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Re: Performance optimization (allocation inside a for loop)

From: John W. Eaton
Subject: Re: Performance optimization (allocation inside a for loop)
Date: Thu, 2 Apr 2009 12:23:01 -0400

On  2-Apr-2009, James Sherman Jr. wrote:

| Its redundant as far as I know, but I always use it because I think it looks
| more readable.
| James
| On Thu, Apr 2, 2009 at 11:58 AM, Elias Assmann <
| address@hidden> wrote:
| > Rob Mahurin wrote:
| > > octave:29> tic; n = 1e5; retval = 1:n; toc
| > > Elapsed time is 0.000756025 seconds.
| > > octave:30> tic; n = 1e5; retval = (1:n)(1:n); toc
| > > Elapsed time is 0.00757694 seconds.
| > > octave:31> tic; n = 1e5; retval = [1:n]; toc
| > > Elapsed time is 0.0125589 seconds.
| >
| > Can someone explain the difference between "1:n" and "[1:n]"?  I would
| > normally write the brackets because I find that way clearer; does this
| > mean "1:n" is actually the "preferred" way?

In Octave, an expression like 1:n creates a range object, which
contains only the base, limit, and increment as double precision
values, so no matter how many elements are in the range, it only takes
a few bytes of storage (24 for the data plus some overhead for the
internal octave_value object itself).

If you write [1:n], you force a Matrix object with N elements to be
created.  It will require 8*N bytes of storage, plus the overhead for
the internal octave_value object itself.

In Octave 3.2, some operations on ranges will preserve the range data
type.  In earlier versions, nearly all operations converted range
objects to matrix objects first.

If you think it looks more readable to have something surrounding the
range expression, then I would recommend writing (1:n) instead of [1:n].

Finally, range dimensions are always [1, N], so transposing a range
will create a matrix object.  For example, try this:

  x = 1:10;
  y = x';

and note the differents in the number of bytes used.

Maybe we should consider fixing that so that we could also have
column-oriented ranges.


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