# Find the sum of the non perimeter elements of a matrix using a for loop

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Vinny on 24 Apr 2016
Answered: Ibrahim Abouemira on 19 May 2019
I have this matrix
B=[-3 4 7;9 11 -13;-17 19 -23;-29 31 37;39 41 47]
and I need to find the sum of the non perimeter elements using a for loop. I can find the sum of a single column or row or of the whole matrix, but I don't know how to write a loop to only sum the 3 non perimeter numbers. Any help is appreciated, thank you.

Image Analyst on 24 Apr 2016
Edited: Image Analyst on 24 Apr 2016
Try it this way:
B=[-3 4 7;9 11 -13;-17 19 -23;-29 31 37;39 41 47]
[rows, columns] = size(B)
theSum = 0;
for col = 2 : columns - 1
for row = 2 : rows-1
theSum = theSum + B(row, col);
end
end
fprintf('The sum = %f\n', theSum);

Vinny on 24 Apr 2016
Thank you! How about if I wanted it to find the sum of the perimeter elements?
Image Analyst on 24 Apr 2016
Hopefully you're not just asking me to do all your homework for you. Here is one way:
B=[-3 4 7;9 11 -13;-17 19 -23;-29 31 37;39 41 47]
[rows, columns] = size(B)
theSum = 0;
for col = 1 : columns
for row = 1 : rows
if row == 1 || row == rows || col == 1 || col == columns
% It's on the perimeter of the array.
theSum = theSum + B(row, col)
end
end
end
fprintf('The sum = %f\n', theSum);

Azzi Abdelmalek on 24 Apr 2016
Edited: Azzi Abdelmalek on 24 Apr 2016
A=B
A(:,[1 end])=0
A([1 end],:)=0
out=sum(A(:))

Ibrahim Abouemira on 19 May 2019