Matrix manipulation, filtering

2012 1 17
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Given a large matrix containing values in range [0,1].
Is it possible to change (as in, set to fixed value, add value, subtract value ect.) elements above a certain threshold, without the use of loops? And without changing the rest of the matrix.

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Sean de Wolski
Sean de Wolski 2012-1-17

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Of course - use logical indexing:
A = rand(5);
A(A>0.5) = 10; %set all values in a greater than 0.5 to 10

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Nicklas Utgaard
Nicklas Utgaard 2012-1-17
Thanks. :)
I see how this would work for setting fixed value. But how about adding and subtracting? Is there an easy way? What I have so far is
B = rand(5);
B = B-(B > 0.5)/5; % subtracts 0.2 from all the values greater then 0.5.
Is there any easier way? And how would this work if I wanted to subtract 50% off of each value greater then 0.5?
Walter Roberson
Walter Roberson 2012-1-17
Normally people would write
idx = B>0.5;
B(idx) = B(idx) - 0.2;
B(idx) = B(idx) / 2;
Sean de Wolski
Sean de Wolski 2012-1-17
What Walter said!

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