# How to sum n lines of a matrix to another matrix?

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Pedro Vicente on 23 Sep 2018
So basically, i have a matrix Apit with the index of (51*n,5,1000) and what i wanted is another matrix TotalApit, with the index (51,5,1000), that sums up 'n' lines by 'n' lines.
like:
if n = 2, Apit(102,5,1000) and TotalApit(51,5,1000)
TotalApit(1,1,1) = Apit(1,1,1)+Apit(2,1,1)
TotalApit(2,1,1) = Apit(3,1,1)+Apit(4,1,1)
So i want that 1 line of TotalApit correspond the sum of n lines of Apit
Can someone help me?
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Image Analyst on 23 Sep 2018
Explain how the first indexes (row number) are determined on the right hand side. Like why row is 1 and 2 in the first equation, and 3 and 3 (the same) in the second equation. What rule are you using to determine which rows to sum?
And, this is not a big array, just a simple for loop would work, though there are more efficient ways.
Pedro Vicente on 23 Sep 2018
Oh, damn, that was my bad. Should be Apit(3,1,1)+Apit(4,1,1)
I want each line of TotalApit to sum n lines of Apit.

Image Analyst on 23 Sep 2018
You could do this:
Apit = rand(102,5,1000);
TotalApit = zeros(51,5,1000);
% Now do the sum
[rows, columns, slices] = size(Apit)
n = 2;
tic
k = 1;
for row = 1 : n : rows-n
thisVolume = squeeze(Apit(row:(row+n-1), :, :));
thisSum = sum(thisVolume, 1);
TotalApit(k, :, :) = thisSum;
k = k + 1;
end
toc
Takes a hundreth of a second on my computer. You might be able to do it even faster with convn(). And it would be only 1 or 2 lines of code.

### More Answers (1)

Kaushik Lakshminarasimhan on 23 Sep 2018
n=2;
TotalApit = reshape(sum(reshape(Apit,[n 102/n 5 1000])),[102/n 5 1000]);