Matrix manipulation, filtering

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.

Answer 1

[Sean de Wolski]

Of course - use logical indexing:

A = rand(5);
A(A>0.5) = 10; %set all values in a greater than 0.5 to 10

[Nicklas Utgaard]

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]

Normally people would write

idx = B>0.5;

B(idx) = B(idx) - 0.2;

B(idx) = B(idx) / 2;

[Sean de Wolski]

What Walter said!