# Sum every i-th column in matrix seperately

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Anya on 12 Jun 2014
Commented: Anya on 12 Jun 2014
Hi all,
I have randomly generated matrix and I want to go through all columns and to sum every i-th column separately. I don't want to sum all the columns but to sum depending on the counter in for loop for example:
[a,b] = size(C);
for i = 1:b
S = sum(C(:,i))
S = 0 %but this doesn't work, result is sum of elements of all columns

Zikobrelli on 12 Jun 2014
Try sum(A(:,[1 4]))
where A is your random matrix.The line above will give you the sums of column 1 and 4.
ex: A=spiral(4)
sum(A(:,[1 4]))
ans = 34 46
Anya on 12 Jun 2014
I can't believe that I missed this last control row . I don't have a lot of experience so I didn't noticed. I change the code now by changing the sum function and it works ! Thanks
sum(C(1:end-1,i))

### More Answers (2)

Roger Stafford on 12 Jun 2014
S = sum(C(:,i:i:end),1);
This results in a row vector, S, consisting of the sum of every i-th column of C, as requested. It is not clear where you want the i-spaced columns to start. This starts at the i-th column. If you want them to start at the first column, change "i:i:end" to "1:i:end".
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Anya on 12 Jun 2014
I think that I was not clear enough. I will try to explain on the example matrix C. Using for loop I want to iterate through the columns and count number of non-zero elements. They are written after the last row. If I found the column with all ones or all zeros I want to skip that column and move on.
C =
1 1 1 1 0 1 1 1 1 0
0 1 1 0 0 0 1 1 1 0
0 1 0 1 0 0 0 1 0 0
0 1 1 0 1 0 0 0 0 0
1 4 3 2 1 1 1 3 2 0

Jos (10584) on 12 Jun 2014
sumColsC = sum(C,1)
NotInteresting = sumColsC == 0 | sumColsC == size(C,2)
sumColsC(NotInteresting) = []
Jos (10584) on 12 Jun 2014
When you change the iterator in a for-loop, it will reset at the end
for k=1:10
disp(k) ;
k = 1 ;
disp(k) ;
end
You should be clearer about your goals. What do you mean with "move on"?