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From: &quot;Yuri Geshelin&quot; &lt;geshelin@hotmail.com&gt;
Newsgroups: comp.soft-sys.matlab
Subject: Re: eval, feval, global
Date: Thu, 27 Mar 2008 03:22:04 +0000 (UTC)
Organization: Bedford Institute of Oceanography
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Thanks to everyone who replied. I agree with John about 
GLOBAL.

As for EVAL, as Peter suggested, let me show you my code, 
which I found useful. 

(Actually, there are two situations, in which I use EVAL, 
but I will start with presenting the 1st one, as it is 
perhaps the more controversial.)

Suppose we have 2 arrays of equal length:

a = [15 23 35 43];
b = [ 2 11  8  3];

and we need an algorithm, which constructs output array c 
as follows:

c = [15 15 &lt;23 repeated 11 times&gt; &lt;35 repeated 8 times&gt; 43 
43 43]

Here is a simple function construct_array1, which 
accomplishes this task.

% *************************** top of construct_array1
function c = construct_array1(a,b)

c = [];
for i = 1 : length(a)
    c = [c ones(1,b(i))*a(i)];
end

% *************************** end of construct_array1

And here is another function construct_array, which 
accomplishes the same task.

% *************************** top of construct_array
function c = construct_array(a,b)

if length(a) ~= length(b)
    error('arrays a and b should be of equal lengths!')
end

k = floor(log10(max(a)))+1;
i = floor(log10(max(b)))+1;
n = length(a);
m = ones(n,1);
c = eval(['[' ...
    reshape([char([m*'o' m*'n' m*'e' m*'s' m*'(' m*'1' 
m*',']) ...
    reshape(sprintf(['%' num2str(i) 'd'],b),i,n)' ...
    char([m*')' m*'*']) ...
    reshape(sprintf(['%' num2str(k) 'd'],a),k,n)' ...
    char(m*' ')]',1,n*(k+i+10)) ']']);
% *************************** end of construct_array

Think this is crazy? However, let us test both functions:

&gt;&gt; t = cputime; construct_array1(1:10,1000001:1000010); 
cputime-t

ans =

     1.1875

&gt;&gt; t = cputime; construct_array(1:10,1000001:1000010); 
cputime-t

ans =

     0.4531

(I was afraid that matlab would crash, but it did not.)

This was experimented on

MATLAB Version 7.0.4.365 (R14) Service Pack 2
Operating System: Microsoft Windows XP Version 5.1 (Build 
2600: Service Pack 2)
Pentuim(R) 4 CPU 3.20 GHz, 1.00 GB of RAM.

My implementation is thus 2.6 times faster than the use of 
a loop. 
I admit that this can be costly in terms of RAM, because 
the text string supplied to eval can 
be huge. But in my practice, the arrays are not too long 
(way shorter than in the test case). 
The cpu time is saved because I call my function many times 
in a loop.

Yuri


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