How do I replace same values in an array?

If I have this couples of values: 1—5; 2—7; 3—4; 2—3; 6—5 and I want that the first column is the x column and the second one is the y column of an xy graph (especially an histogram), how can I make that for the same value of x, y is the sum of y values for the same x as follow? 1—5; 2—10; 3—4; 6—5.

Thank you

Fabio

Answer 1

[dpb]

If

xy=your array;
y=accumarray(xy(:,1),xy(:,2));

See

doc unique
doc accumarray

for how the above works...

>> xy=[1 5;2 4;3 8; 2 6];
>> [unique(xy(:,1)) accumarray(xy(:,1),xy(:,2))]
ans =
   1     5
   2    10
   3     8

[Fabio]

Thank you dpb.

Fabio

[Fabio]

There is a problem when the first column values aren't continuos, for example for xy=[1 2; 2 3; 3 4; 2 7; 8 1]. so to avoid error, that's the correct way:

xy=[1 2; 2 3; 3 4; 2 7; 8 1];

ac=accumarray(xy(:,1),xy(:,2))

ac_modified=ac(ac~=0); % to delete zeros

result= [unique(xy(:,1)) ac_modified]

[dpb]

Or in place without a temporary,

ac(ac==0)=[]; % to delete zeros

You can also get more clever in using unique and accumarray together with the alternate outputs of unique as indices and anonymous functions to only sum the nonzero entities. I'll leave as "exercise for the student"; the nuances of accumarray are legion and an area of expertise to be gained all of its own. :)

[Fabio]

Thank you again :)