create matrix from loop in matlab

from the following code,

    h=200; k=200;
    for i=1:400;
        for j=1:400;
          a= round(atan2(h-i,-(k-j))*180/pi);
          b(i,j)=[a];
        end
    end

how can I get (i,j) values for 0<= a <=5 because I want use those indexes to read elements from a "data(i,j)" matrix. Moreover, I want to get (i,j) for several ranges, say 5<= a <=10, 10<= a <=15,...85<= a <=90. How can I perform this?

Answer 1

[Andrei Bobrov]

[ii,jj] = ndgrid(1:400);
x = [ii(:),jj(:)];
a = round(atan2d(200 - ii(:), jj(:) - 200));
idx = discretize(a,0:5:90);
t = ~isnan(idx);
xx = x(t,:);
aa = a(t);
out_idx = accumarray(idx(t),(1:numel(aa))',[],@(x){xx(x,:)});

Answer 2

[Jan]

The linear index is usually more efficient than the row and column indices:

% 0<= a <=5
index = (0 <= b) & (b <= 5);

But you can get the different indices also:

[i1, i2] = find((0 <= b) & (b <= 5));

For the set of ranges see

doc discretize