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# how can i genaralise this code for any m*n matrix?{the original question is to sum all the elements in the perimeter of a given matrix}?

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Subramanian Mahadevan on 25 Sep 2016
Closed: MATLAB Answer Bot on 20 Aug 2021
This was my code;which works good for a 3*3 matrix.
function sum1 = peri_sum(A);
q=0;r=0;s=0;t=0;u=0;v=0;w=0;
A=[1:3;4:6;7:9];
q = A([1,end],2);
r = A(:,[1,end]);
s = reshape(q,1,2);
t = reshape(r,1,6);
u=sum(s);
v=sum(t);
w=u+v;
sum1 = w;
end

Image Analyst on 25 Sep 2016
Try this:
topSum = sum(A(1, 1:end))
bottomSum = sum(A(end, 1:end))
leftSum = sum(A(2:end-1, 1))
rightSum = sum(A(2:end-1, end))
perimeterSum = topSum + bottomSum + leftSum + rightSum
Image Analyst on 26 Sep 2016
The loop was just to try different input matrices to make sure I didn't pick a sample test matrix that just happened to work.
So anyway, using [1,1;1,1], which has 1's all the way around the perimeter, and therefore has a sum of 4:
A = [1,1;1,1];
topSum = sum(A(1, 1:end));
bottomSum = sum(A(end, 1:end));
leftSum = sum(A(2:end-1, 1));
rightSum = sum(A(2:end-1, end));
perimeterSum = topSum + bottomSum + leftSum + rightSum
This gives perimeterSum = 4 as expected. So, you forgot to tell me what you expect the sum of the outer perimeter values of that to be. Obviously you don't think it's 4, but what do you think it should be, and why?

Andrei Bobrov on 26 Sep 2016
a = randi(9,3,5)
s = size(a);
t = true(s);
t(2:end-1,2:end-1) = false(s-2);
out = sum(a(t));
##### 2 CommentsShow 1 older commentHide 1 older comment
James Tursa on 26 Sep 2016
Andrei's code and your code give the same result for this A matrix, 40.

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