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Novice C student here. I am trying to write a function called seriesSum that accepts one integer parameter and returns the sum n numbers following the pattern below.

// seriesSum(1) => 1 = "1.00"
// seriesSum(2) => 1 + 1/4 = "1.25"
// seriesSum(5) => 1 + 1/4 + 1/7 + 1/10 + 1/13 = "1.57"

Simply summing n terms together with each terms' denominator increasing by three each term. The program should return the sum to two decimal places and return 0.00 if n = 0. n will only be given natural numbers (positive inters and 0).

However, any time I enter a value for n, I get 1.00 as output every time. I am not trying to re write the whole logic to my program; I am just hoping someone can point out the fallacy and why I get 1.00 every time. My code is below. Thanks in advance.

#include <iostream>
#include <string>
using namespace std;

double seriesSum(int n);

int main()
{
    int n = 2;
    seriesSum(n);
}

// write a function which returns the sum of following series up to nth
// parameter.

// need to round the answer to 2 decimal places and return as a string
// if the given value is 0, must return 0.00
// will only be given natural numbers as arguments

double seriesSum(int n)
{
double nDouble[n];

if (n == 0)
{
    nDouble[0] = 0;
    cout.precision(2);
    cout << fixed << nDouble[0] << endl;

    return nDouble[0];
}
else
{
    nDouble[0] = 1;

    int i;
    for (i = 1; i < n; i++)
    {
        nDouble[i] = (1) / (1 + (3 * i));
    }

    double sum = 0;
    int j;
    for (j = 0; j < n; j++)
    {
        sum += nDouble[j];
    }

    cout.precision(2);            // setting to 2 decimal places
    cout << fixed << sum << endl; // not sure what fixed means but it works

    return sum;
}

}

coolguysyndrome
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  • https://stackoverflow.com/questions/3602827/what-is-the-behavior-of-integer-division , cast numerator to `1.0` - double. – rafix07 Mar 02 '20 at 16:01
  • Technically your program is not a valid C++ program, as it uses [variable-length arrays](https://en.wikipedia.org/wiki/Variable-length_array). Use [`std::vector`](https://en.cppreference.com/w/cpp/container/vector) instead. – Some programmer dude Mar 02 '20 at 16:04
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    As for your problem, all values in `(1) / (1 + (3 * i))` are *integers*, so the result will be an integer as well. – Some programmer dude Mar 02 '20 at 16:04
  • Your second for loop should also run the other direction. Compute the sum adding the number from smallest to largest. That way you get a few extra bits precision. Might make a difference if the result is 1.494... vs. 1.495... PS: why use an array at all? Just sum up the parts in a single pass. – Goswin von Brederlow Mar 02 '20 at 16:16
  • Apologies for the naivety. Thank you both for your insight – coolguysyndrome Mar 02 '20 at 16:17
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    The easiest fix is change the numerator from the integer "1" to the double "1.0" – Kenny Ostrom Mar 02 '20 at 16:27
  • I also want to make a review comment -- Do not print it inside the function. Have the function return a double, and let the calling code print it. (Also don't do a special case for n==0, the loop should handle that just fine.) – Kenny Ostrom Mar 02 '20 at 16:30

0 Answers0