Trouble with loop program

I have been asked to write a program that uses the formula series
∏ = 4 - 4/3 + 4/5 - 4/7 + 4/9 - 4/11 +... to calculate the value of Pi given a specified number of terms.
The example output that I was given is giving me different output numbers than the program I have written.
Can someone please take a look at my code and help me figure out why I am getting a different output than I should be.

Here is the example output that I was given:

Program will approximate Pi

Enter the number of terms to use: 5

Display Pi after every how many steps? 1

RESULTS:

   1: Pi = 4.000000000
   2: Pi = 2.666666667
   3: Pi = 3.466666667
   4: Pi = 2.895238095
   5: Pi = 3.339682540

Final Pi = 3.339682540

Press any key to continue . . .

and here is what I have so far:

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

void input(string& x, int& y, int& z);

int main()
{
    int denom = 3;
    int terms;
    int steps;
	int i;
    double newPi = 4.0;
    bool condition = true;


	cout << " This program will approximate Pi " << endl << endl;

    string prompt= " Enter the number of terms to use: ";

    input( prompt, terms, steps );

    cout << "Results:";
    cout << endl << endl;


    for ( int i = 1; i <= terms; i++ )
    {
        if (condition) 
        {
            newPi -= ( 4.0 / denom );
        } 
        else 
        {
            newPi += ( 4.0 / denom );
        }
        condition = !condition;
        denom += 2;

        if ((i % steps) == 0)
        {
            cout << fixed << showpoint << setprecision(9); 
	    cout << steps << ": Pi = " << newPi << endl;
        }
    }

    cout << endl << endl;
    system ("PAUSE");
    return 0;
}

void input(string& x, int& y, int& z)
{
    do 
    {

        //string x = "Enter the number of terms to use: ";

        cout << x;
        cin >> y;
        cout << endl;

        if ( y <= 0 )

        cout << " Error -- invalid input" << endl << endl;

    } while ( y <= 0 );

    do 
    {
        cout << " Display Pi after every how many steps? ";
        cin >> z;
        cout << endl;

        if ( z <= 0 )

        cout << " Error -- invalid input" << endl << endl;

    }while ( z <= 0);
}

firedraco (6243)

What is the output you are getting?

I don't see anything wrong with the code at first glance.

closed account (zb0S216C)

What compiler vendor are you using? My results are far from yours. I'm using GNU GCC by the way.

closed account (D80DSL3A)

I ran your code and it gave the right numbers but the output form was a little off. It skips the first case and reports one extra. This is because the output follows evaluation of the next term. Moving lines 41-45 to the top of the for loop fixed this. The other problem is that every output line starts with 1: (instead of 1: then 2: then 3: , etc.). Use i instead of steps in this line cout << steps << ": Pi = " << newPi << endl; which is presently line 44.

mgilchrist (7)

Thanks so much everyone! I could not have figured that out myself.

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