CODEWITHSUNDEEP
arraysmatlab
Rashid Bagheri
Rashid Bagheri

Reputation: 89

Fast column by column array division

Suppose that M and N are two Arrays. In simplest case, M and N like this:

1 14 7 80

2 15 8 12

3 16 9 11

(3 Rows and 4 Columns)

I want to divide column 1 by All three Columns, then divide column 2 by All three Columns and then divide column 3 by All three Columns.

What is the fastest way to do it? ( Surely, using for-loop is not a good algorithm. )

EDIT:

here is my for-loop code:

idx = 1;
for i = 1 : size(N,2)
   for j = 1 : size(M,2)
       u(:,idx)=N(:,i) ./ M(:,j);
       idx = idx + 1;
   end
end

Upvotes: 3

Views: 81

Answers (2)

Dan
Dan

Reputation: 45752

If 

A = [1 14 7 80

     2 15 8 12

     3 16 9 11]

Then

bsxfun(@ldivide, prod(A,2), A).*A

returning

ans =

    0.0001    0.0250    0.0062    0.8163
    0.0014    0.0781    0.0222    0.0500
    0.0019    0.0539    0.0170    0.0255

So the idea is to just divide every element by ALL the other elements in that row (i.e. by the product of the row, prod(A,2)) and then just multiply back by the original number so cancel the fact that you've divided by it (i.e. the .*A at the end). So ans(2,3) above is 0.0222 which equals (8/(2*15*8*12))*8 where (2*15*8*12) is the product of row 3.

NOTE this answers the original question (i.e. the question you describe) and does NOT answer the question that your code implies

Upvotes: 3

Santhan Salai
Santhan Salai

Reputation: 3898

How about using bsxfun and permute

Assuming M and N are same and equal to A

out = bsxfun(@rdivide, permute(A,[1 3 2]), A)

Input:

A =

 1    14     7    80
 2    15     8    12
 3    16     9    11

Results for your Sample Input:

out(:,:,1) =

1.0000    0.0714    0.1429    0.0125
1.0000    0.1333    0.2500    0.1667
1.0000    0.1875    0.3333    0.2727


out(:,:,2) =

14.0000    1.0000    2.0000    0.1750
7.5000    1.0000    1.8750    1.2500
5.3333    1.0000    1.7778    1.4545


out(:,:,3) =

7.0000    0.5000    1.0000    0.0875
4.0000    0.5333    1.0000    0.6667
3.0000    0.5625    1.0000    0.8182


out(:,:,4) =

80.0000    5.7143   11.4286    1.0000
6.0000    0.8000    1.5000    1.0000
3.6667    0.6875    1.2222    1.0000

Upvotes: 5

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