Matlab, if condition without loop

Question

There are matices A(n,1), B(n,1) and the following condition inside a loop

for i=1:m
   if ( A(i, 1) > error )
      B(i,1) = 0;
   else
      B(i,1) = exp (-A(i,1) / 100)
   end
end

How to rewrite this condition without using any loop? Is it possible something like that

 if ( A(:, 1) > error )
      B(:,1) = 0;
   else
      B(:,1) = exp (-A(:,1) / 100)
 end

Oliver Charlesworth · Accepted Answer · 2012-02-26 15:50:21Z

7

Use logical indexing:

idxs = (A > error);
B( idxs) = 0;
B(~idxs) = exp(-A(~idxs) / 100);

answered Feb 26, 2012 at 15:50

Oliver Charlesworth

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Comments

tmpearce · Accepted Answer · 2012-02-26 16:02:00Z

2

You were close with your suggestion. The key is to form a "logical index."

i = A(:,1) > error;

B(i,:) = 0;
B(~i,:) = exp (-A(:,1) / 100);

Since your matrices A and B are vectors (one-dimensional matrices), the (:,1) and (i,:) aren't necessary in this case but as they were in your initial formulation, I left them in. If you were using multi-dimensional matrices instead (m * n) you could form an (m * n) logical index rather than (m * 1) by doing i = A > error; instead of i = A(:,1) > error;

edited Feb 26, 2012 at 16:02

answered Feb 26, 2012 at 15:56

tmpearce

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