help with find() or with logical indexing

I want to find array idx1, which is the values of array idx such that S(idx1) == j.

Example: Suppose

idx=[2 3 4 6 7 9 10 11], S=[1 1 1 1 2 2 3 3 3 4 4 4 4]

When j=1, I want idx1=[2 3 4];

when j=2, I want idx1=[6];

when j=3, I want idx1=[7 9];

when j=4, I want idx1=[10 11].

I tried idx1=idx(S==j)) and other ideas, but have not found the answer.

Thank you.

[MSP]

According to the question aren't you supposed to get idx1=[10 11 12] when j=4

[William Rose]

I found an ugly answer to my question. If we assume that

j=1; idx=[2 3 4 6 7 9 10 11];
S=[1 1 1 1 2 2 3 3 3 4 4 4 4];

then

idx1=[];
for k=1:length(idx)
    if S(idx(k))==j
        idx1=[idx1 idx(k)];
    end
end

gives the desired value for idx1. And it works for j=2, 3, 4 also. I suspect there is a much more elegant answer involving the clever use of array indexing.

[Image Analyst]

Well if idx1 = [2,3,4], it's true that S(idx1) will equal 1. HOWEVER, if idx1 = [2,3,4,6] then S(idx1) will ALSO equal 1 for each value of idx1. What if idx1 = [2,3,4,6] - the first 4 values, which will return the first 4 values of S which are all 1. Why do you want idx1 = [2,3,4] instead of [2,3,4,6]?

And when you say that idx1 = 6, then S(idx1) equals S(6) which is 2, which is j for that case, but with a j of 2, it would also work to have idx1 = 5, because S(5) = 2. Why is idx1 not equal to [5,6]?

[Walter Roberson]

"According to the question aren't you supposed to get idx1=[10 11 12] when j=4"

No, only index values in idx can be returned, and 12 is not in idx.

[Walter Roberson]

I just tested the Answer I posted a couple of hours ago, and it produced exactly the desired output...

[William Rose]

Thank you, Image Analyst and Walter, for your help. Context: Suppose I do 13 experimental trials: 4 trials on subject 1, 2 trials on subject 2, 3 trials on subject 3, and 4 trials on subject 4. Then array S(1:13) is the subject number for each trial. Eight trials yielded valid data, and those trial numbers are listed in idx(1:8). I want "idx1", which is a list of all the valid trials in subject 1. Then I want all the valid trials from subject 2, etc. The actual numbers of trials and subjects is much larger, and still growing. Walter provided an elegant solution.

Answer 1

[Walter Roberson]

intersect( find(S == j), idx )

[William Rose]

Thank you Walter! Great answer. I did not know there was an intersect() function.