MATLAB: Produce equality matrix based on elements in vector.

MATLABmatrixvector

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?

Best Answer

Trivially done.

%assuming A and B are both row vectors:
C = A.' == B;

If they're both column vectors, transpose B instead.

Related Solutions

MATLAB: Gather data from different sections of each column

You only need numelselect values of rows:

 B = randi(90,numselect,1);

Then use

 for i = 1:numel(B), D(:,i) = A(B(i):B(i)+9, C(i)); end

MATLAB: Compute the difference matix between a marix and another matrix in MATLAB

Hello Jesujoba,

it looks like A and B must have the same number of columns but can have a different number of rows. The for-loop method and the repmat method (below) agree, although for the integer data I used, repmat is exact but for-loop has some tiny numerical blips. That's because in for-loop, norm takes a square root and you square that out again.

For loops aren't all bad. Repmat may be faster and more efficient when A and B are large, but with for-loop the intent is clearer from just looking at the code. Repmat would require annotation to even know what its task is.

If there are 10 columns and A and B each have 100 rows, then on my PC the calculation by for-loop takes about 140 millisec. Not bad. For 800 rows, for-loop takes less than a sec. Repmat in that case comes in at about .05 sec, and while it can be a point of pride to make things faster, speed and vectorization aren't everything.

To me one of the biggest values of the repmat method is not speed, but as a particular example of messing around with matrices in Matlab. For a small matrix like in this problem, taking out the semicolons on each line illustrates the process.

% data
% sizes of rows and columns
ca = 2;
cb = ca;   % required
ra = 3;
rb = 2;
A = randi(10,ra,ca);
B = randi(10,rb,cb);
% method 1
clear C
for i = 1:ra
    for j = 1:rb
       C(i,j) =  (norm(A(i,:)-B(j,:)))^2;
    end
end
% method 2
Arep = repmat(A,rb,1);
% make multiple copies of each row of B, stacked vertially
Brep = repmat(B',ra,1);
Brep = reshape(Brep,ca,[])';    % using [] auto calculates the number of columns
Cvals = (Arep-Brep).^2;
C2 = sum(Cvals,2);
C2 = reshape(C2,ra,rb);
C
C2

Best Answer

Related Solutions

MATLAB: Gather data from different sections of each column

MATLAB: Compute the difference matix between a marix and another matrix in MATLAB

Related Question