What broke?

[x1ionu]

l made a script to calculate pi, and it worked just fine, but I tried to make it less messy with a for loop, but then it broke and started giving weird, seemingly random outputs like -2.468526e+142, stuff like that (not an actual example)

So I'm pretty sure it has something to do with the for loop. Could someone take a look?

//This calculation uses the equation:
//(1 * -1/3 * 1/5 * -1/7...) * 4


#include <iostream>
using namespace std;

int main() {
    
    double n = 1;
    double bank = 0;
    double d = 1;
    double f;
    double pi;
    int i;
    
    for (int i = 1000000; i = 0; i--){
        f = 1/i;
        bank = bank + f * d;
        d = d * -1;
        n = n + 2;
        pi = bank * 4;
    }
    
    if (i <= 0){
       cout<<"Pi is aproximately "<<pi<<"!";
    }
    return 0;
}

[keskiverto]

Line 17 has int/int. The result of integer division is integer. It does not help that the result is stored into a double. You should have f = 10.0/i;

What is the end condition of the loop? You currently have an assignment on line 16.

[Chervil]

Rather than 1/i, shouldn't it be 1.0/n ?

Line 22 can be moved outside the loop, do it just once after the loop has completed.

The if statement at line 25 is not needed.

[E0p9LyTq]

Your loop never executes because of your test condition (i = 0). Even if your test condition were syntactically correct (i == 0) the for loop would never execute.

You never assign a starting value to pi when you create it. Because your loop never executes you print out whatever garbage value was in the memory location assigned to pi.

One side note: are you sure you want your loop variable to ever be 0? Dividing by zero is BAD NEWS!

#include <iostream>

int main()
{
   double n = 1;
   double bank = 0.0;
   double d = 1;
   double f;
   double pi = 0.0;
   // int i;

   for (int i = 1000000; i > 0; i--)
   {
      f = 1.0/i;
      bank = bank + f * d;
      d = d * -1;
      n = n + 2;
      pi = bank * 4;
   }

   std::cout << "Pi is approximately " << pi <<"!\n";
}

Pi is approximately -2.77259!

Well, we now have output because the loop is executed, but we still get a very wrong result.

Something in how you calculate PI in your loop is mathematically flawed.

[E0p9LyTq]

A for loop test condition (VERY simplified) acts as an if statement:

#include <iostream>

int main()
{
   int i = 100000;

   if (i == 0)
   {
      // do something!!
      std::cout << "I'm doing something!\n";
   }

   std::cout << "Did I just do something?\n";
}

Did I just do something?

[x1ionu]

Thanks, Chervil! I didn't realize I forgot to change the i to n, oops! And thanks for the extra tip! (If you ever read this)

[x1ionu]

I figured it all out (and made it look nicer):

#include <iostream>
using namespace std;

int main() {
    
    double s, f, pi;
    double d = 1;
    double x = 1;
    
    for (int i = 0; i < 1000000; i++){
        f = 1/d;
        s += f * x;
        x *= -1;
        d += 2;
        pi = s * 4;
    }
    
    cout<<"Pi is ~"<<pi;
    return 0;
}

//This calculation uses the equation:
//(1 * -1/3 * 1/5 * -1/7...) * 4 

[keskiverto]

That has still undefined behaviour:

double foo; // What is the value of foo? Unknown.
foo += 42;  // How much is unknown+42?   Unknown. 

[x1ionu]

#include <iostream>
using namespace std;

int main() {
    
    double s = 0, f = 0, pi = 0;
    double d = 1, x = 1;
    
    for (int i = 0; i < 1000000; i++){
        f = 1/d;
        s += f * x;
        x *= -1;
        d += 2;
        pi = s * 4;
    }
    
    cout<<"Pi is ~"<<pi;
    return 0;
}

//This calculation uses the equation:
//(1 * -1/3 * 1/5 * -1/7...) * 4 

There. Happy?

[fredk]

Don't end your loop at a fixed number (1000000). You should exit the loop when the new term (f*x) is so small that it makes no significant change to answer.

[x1ionu]

Oh, thanks that's a good idea

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

int main() {
    
    double s=0, f=1, pi=0;
    double d=1, x=1;
    
    while (abs(f)>0.000001){
        f=1/d;
        s+=f*x;
        x*=-1;
        d+=2;
        pi=s*4;
    }
    
    cout<<"Pi is ~"<<pi<<endl;
    return 0;
}

//This calculation uses the equation:
//(1 * -1/3 * 1/5 * -1/7...) * 4