Find elements of an array in another array

Hello,

Let's say I have 2 arrays of double, call then A and B. If both have unique entries and I want to find the position of each element of A in array B I can do:

[~, pos] = ismember(A,B);

What if the elements of A show up multiple times in B and I want to get the first time they show up or the last time they show up? I know I can do

pos = zeros(length(A),1);

for k = 1:length(A)

    pos(k) = find(B == A(k),1,'first');

end;

But is there a better, more efficient way of doing it? For loops are not exactly in the spirit of Matlab as far as I know.

Thanks, Alex

[Azzi Abdelmalek]

If

a=[1 2 3 4 5 6 7]
b=[12 13 2 4 3 2 4 2 25]

what is the expected result?

Answer 1

[Azzi Abdelmalek]

Maybe in your case, all element in A are present in B

A=[1 2 3 4 5 6 7]
B=[12 13 2 4 3 2 4 2 25 1 6 7 5]
pos=arrayfun(@(x) find(B==x,1),A)

[Alexandru]

Thanks Azzi! If I do find(B==x,1,'first') or find(B==x,1,'last') in your code I get exactly what I want.

One more question. Suppose I want to get the second occurrence when it exists. So for the second element of A which is 2 the positions it shows in B are 3, 6 and 8. Suppose I want the code to return 6. How would I modify it?

[Azzi Abdelmalek]

Use in the loop

id=find(B==A(k),2)
id=id(2)

Answer 2

[Azzi Abdelmalek]

I prefer this one. It should be much faster

A=[1 2 3 4 5 6 7]
B=[12 13 2 4 3 2 4 2 25 1 6 7 5];
[ii,jj]=unique(B,'stable');
n=numel(A);
pos = zeros(n,1);
for k = 1:n
     pos(k)=jj(find(ii == A(k)));
end;

[Alexandru]

Ok, so in this case it would come down to a for loop.

Thanks again!

[Azzi Abdelmalek]

This answer is more efficient then the first one