# compare two matrix and keep only the elements in common

13 views (last 30 days)
Aude Rapet on 21 Nov 2016
Commented: Aude Rapet on 22 Nov 2016
Hi, I am a beginner in matlab, and I need to compare to matrix and then keep only the elements present in both matrix in my 1st matrix (s(1).r). For this I compare the first column of each matrix. I've read the documentation and tried to write a code with ismember:
Lia = ismember(s(1).r(:,1), res_select(:,1)) ;
for ind = length(s(1).r)-1 : -1 : 1
if Lia(ind)==0
s(1).r(ind,:)=[];
end
end
I think something is wrong, because I am supposed to find 1987 elements in common and I found 1988...so close! Where is my mistake, or is there an easier way to do this? Thank you!! Aude

KSSV on 21 Nov 2016
Edited: KSSV on 21 Nov 2016
We don't have data to check your code. Note that, You need not to run a loop...simply use, s(1).r(Lia,:) to get the common elements.
As you have two sets and you want to pick the common elements, you can find intersection of both the arrays. Intersection gives you common elements. doc intersect.
Sometimes depending on the floating point numbers, ismember/ intersect may not work. That case, you need to fix a tolerance value and use ismemebrtol. doc ismembertol.

Aude Rapet on 21 Nov 2016
Thank you very much KSSV!
I used intersect:
[Cc,ia,ib] = intersect(res_select(:,1),s(1).r(:,1)) ;
and then I tried to remove the non-in common values:
s(1).r(ib,:)=[];
But it is not working, at the end I find 48 values and I should find 1987. Do you know another way to remove the values I don't want to keep after intersect?