Find indices of elements in an array based on a search from another array

Question

Imagine that i have two arrays:

a = [1, 2, 5, 7, 6, 9, 8, 3, 4, 7, 0];  b = [5, 9, 6]; 

I want to find the indices of the values of b in a (only the first hit) ie:

c = [3, 6, 5]; 

Is there an easy Matlab native way to do this without looping and searching.

I have tried to use find() with:

find(a == b) 

and it would work if you did this:

for i = 1:length(b)     index = find(a == b(i));     c = [c, index(1)] end 

But it would be ideal for it to be easier then this.

Answer 1

You can compact your for loop easily with arrayfun into a simple one-liner:

arrayfun(@(x) find(a == x,1,'first'), b ) 

also see Scenia's answer for newer matlab versions (>R2012b).

Answer 2

This is actually built into ismember. You just need to set the right flag, then it's a one liner and you don't need arrayfun. Versions newer than R2012b use this behavior by default.

Originally, ismember would return the last occurence if there are several, the R2012a flag makes it return the first one.

Here's my testing results:

a = [1, 2, 5, 7, 6, 9, 8, 3, 4, 7, 0, 6]; b = [5, 9, 6];  [~,c] = ismember(b,a,'R2012a'); >> c c =      3     6     5