How to add values in matrix if in a conditions is met?

Hey everybody,

I'm processing some data and i'm stuck at the moment. I have a matrix consisting of 2 columns and about 4000 rows. A simplified example is given below:

data =  0       1.5
        0.2     0.3
        0.4     4 
        0.6     0.2 
        0.8     0.9 
        1       1.5
        0       2
        0.2     1.2
        0.4     0.8
        0.6     2.3
        0.8     4.5
        1       1.3

And what I need is that when values in column 1 are the same, then values of the second column are added to get one answer for one value in column 1. For the data above this would give:

For 0: 1.5 + 2 = 3.5

0.2: 0.3 + 1.2 = 1.5

0.4: 4 + 0.8 = 4.8

etc.

Could anyone help me with how to do this? Cause at the moment, I really don't know how. I tried a if loop but it didn't work.

Answer 1

[dpb]

 >> accumarray(findgroups(data(:,1)),data(:,2))
ans =
    3.5000
    1.5000
    4.8000
    2.5000
    5.4000
    2.8000
>> 

or, more completely

>> [ig,g]=findgroups(data(:,1));
>> [g accumarray(ig,data(:,2))]
ans =
         0    3.5000
    0.2000    1.5000
    0.4000    4.8000
    0.6000    2.5000
    0.8000    5.4000
    1.0000    2.8000
>> 

Answer 2

[Zezheng Wu]

splitapply(@(x) [x(1, 1), sum(x(:, 2))], data, findgroups(data(:, 1)));

[Danny Helwegen]

Thanks for the reply, if I run this I get als negative values whilst I don't have them in my data set. Any idea how I can solve that?

[Zezheng Wu]

This piece of code works just fine with your example data:

data =  [0       1.5;
    0.2     0.3;
    0.4     4 ;
    0.6     0.2;
    0.8     0.9 ;
    1       1.5;
    0       2;
    0.2     1.2;
    0.4     0.8;
    0.6     2.3;
    0.8     4.5;
    1       1.3];

splitapply(@(x) [x(1, 1), sum(x(:, 2))], data, findgroups(data(:, 1)))

ans =

         0    3.5000
    0.2000    1.5000
    0.4000    4.8000
    0.6000    2.5000
    0.8000    5.4000
    1.0000    2.8000
    

I'm not sure how can there be negative values.