sum of complicated numbers

how can I add these numbers

1:2:101 , 1:-2:101

[Walter Roberson]

What is 1:-2:101 ? 101 is less than 1 so normally 1:-2:101 would be empty.

[DGM]

... it used to be an alternating series of odd integers from 1 to 101

[Image Analyst]

Do you mean he had said previously

1 -3 5 -7 9 -11 13 -15 17 -19 21

and then changed it to the null vector 1 : -2 : 101 ?

Answer 1

[DGM]

Consider the reduced example.

N = 21;
n = 1:2:N

n = 1×11

1 3 5 7 9 11 13 15 17 19 21

n = (1-2*mod((n-1)/2,2)).*n

n = 1×11

1 -3 5 -7 9 -11 13 -15 17 -19 21

s = sum(n)

s = 11

If you must absolutely do it with a loop, you can use the above arithmetic to generate the sequence.

EDIT:

The revised question does suggest its own method which works just as well:

N = 21;
n = sum([1:4:N -(3:4:N)])

n = 11