Consider the following arrays:
A = [1 3 4 5 6 7 1 3 7];
B = [1 4 7];
I want to find all elements of B in array A. So, my final output array will look something like:
C = [1 7 3 6 9];
First element of B is at locations 1 and 7 in array A, so C has 1 and 7 as first two elements. Element 4 of B is at location 3, so array C has 3 as its third element and so on.
The order of C is required?
Fast, but another order:
find(ismember(A,B))
Slower, but the order you want:
cell2mat(arrayfun(@(x)(find(A==x)),B,'UniformOutput',false))
Basically, the second solution iterates over all elements of B and applies find(A==x) in a loop.
You may also delete the cell2mat, then a cell is returned. First element -> Occurrences of 1, second element-> occurrences of 4 etc.
If you need the result in that order: you can use the two outputs of ismember. This may be faster than Daniel's answer (second part) as it avoids arrayfun:
[tf, loc] = ismember(A,B);
loc = loc(tf);
ind = find(tf);
[~, order] = sort(loc);
C = ind(order);
Second output of ismember will give a map for each element of B
>> [~,ic] = ismember(A,B)
ic =
1 0 2 0 0 3 1 0 3
Then element-wise test against each element of B:
>> [C,~] = find(bsxfun(@eq,ic.',1:numel(B)))
C =
1
7
3
6
9
And because I'm require to do so, an alternative method following ismember:
c = accumarray(nonzeros(ic),find(ic),[],@(x) {sort(x)});
C = vertcat(c{:})
bsxfun. But it may consume much memory for large A and B.bsxfun can be quite memory heavy.