Implementing the matlab find function [closed] - matlab

Question

Closed. This question needs details or clarity. It is not currently accepting answers.
Want to improve this question? Add details and clarify the problem by editing this post.
Closed 8 years ago.
Improve this 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))=[];