CODEWITHSUNDEEP
matlabloopssumconditional-statements
Thirst for Knowledge
Thirst for Knowledge

Reputation: 1628

Matlab - Summing values equal to a condition in a loop and storing output

Each of my statements work individually, however, I'm having problems with my loop. I have substation data such as this:

1  4
1  5
1  6
2  2
2  8
2  9
3  1
3  5
3  8

I then try to use the following loop to sum the data in the second column grouped by the number in the first column, and then store it in a matrix. 

for region = 1:Nnuts3
idx = find(substations(:,1)==Nnuts3);
output = sum(substations(idx,2),1);
mat(Nnuts3,1) = output;
end

Each of the statements in here works fine as an individual line of code when I remove Nnuts3 and place in a number, but it does not work as a whole loop.

What am I doing wrong? I merely want to sum the data using the index in the first row as a condition, and then store the output.

Upvotes: 0

Views: 52

Answers (2)

obchardon
obchardon

Reputation: 10792

You should use the function accumarray which is specifically designed for your problem:

data   = [1, 4; 1, 5; 1, 6; 2, 2; 2, 8; 2, 9; 3, 1; 3, 5; 3, 8];
result = accumarray(data(:,1),data(:,2),[],@sum) %accumarray(index,data,[],@function)

We obtain:

[[1:max(data(:,1))]',result(:)] =

1   15
2   19
3   14

PS: result = accumarray(data(:,1),data(:,2)) %accumarray(index,data) will give you the same result, but in my opinion it's more clear to precise the desired "grouping" function.

Upvotes: 3

I_am_Grand
I_am_Grand

Reputation: 106

It looks like you were not using the variable region on the line idx = find(substations(:,1) == region);

The following works for me:

clear all;

substations = [1, 4; 1, 5; 1, 6; 2, 2; 2, 8; 2, 9; 3, 1; 3, 5; 3, 8];

Nnuts3 = 8; % or whatever it needs to be

for region = 1:Nnuts3
    idx = find(substations(:,1) == region);
    output = sum(substations(idx,2), 1);
    mat(region,1) = output;
end

Giving the output:

mat =

    15.0000e+000
    19.0000e+000
    14.0000e+000
     0.0000e+000
     0.0000e+000
     0.0000e+000
     0.0000e+000
     0.0000e+000

I hope that helps :)

Upvotes: 0

Related Questions