# Produce equality matrix based on elements in vector.

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Alexander Holmes on 27 Mar 2020
Answered: darova on 27 Mar 2020
Given two equally sized vectors A and B, is there any way to make a matrix C of 1's and zero's such that the kth row of C contains 1's wherever elements of B equal the kth element of A?
I can do it by looping through elements of A, but I want to know if there's a vectorised way of doing this to speed it up?

Alexander Holmes on 27 Mar 2020
Actually, think I figured out an answer. I can just use repmat to create a matrix of n copies of the vector, and use == with the vector to compare it that way.
Guillaume on 27 Mar 2020
"I can just use repmat"
You don't need repmat. Implicit expansion will take care of repeating the elements for you and will be faster. See my answer.

Guillaume on 27 Mar 2020
Trivially done.
%assuming A and B are both row vectors:
C = A.' == B;
If they're both column vectors, transpose B instead.

#### 1 Comment

Alexander Holmes on 27 Mar 2020
Ah yeah, that does it perfectly. That's a much better method.
Thank you!

Fangjun Jiang on 27 Mar 2020
Like this?
A=randi(10,5,1);
B=randi(10,5,1);
k=3;
C=(B==A(k))

Bernd Wagner on 27 Mar 2020
Does the Logical opperator C= A==B not do that work?
It compares values in Vector A and responds a logical value 1 if the Value is also on the same line in B. Hence your C vector will be a vector of 0 and 1 with 1 if A==B.

#### 1 Comment

Alexander Holmes on 27 Mar 2020
That wasn't quite what I wanted. I wantred to compare the whole vector to each element of another vector, which would give me a matrix at the end rather than a vector. But the user above has given me what I needed.
Thank you though!

darova on 27 Mar 2020
Try bsxfun
% make all combinations using bsxfun
C = bsxfun(@minus,b(:),a(:)'); % b - rows, a - columns
[i,j] = find(~C); % find 'zero'
C1 = C*0;
C1(i,:) = 1; % make entire row '1' if any element a==b