How to subtract vectors with different number of entries?

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]

Post what should be the result

Answer 1

[Sean de Wolski]

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]

Thanks! :)

Answer 2

[Azzi Abdelmalek]

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)

Answer 3

[Sandy]

Both of these work perfectly. Thanks!