matrix addition different dimension using for loops.

Hi,

I am trying to do matrix addition of 2*4 to fit in 4*4 matrix. I am using for loops to get me familiarize with loops so that i can use it for image matrix manupulation. But with my code my output is 2*4 instead of 4*4.. could someone explain me the concept or the logic i am lacking here ..

a = [11:14; 15:18]; 
 b = [3 2 3 1; 2 1 1 1;1 3 3 2; 2 1 1 3];
for i = 0
    for j = 0
        for  x = 1:2
            for y = 1:4
                c(i+x,j+y) =b(i+x,j+y)+a(x+i,y+j);
            end 
        end
    end
end   

[Walter Roberson]

If the first matrix were

1 2 3 4
5 6 7 8

and the second matrix was all zero, then what would you want the result of the addition to be ?

[Malini Bakthavatchalam]

1 2 3 4
5 6 7 8
1 2 3 4
5 6 7 8

This would be my resultant matrix

Answer 1

[Walter Roberson]

%this code will fail if a or b is empty, or if one size is not an exact multiple of the other
asz = size(a);
bsz = size(b);
maxsz = max(asz,bsz);
arep = maxsz ./ asz;
brep = maxsz ./ bsz;
ar = repmat(a, arep);
br = repmat(b, brep);

c = ar + br;

That is, create new matrices that contain the old matrices copied as many times as needed.

[Malini Bakthavatchalam]

sir, I am in the processing of learning for loops, cannot i use for loops to get the desired 4*4 matrix? I am basically stuck in the logic of how to iterate after 2 nd row if I run the above mentioned code my answer s

14 14 16 15

17 17 18 19

But my desired output is this

14 14 16 15

17 17 18 19

12 15 16 16

17 17 18 21

[Walter Roberson]

ar = repmat(a, arep);

can be replaced with

[arows, acols, apanes] = size(a);
ar = zeros(arows*arep(1), acols*arep(2), apanes, class(a));
for J = 1 : arep(1)
    arows = (J-1)*arows+1 : J*arows;
    for K = 1 : arep(2)
        acols = (K-1)*acols+1 : K*acols;
        ar(arows, acols, :) = a;
    end
end

[Walter Roberson]

I fixed a typing mistake in setting the class of ar