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matlabmatrixindexingfind
Spacey
Spacey

Reputation: 3019

MATLAB: I want to threshold a matrix, based on thresholds in a vector, without a for loop. Possible?

Let us say I have the following:

M = randn(10,20);
T = randn(1,20);

I would like to threshold each column of M, by each entry of T. For example, find all indicies of all elements of M(:,1) that are greater than T(1). Find all indicies of all elements in M(:,2) that are greater than T(2), etc etc.

Of course, I would like to do this without a for-loop. Is this possible?

Upvotes: 2

Views: 1256

Answers (3)

MZimmerman6
MZimmerman6

Reputation: 8593

You can use bsxfun to do things like this, but it may not be faster than a for loop (more below on this). 

result = bsxfun(@gt,M,T)

This will do an element wise comparison and return you a logical matrix indicating the relationship governed by the first argument. I have posted code below to show the direct comparison, indicating that it does return what you are looking for. 

%var declaration
M = randn(10,20);
T = randn(1,20);
% quick method
fastres = bsxfun(@gt,M,T);

% looping method
res = false(size(M));
for i = 1:length(T)
    res(:,i) = M(:,i) > T(i);
end

% check to see if the two matrices are identical
isMatch = all(all(fastres == res))

This function is very powerful and can be used to help speed up processes, but keep in mind that it will only speed things up if there is a lot of data. There is a bit of background work that bsxfun must do, which can actually cause it to be slower.

I would only recommend using it if you have several thousand data points. Otherwise, the traditional for-loop will actually be faster. Try it out for yourself by changing the size of the M and T variables.

Upvotes: 3

Cape Code
Cape Code

Reputation: 3574

You can replicate the threshold vector and use matrix comparison:

s=size(M);
T2=repmat(T, s(1), 1);
M(M<T2)=0;
Indexes=find(M);

Upvotes: 1

jmbond
jmbond

Reputation: 88

You can use bsxfun like this:

I = bsxfun(@gt, M, T);

Then I will be a logcial matrix of size(M) with ones where M(:,i) > T(i).

Upvotes: 5

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