MATLAB: Finding the indices of the elements of one array in another

matlab functionvectorization

Given two vectors A and B, find the index, idx into A of the element of B so that

A(idx)=B.

Now I know there must be many ways it can be done, but is there a one-liner?

For example if

A=[3 4 5 6 7];
B=[6 4 7];

then

[tf,loc]=ismember(A,B);
idx=[1:length(A)];
idx=idx(tf);
idx=idx(loc(tf));
disp(A(idx))

will do it but that is four steps. Is there a more elegant way?

There are a few options to get the indices you are looking for. The following output indices (idx) preserve the order in A of the shared values:

[sharedvals,idx] = intersect(A,B,’stable’)

You can also use the following command if the order in A is not necessary:

[tf,idx] = ismember(B,A)

A = [5, 4, 6, 7, 3];
idx=find(mod(A,3)==0); % Gives the index number / Position
disp(A(idx)); % Gives the those idx number in A

A = [1 2 3];
B = [2 4 6];
i2 = cumsum(B);
idx = zeros(1,i2(end));
idx(i2 - B + 1) = 1;
C = A(cumsum(idx));

C = cell2mat(arrayfun(@(x,y)x(ones(1,y)),A,B,'un',0));

i2 = cumsum(B);
i1 = i2 - B + 1;
C = zeros(1,i2(end));
for jj = 1:numel(A)
    C(i1(jj):i2(jj)) = repmat(A(jj),1,B(jj));
end