Question about summations from excel (backwards sum)

Hello, I have an imported excel file. Say I have 2, 1x20 Matrix (set as A and B respectively). How can I make an upside down sum between my matrices? Instead of going from i=0 to i=20 , I want to go from i=20 to i=0. An easy fix could be re-arranging the excel files but I wanna know if theres a way around with matlab.

Answer 1

[Roger Stafford]

 s = sum(fliplr(A)+fliplr(B));

Note: Whatever order 'sum' would have taken with A+B will be reversed.

Answer 2

[Wayne King]

There are a couple issues here. A sum that goes from 0 to 20 has 21 elements not 20 as you indicate. Further, summation is commutative so why does it matter what order you sum in?

A = randn(2,20);
sum(A,2)

B = fliplr(A);  % reverse the elements in the rows of A
sum(B,2)

What have I misunderstood?

fliplr() reverse the rows of A (reverses each row of A)

[Omar]

Small mistake on my part for the 20 terms. Im aware that its commutative but my true task is to find the summation of the average of the lowest 2 terms of my matrix * the sum of the upper 2 terms of my other matrix and so on.

F=fliplr(A);

S=sum(((F(1:20)+F(2:20))/2).*(B(1:20)+B(2:20)))

Will this do?

Answer 3

[Azzi Abdelmalek]

Your question is not clear, maybe you want

A=[ 1 2 3 4]
B=[10 20 30 40]
out=cumsum(fliplr(A+B))