What's wrong with my code? (Beginner's question; geometric sum)

This is the sum I'm trying to use:

1 + r^2 + r^4 + r^6 + ... + r^n

If n isn't even, I'm supposed to display an error.

Here's my code:

function [sum] = series(r,n)
sum = 1;
if mod(n,2) == 0
    for i=2:n
        sum = sum + r.^i;  
    end
else
    disp('Error!'); 
end
end

Keeps returning the wrong value. What's wrong with my code?

Answer 1

[Roger Stafford]

Replace

   for i=2:n

by

   for i=2:2:n

Or you could always just write

   (1-r^(n+2))/1-r^2)

for even n.

[n]

Thank you!

[Image Analyst]

Here is the question Roger answered so people will know after n deletes his question, which he has a bad habit of doing:

This is the sum I'm trying to use:

1 + r^2 + r^4 + r^6 + ... + r^n

If n isn't even, I'm supposed to display an error.

Here's my code:

function [sum] = series(r,n)
sum = 1;
if mod(n,2) == 0
    for i=2:n
        sum = sum + r.^i;  
    end
else
    disp('Error!'); 
end
end

Keeps returning the wrong value. What's wrong with my code?

Answer 2

[Image Analyst]

Don't use sum as the name of a variable since it's a built-in function. You want

    for k = 2 : 2 : n
           theSum = theSum + r^k;