# Indexing with two matrices

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Tchilabalo on 21 Jan 2020
Commented: Tchilabalo on 23 Jan 2020
I have two matrices A(1000 by 100) and B(10 by 100).
A=
1 4 20
3 5 15
2 1 24
7 9 42
B=
3 4 15
7 1 42
I want to return a matrix C such that, for each column of B i find the indexes in the coresponding column in A.
For this case:
C=
2 1 2
4 3 4
I have tried the code below, but i am not getting what I want.
for i=1:size(A,2)
C(:,i)=intersect(A(i),B(i));
end

KALYAN ACHARJYA on 21 Jan 2020
Edited: KALYAN ACHARJYA on 21 Jan 2020
A=[1 4 20
3 5 15
2 1 24
7 9 42];
B=[3 4 15
7 1 42];
[r1,c1]=size(B);
c=zeros(r1,c1);
for i=1:r1
for j=1:c1
r=find(B(i,j)==A(:,j))
c(i,j)=r
end
end
c
#
c =
2 1 2
4 3 4

Tchilabalo on 21 Jan 2020
Thanks Kalyan for your answer. The code is working but i have on special case: what if i have one value repeated multiple times in a column? Here is an example:
A=[1 4 20
3 5 15
2 1 24
2 5 15
3 4 15
7 9 42];
B=[3 4 15
7 1 42];
[r1,c1]=size(B);
c=zeros(r1,c1);
KALYAN ACHARJYA on 21 Jan 2020
but i have on special case: what if i have one value repeated multiple times in a column
The logic, you have to find out, we are here to help you to implementation in MATLAB.

Stephen Cobeldick on 22 Jan 2020
>> [C,~] = find(bsxfun(@eq,permute(A,[1,3,2]),permute(B,[3,1,2])));
>> C = reshape(C,size(B))
C =
2 1 2
4 3 4

#### 1 Comment

Tchilabalo on 23 Jan 2020