Thread: Quick question on geometric series'

How would one write the geometric series
a + a r + a r² + … + a r^N-1
as an expression in C?

At the moment I have the code

Code:

     sum = a * (pow(r, N - 1));

where a is the first value in the series, N is the number of values in the series and r is the common ratio, but this only prints the last value of the series.

Code:

int i,n;
for (i=0; i<n; i++)
    printf("%d\t", a * (pow(r,N-1)));

@chongmet:
Please don't give out solutions. It's against our policy and it robs the OP of the opportunity to solve the problem on their own and learn from their efforts. Furthermore, your posted code sample does not actually solve the OP's problem.

@bevin:

1. You will need to use loops. A for loop is a good loop to use here; they are good for counting loops, and you are basically counting through the possible values of the exponent.
2. Start simple and build on that. Make sure you test the building blocks, don't build on a broken foundation. Start with a loop that just prints all the exponent values you will need to use. Then modify it to print rⁱ where i is the exponent value in your loop. Then, print each term a*rⁱ.
3. The last step you need is to sum these values. Thus, you should have a sum (addition) operator in there somewhere.

Thanks all, I have now got the following for loop which prints the correct values:

Code:

for (i = 0; i < N; i++)
    {
        sum = sum + a*pow(r, i);
    }

@grumpy any ideas on what to use instead of pow()?

Originally Posted by grumpy

Read my previous post. I already answered that.

Indeed you did Thank you!

I've got the loop to work without the use of the pow() function. Thank you all!

Code:

    for (i = 0; i < N; i++)
    {
        sum1+=(a*t);
        t*=r;
    }