Implementing the matlab find function [closed]

Question

I am trying to find the coordinates of all non zero elements in a matrix, in MATLAB. I know that find makes this very easy. The problem is that I need to define the length of my matrix before I can fill it up. I believe this is impossible using find, since it will make its own vector.

So I have to find another way. If I'm looking for 1 element it will be easy as well. Let me use an example. Say I have a matrix M of 500×500, with 60 ones somewhere. I want to find the coordinates. So I can start with:

for i = 1:500
  for j = 1:500
    if M(i,j) == 1
       row = i;
       col = j;
    end
  end
end

But I want more then one point, and I need to define the length of a vector before filling it. So I'll make the gamble that there are less then 100 ones in the matrix:

v = zeros(1,100)
w = zeros(1,100)

And how I essentially want to fill this vector is as follows. Let's say I have already filled N elements of the vector, so the next one will be:

for i = 1:500
  for j = 1:500
    if M(i,j) == 1 && i ~= v(1) && i ~= v(2) && ... && i ~= v(N) && j ~= v(1) && ... && i ~= v(N)
     v(N+1) = i
     w(N+1) = j
    end
  end
end

So I need another for loop containing all this, which will run through the elements of the vectors v and w, and I need some loop that will make sure the right if statement is being used.

Does anyone have any idea of how to do this?

Kind regards,

Jesse

Answer 1

Here's another way, nice & wasteful:

[jj,ii] = meshgrid(1:size(M,2),1:size(M,1));        
inds = M~=0;
ii   = ii(inds);
jj   = jj(inds);

Answer 2

I honestly cannot understand what's wrong with find.
But, if you insist on your own implementation using nested loop, then

v = NaN(100,1);
w = NaN(100,1); % pre-allocate
ii=1;
for row=1:size(M,1)
    for col=1:size(M,2)
        if M(row,col) == 1
           v(ii)=row;
           w(ii)=col;
           ii=ii+1; %counts howmany elements were found
        end
    end
end
v(isnan(v))=[]; %discard redundent elements
w(isnan(w))=[];