CODEWITHSUNDEEP
matlabmatrix
Butti
Butti

Reputation: 25

Calculate the square root for only specific entries inside a matrix

I have a matrix A of an image with elements from 0 to 255. Now I want only the elements which are > 48 and < 200 to be changed to their square root.

I know I can find and replace elements like this:

A(A>48 & A<200) = 3;

But I don't want to set the elements to a number, I want to use the elements value for the new value.

Something like this: 

A(A>48 & A<200).^(1/2)

The above syntax is obviously not correct, but I would like to calculate the square root of the elements which are > 48 and < 200

Is there a way without loops?

Upvotes: 1

Views: 266

Answers (1)

rayryeng
rayryeng

Reputation: 104535

You're pretty close:

A(A>48 & A<200) = A(A>48 & A<200).^(1/2);

A > 48 & A < 200 creates a logical mask to apply an operation only to specific entries in A. Therefore, if you only want to select out those elements that are > 48 and < 200, do so but then when you apply the operation make sure you assign back to only those positions.

If you want less typing, create the mask separately, then do the assignment:

mask = A > 48 & A < 200;
A(ind) = A(ind).^(0.5);

Minor Note

As recommended by Troy Haskin, it is better to actually use sqrt instead of taking the half-power as sqrt is a specialized operation and is optimized.

Therefore, do this instead:

mask = A > 48 & A < 200;
A(ind) = sqrt(A(ind));

Upvotes: 4

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