How to find the indeces of same values within two vectors with repetitive values?

Hi all,

I've got two vectors:

A=[1000 2000 2000 5000 20000 20000];

B=[1000 2000 2000 5000 10000 20000 20000 60000 60000 60000 60000 0 0 0 0 0 0 0 0 0 0 0 0 0];

and I want to find the indeces of the elements of B that appears also in A.

I've tried this:

[~,idx]=ismember(A,B);

and it gives me idx=[1 2 2 4 6 6] but I want idx to be [1 2 3 4 6 7].

The problem is given by the repetitive values but I don't know how to fix it. Any ideas? Thanks in advance.

Answer 1

[Stephen23]

The example data from your question:

>> A = [1000,2000,2000,5000,20000,20000];
>> B = [1000,2000,2000,5000,10000,20000,20000,60000,60000,60000,60000,0,0,0,0,0,0,0,0,0,0,0,0,0];
>> X = A(:)==B; % requires MATLAB >=R2016b, for earlier versions replace == with BSXFUN.
>> [~,Y] = find(X & cumsum(X,2)==cumsum(X,1))
Y =
   1
   2
   3
   4
   6
   7

The example data from your comment:

>> A = [500,500,5000];
>> B = [250,500,500,1250,2500,5000,5000,15000,15000,15000,15000,8166,8926,9796,10800,11967,13333,14948,16875,16875,19200,22041,22041,25562];
>> X = A(:)==B; % requires MATLAB >=R2016b, for earlier versions replace == with BSXFUN.
>> [~,Y] = find(X & cumsum(X,2)==cumsum(X,1))
Y =
   2
   3
   6

[Valentina Mazzoni]

Thank you Stephen, it's the second time that you save me!! It works perfectly!!

[Akihumi]

Wow that's a brilliant way to do it Stephen! Amazing!

Answer 2

[Mario Malic]

Values=ismember(B,A) % Will return true/false for each value of B found in A
Ind_Values = Values .* [1:1:length(B)] % Multiply it by a vector to get actual indices from B
Ind_Values = nonzeros(Ind_Values)' % Remove zeros 

Certainly, there is more elegant way to do this.

[Valentina Mazzoni]

Thank you Mario for your answer. Your code works perfectly with the vectors that I've written in the question but if I change them, for example if I have:

A=[500 500 5000];

B=[250 500 500 1250 2500 5000 5000 15000 15000 15000 15000 8166 8926 9796 10800 11967 13333 14948 16875 16875 19200 22041 22041 25562];

I obtain Ind_Values =[ 2 3 6 7] but I'd like to have Ind_Values=[2 3 6] in order to take each value once. Do you have any suggestions? Thanks.

[Mario Malic]

Akihumi's answer is great. Maybe you can add if condition to his code to remove extra numbers?

Another suggestion is a function intersect, but it requires that matrix A does not have the same values, otherwise it will not work as you want.

[C,ia,ib] = intersect(A,B) %ib contains indices in B

[Valentina Mazzoni]

Thank you very much Mario, I've already tried with intersect as you suggest but it didn't work as I wanted. Stephen Cobeldick's code is what I need. Thank you for your time!

Answer 3

[Akihumi]

flip it to another way should work:

idx = find(ismember(B,A)==1);

[Valentina Mazzoni]

Thank you Akihumi, your code works perfectly too with the vectors in the question but if I change them, I obtain the same result that I have with Mario's code. Of course if you have any ideas, I'll be very happy to read them.