# How to subtract vectors with different number of entries?

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Sandy on 28 Feb 2013
I have two data files with two lines of data each.
EX:
Data Set 1:
1970 254.5
1980 264.5
1990 234.5
2000 234.7
Data Set 2:
1970 234.5
1980 234.6
2000 456.7
2010 234.7
I have to subtract Column 2 of data set one from Column 2 of data set 2. But in order to to this the first columns must match up. How can I write a code where it skips the entries where the entries in column one don't match up.
Azzi Abdelmalek on 28 Feb 2013
Post what should be the result

Sean de Wolski on 28 Feb 2013
D1 = [
1970 254.5
1980 264.5
1990 234.5
2000 234.7 ];
D2 = [
1970 234.5
1980 234.6
2000 456.7
2010 234.7];
%Where do they intersect?
[~,ia,ib] = intersect(D1(:,1),D2(:,1));
%Subtract intersecting parts:
dD = D1(ia,2) - D2(ib,2)
Sandy on 28 Feb 2013
Thanks! :)

Azzi Abdelmalek on 28 Feb 2013
a=[1970 254.5
1980 264.5
1990 234.5
2000 234.7]
b=[1970 234.5
1980 234.6
2000 456.7
2010 234.7]
idx=find(a(:,1)-b(:,1)==0)
c=b(:,2)
c(idx)=b(idx,2)-a(idx,2)

Sandy on 28 Feb 2013
Both of these work perfectly. Thanks!