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}?

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

Answer 1

[Image Analyst]

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

[Subramanian Mahadevan]

this doesn't work for a two by two matrix:(

[Image Analyst]

I tried it for a million 2x2 matrices. Several times in fact so probably around 10 million matrices and they all worked. Tell me the matrix where it did not work on.

for k = 1 : 1000000
  A = randi(9,2);
  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;
  if perimeterSum ~= sum(A(:))
    msgbox('Does not match');
    A
  end
end

[Subramanian Mahadevan]

why is there a for loop for k?where is it being used?Is it possible to do it without using loops? and the matrix it didn't work for was [1 1;1 1].

[Image Analyst]

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?

Answer 2

[Andrei Bobrov]

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));

[Subramanian Mahadevan]

this doesn't hold true for a 3*3 matrix [1 2 3;4 5 6;7 8 9]

[James Tursa]

Andrei's code and your code give the same result for this A matrix, 40.