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

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Danny Helwegen
Danny Helwegen on 8 Dec 2020
Edited: dpb on 8 Dec 2020
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.

Accepted Answer

dpb
dpb on 8 Dec 2020
Edited: dpb on 8 Dec 2020
>> 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
>>

More Answers (1)

Zezheng Wu
Zezheng Wu on 8 Dec 2020
splitapply(@(x) [x(1, 1), sum(x(:, 2))], data, findgroups(data(:, 1)));
  2 Comments
Zezheng Wu
Zezheng Wu on 8 Dec 2020
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.

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