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Subject: Re: Speeding Up Code
Date: Tue, 7 Jun 2011 15:51:20 +0000 (UTC)
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"Yvette Hammond" wrote in message <isk6s3$r2a$1@newscl01ah.mathworks.com>...
> Hi,
> 
> I have the folllowing type of loop several times in my code and three of the lines are slowing my code down massively:
> 
> B=zeros(length(ID),1);
> r_i=zeros(1,1);
> for i=1:1:length(ID)
> 
> r_i=find(mat1(:,1)==ID(i,1));
> if isempty(r_i)
>       B(i,1)=0;
>       
>    else
>        
>        B(i,1) = r_i;
>        
>        if max(B)==0
>            B1=0;
>        else
>        B1=B(B~=0);
>        end
> end
> end
> 
> The <mx1> matrix 'ID' contains a list of numbers that I want to search for in the <px2> matrix 'mat1'. I want B1 to be an ordered list of the rows in 'mat1' where each number in 'ID' occurs (if it occurs at all). All numbers in 'mat1(:,1)' are unique and occur only once, the numbers in 'ID' may repeat. 'mat1' may not contain every number in 'ID'.
> 
> The three lines that are taking the longest to compute are (in order of longest to shortest) 'B1=B(B~=0);', 'r_i=find(mat1(:,1)==ID(i,1));' and 'if max(B)==0'.
> 
> Any ideas on how to speed up the above code would be very much appreciated, I have been struggling with this for a long time!
> 
> Thanks in advance,
> 
> Yvette
- - - - - - - - -
  The five-line if-else-end sequence involving B1 looks completely misplaced.  It keeps overwriting all its previous computations in the for-loop, so therefore it ought to be done after the for-loop finishes where it would only be performed once.

  However, there is a function, 'ismember', that can calculate B much more efficiently:

 [t,B] = ismember(ID,mat1(:,1));
 B1 = sort(B(t));

I have interpreted your phrase "I want B1 to be an ordered list" to mean you wanted the B1 indices of B to be in ascending order.  That would not be accomplished in your code.

Roger Stafford