CODEWITHSUNDEEP
matlab
user3527975
user3527975

Reputation: 1773

Comparing columns of matrix and giving boolean output

I have checked other questions. I didn't find my answer. I have a matrix of n * 2 size. I want to compare the 1st and 2nd column and based on which is greater I want to assign 0/1 to the respective index. Suppose I want an output as 

a = 1 2
    4 3
    7 8

I want the output like this

out = 0 1
      1 0
      0 1

I did this : 

o1 = a(:,1) > a (:,2)
o2 = not(o1)
out = [o1, o2]

This does the job but I am sure there's a better way to do this. Need suggestions on that/. Forgot to mention, the datatype is float in the matrix.

Upvotes: 0

Views: 63

Answers (2)

Mohsen Nosratinia
Mohsen Nosratinia

Reputation: 9864

A more generic solution that can handle matrices with more than two columns:

out = bsxfun(@eq, a, max(a,[],2));

Upvotes: 1

Jonathan H
Jonathan H

Reputation: 7943

What you did is good. The number of lines doesn't really matter, what matters is the complexity of the operation in each line. Following the comments, I think you could gain some time as well by avoiding copy and multiple allocations:

out = false(size(a)); out(:,1) = (a(:,1) > a(:,2)); out(:,2) = ~out(:,1);

It is good practice to preallocate in Matlab, and in general to avoid copies in any programming language. Optimizing further the runtime of this by using different operations is pointless IMO. If you really need speed you could Mex it to spare one iteration through the rows (second assignment), it's literally a dozen C lines, although you'd have to be careful about how you write the loop (the naive way would cause cache-miss at each iteration).

Upvotes: 1

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