Finding maximum y value corresponding to same x values.

I have a n by 2 array.

x y
1 5
1 7
1 9
2 8
2 22
2 11
5 5
5 10
5 20

I'd like to spit out an array of the max y coordinate that corresponds to each x coordinate. So I'd have:

1 9
2 22
5 20

Thanks for any help!

Answer 1

[Sean de Wolski]

xy = [
1 5
1 7
1 9
2 8
2 22
2 11
5 5
5 10
5 20];

[uv,~,idx]  = unique(xy(:,1)); %what do they correspond to?
xmax = accumarray(idx,xy(:,2),[],@max); %build matrix
[uv xmax]

Answer 2

[Azzi Abdelmalek]

[a,b]=unique(x);
for jj=1:numel(a);
  e(jj)=max(y(x==a(jj)));
end
result=[a e]

Answer 3

[Shashank Prasanna]

Quickest way I can think of is using accumarray:

% let xy be your array

A = accumarray(xy(:,1), xy(:,2), [], @max)
[unique(xy(:,1)) A(A~=0)]
ans =

       1     9
       2    22
       5    20

[Sean de Wolski]

If you run the unique command first, you skip having to remove the zeros. With 1,2,5 whatever, with 1,2, 5000000 you run out of memory...

With my approach, you build a 1x3 vector regardless of the values, even if they're non-integer :)

[Shashank Prasanna]

That's good to note! I didn't see your reply before i put mine in. Thanks for the tip!