How can I divide each row of a matrix by a fixed row?

Question

Suppose I have a matrix like:

100 200 300 400 500 600
  1   2   3   4   5   6
 10  20  30  40  50  60
...

I wish to divide each row by the second row (each element by the corresponding element), so I'll get:

100 100 100 100 100 100
  1   1   1   1   1   1
 10  10  10  10  10  10
...

Hw can I do it (without writing an explicit loop)?

Answer 1

Use bsxfun:

outMat = bsxfun (@rdivide, inMat, inMat(2,:));

The 1st argument to bsxfun is a handle to the function you want to apply, in this case right-division.

Answer 2

Here's a couple more equivalent ways:

M = [100 200 300 400 500 600
     1   2   3   4   5   6
     10  20  30  40  50  60];

%# BSXFUN
MM = bsxfun(@rdivide, M, M(2,:));

%# REPMAT
MM = M ./ repmat(M(2,:),size(M,1),1);

%# repetition by multiplication
MM = M ./ ( ones(size(M,1),1)*M(2,:) );

%# FOR-loop
MM = zeros(size(M));
for i=1:size(M,1)
    MM(i,:) = M(i,:) ./ M(2,:);
end

The best solution is the one using BSXFUN (as posted by @Itamar Katz)

Answer 3

You can now use array vs matrix operations.

This will do the trick :

mat = [100 200 300 400 500 600
     1   2   3   4   5   6
     10  20  30  40  50  60];

result = mat ./ mat(2,:)

which will output :

result =

   100   100   100   100   100   100
     1     1     1     1     1     1
    10    10    10    10    10    10

This will work in Octave and Matlab since R2016b.