"Unable to perform assignment because the left and right sides have a different number of elements error" using find command

reference=[1 2 3 4 5 6 8 9 10];
target=[1 2 3 4 6 7 8 9 10];
for i=1:9
index(i)=find(reference(i)==target);
end

Here, after i reaches 4, "unable to perform assignment" error occurs because:

find(reference(5)==target) = 0×1 empty double column vector

. How I can create the index array without encountering this error such as:

index=[1 2 3 4 0 0 1 1 1]; %or 
index=[1 2 3 4 NaN NaN 1 1 1];

My matlab version is R2019a.

Answer 1

[Walter Roberson]

reference=[1 2 3 4 5 6 8 9 10];
target=[1 2 3 4 6 7 8 9 10 3 8];
index = [];
complete_reference = [];
for i=1:9
    thisref = reference(i);
    match = find(thisref==target);
    if isempty(match)
        complete_reference(end+1) = thisref;
        index(end+1) = NaN;
    else
        index = [index, match];
        complete_reference = [complete_reference, repmat(thisref, 1, numel(match))];
    end
end

complete_reference

complete_reference = 1×11

1 2 3 3 4 5 6 8 8 9 10

index

index = 1×11

1 2 3 10 4 NaN 5 7 11 8 9

We must assume that there could be multiple matches between the target and the reference, not just none or exactly 1, so we created an extended reference that repeats the reference number as often as needed to put in all of the matches.