matlab - Find elements meeting any of a number of criteria

Question

I am trying to find the indices of elements in a vector that correspond to another vector, preferably without using loops. For example, my input might be:

DJiSet = [5 7 8];                   % elements of which I need the indices
JiSet = [3 4 5 6 7 8 9 11 12 20];   % vector to search

The output here would be:

[3 5 6]

The fastest I've come up with so far is this:

Output = find(ismember(JiSet,DJiSet));

However I have a feeling this could be done faster, especially since I thought the find command is rather slow.

Things to note:

• The values in DJiSet are guaranteed to all be present in JiSet
• JiSet is always sorted in ascending order, without repeated entries
• The vector DJiSet is not guaranteed to be found contiguously in JiSet

Answer 1

Approach #1

You can avoid find by reversing the places of DJiSet and JiSet inside ismember and then use the second output that gives us the matching indices -

[~,out] = ismember(DJiSet,JiSet)

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Approach #2

Loopy approach catering to the specific conditions set in the question could be tried out, not sure if this will be more efficient though -

intv_idx = zeros(1,numel(DJiSet));
intv_idx(1) = find(JiSet==DJiSet(1),1);
start = intv_idx(1)+1;
for k = 2:numel(DJiSet)
    idx = find(JiSet(start:end)==DJiSet(k),1);
    start = idx+start;
    intv_idx(k) = idx;
end
out = cumsum(intv_idx);