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yinki23
0

Sum specific rows of an array

Asked by yinki23
on 23 Jan 2019
Latest activity Answered by s
on 23 Jan 2019
Hi,
I have an array like:
0.2 2 3
1 3 4
1 4 6
0.8 4 6
2 5 1
1 6 2
0.4 6 2
Now I want to code that MATLAB sums up the values of the 1st column in the rows where the value of the 2nd and 3rd column match.
So the result should look like this below:
0.2 2 3
1 3 4
1.8 4 6
2 5 1
1.4 6 2
I hope someone can help me, thank you!

  3 Comments

"where the value of the 2nd and 3rd column match."
How do the values of the 2nd and 3rd column match in any of these rows?
In general form this should be fairly easy with logic indexing.
Sorry if I was unspecific.
For example, rows no.3 and no.4 both have 4 and 6 in columns no.2 and no.3. So I want to sum these 2 rows up in column 1 (1+0.8). Same with rows no.6 and no.7.
I hope this helps you understand my question.
Are the values in columns 2 and 3 always positive integers?

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2 Answers

Answer by Andrei Bobrov
on 23 Jan 2019
 Accepted Answer

A = [0.2 2 3
1 3 4
1 4 6
0.8 4 6
2 5 1
1 6 2
0.4 6 2];
[a,~,c] = unique(A(:,2:3),'rows','stable');
out = [accumarray(c,A(:,1)),a];

  1 Comments

Thank you so much! Works perfect for me!

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Answer by s
on 23 Jan 2019

clc;
clear all;
A = [0.2 2 3;1 3 4;1 4 6;0.8 4 6;2 5 1;1 6 2;0.4 6 2];
B = size (A,1);
Result = [];
n = 0;
for i = 1:1:B-1
if (A(i,2) == A(i+1,2) && A(i,3) == A(i+1,3))
n = n+1;
end
end
for j = 1:1:B-n
if (A(j,2) == A(j+1,2) && A(j,3) == A(j+1,3))
Sum = A(j,1)+A(j+1,1);
Result = [Result; Sum A(j,2) A(j,3)]
A([j+1],:) = [];
else
Result = [Result; A(j,1) A(j,2) A(j,3)]
end
end
This should help

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