How would you write this series into a for loop?

Question

$$\sum_{i=1}^5 (-1)^{i+1}*i^2 $$ I thought it would be something like this but i always get 36 or -36. This is c also.

for(i = 1; i <= n; i++)
    if(i++ % 2 == 0) {
        count = pow(-1,n+1)*pow(n,2);
    } else{
        count = pow(-1,n+1)*pow(n,2);
    }

It would be better to ask such questions on stackoverflow – 2017-02-23 — 2017-02-23

user id:363369 361 · Accepted Answer

This should suffice

int count = 0;
for(i=1;i<=n;i++)
{
    count += pow(-1,i+1)*i*i;
}

n is just the stop condition, it doesn't need to be referenced explicitly in the loop.

Also you can probably see that you don't need to break the addition into conditions since the arithmetic is the same for both.

Omg thank you soo much, ive been at it for the entire day and i didnt was just stuck. — 2017-02-23

user id:415626 727 · Accepted Answer

First thing you have to find sum for 5 terms. So loop should be from n=1 to 5.

Second here is no need of even odd concept.

user id:278967 22k · Accepted Answer

$2$ things I would like to point out:

First of all, the if statement will be
if(i%2 == 0)
Secondly, the body of both the if and else statements should contain $\mathrm{pow(i,2)}$ rather than $\mathrm{pow(n,2)}$.
Thirdly, the body of both the if and else statements should also contain $\mathrm{count +=}$ rather than $\mathrm{count=}$

Hope this helps.

3 Answers 3