How to find where indices equal?

Hello,

I have a 45x3 double. I split those up into three 45x1 doubles. I then used conditions on each of the three 45x1 doubles to get their index, and got out three different sized Nx1 doubles of indexes. My question is how do I extract the indices where all three Nx1 doubles have the same value (index)?

Example: Column 1 = [1,3,4,5] Column 2 = [1,2,3,4,5] Column 4 = [1,3,5]. How would I go about extracting the values into an array which would be [1,3,5]?

Answer 1

[Stephen23]

This would be much easier if you worked with logical indices rather than subscripts, because then you could just use &, e.g.:

M = your matrix
X = M(:,1)>99 & M(:,2)<0 & isnan(M(:,3))

and you would have your answer already.

But now that you have subscript indices, you can use intersect on each of those index vectors:

X1 = [1,3,4,5];
X2 = [1,2,3,4,5];
X3 = [1,3,5];
intersect(X1,intersect(X2,X3))

You can see how this will get ungainly for more vectors, which is why using logical indexing is much preferred. Note that numbered variables are a sign that you are doing something wrong, and that you should re-think your approach.