Help using find function

This is pretty basic but I'm just trying to properly get subcript indices after edge detection (logical matrix E). For some reason, linear indexing seems to be working, but whe I try to convert to subscript indices it goes all wrong. Any help would be appreciated.

indx = find(E);
mask=false(size(E));
mask(indx)=true;
imshow(mask);
[y,x] = ind2sub(size(E),indx);
mask=false(size(E));
mask(y,x)=true;
imshow(mask);

Answer 1

[Walter Roberson]

mask(y,x)=true;

That means the same as

for X = x
    for Y = y
        mask(Y, X) = true;
    end
end

Which is to say that it sets all combinations of x and y.

It does not mean the same as

for index = 1 : length(y)
    mask(y(index), x(index)) = true;
end

In order to do the operation you were hoping for, leave the indices as linear instead of breaking them out to row and column.

It looks to me as if the whole thing could be replaced with

mask = E ~= 0;
imshow(mask)

[Hannah Rosenberg]

Thank you! The for loops were very helpful for explaining what's actually going on inside of those operations.