also contains timing information and code which demonstrates a performance gain of over ten times for large lists.

~Copyright

This document has been placed in the public domain.