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approximate the value of PI

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approximate the value of PI

Sep 4th, 2008

Hi guys. I'm pretty new to C program. I've been given this assignment to write a program that asks the user thow many terms of the serires equation to use in approximating PI. The formula given is pi=4X(1- 1/3 + 1/5 - 1/7 + 1/9 - 1/11 + 1/13 - ....)

Here is the script that I've written but I'm stuck. Please advice. My alogarithm might be wrong

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 C Syntax (Toggle Plain Text) 
#include <stdio.h> 
int main (void)
{
	int x,
		i;
	//int sum=0;
	double PI;
	float sum=0, y;
 
	printf("Please enter the terms of series that should be included> ");
	scanf("%d", &x);
 
	for (i=1; i<=x; i++){
		i=y;
 
		if (i%2==1) //odd
 
			if (sum%2==0)
				sum=sum+1/y;
			else
				sum=sum-1/y;
				}
 
 
			PI= 4*sum;
			printf("The sum is %f\n", sum);
			printf("Approximate value of PI is %f\n", PI);
 
			return 0;
}
#include <stdio.h> 
int main (void)
{
	int x,
		i;
	//int sum=0;
	double PI;
	float sum=0, y;

	printf("Please enter the terms of series that should be included> ");
	scanf("%d", &x);

	for (i=1; i<=x; i++){
		i=y;

		if (i%2==1) //odd

			if (sum%2==0)
				sum=sum+1/y;
			else
				sum=sum-1/y;
				}


			PI= 4*sum;
			printf("The sum is %f\n", sum);
			printf("Approximate value of PI is %f\n", PI);

			return 0;
}

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Re: approximate the value of PI

Sep 4th, 2008

Hi,

First of all welcome to the world of computer programming.
Here is ur solution :

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
for (i=1; i<=x; i++)
{
 
		if (i%2!=0)
		sum=sum+1/y;
		else
		sum=sum-1/y;
 
	y=y+2;
}
for (i=1; i<=x; i++)
{

		if (i%2!=0)
		sum=sum+1/y;
		else
		sum=sum-1/y;

	y=y+2;
}

Initial value of sum and y should be 0 and 1 respectively. Rest of ur program was ok. I haven't compiled this code. I have just put logic. Please let me know if this code doesn't work.

Last edited by cscgal; Sep 4th, 2008 at 2:21 pm. Reason: Added code tags

Thanks & Regards
Techmighty

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Re: approximate the value of PI

Sep 4th, 2008

I could run it but the answer is wrong. I think i know where went wrong but i can't figure out the solution. The sum should be less than 1. But since the sum is declared as int. So it cannot take decimal places. So what should i change to convert sum from int to float?

Last edited by MadnessCow; Sep 4th, 2008 at 7:26 am.

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Re: approximate the value of PI

Sep 4th, 2008

Use double (not float) type for math calculations!

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
double x, y, z, sum, pi;
...
sum = 1.0;
for (i = 0, y = 3.0; i < n; ++i, y += 2.0)
{
    z = 1.0 / y;
    if ((i & 1) == 0)
        sum -= z;
    else
        sum += z;
}
pi = 4.0 * sum;
double x, y, z, sum, pi;
...
sum = 1.0;
for (i = 0, y = 3.0; i < n; ++i, y += 2.0)
{
    z = 1.0 / y;
    if ((i & 1) == 0)
        sum -= z;
    else
        sum += z;
}
pi = 4.0 * sum;

Last edited by ArkM; Sep 4th, 2008 at 8:16 am.

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Re: approximate the value of PI

Sep 4th, 2008

Maybe you need to do something like:

1.0/y;

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Re: approximate the value of PI

Sep 4th, 2008

Thank you guys. I've manage to work it out. Here is the code:

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#include <stdio.h>
int main (void)
{
int x;
float sum=0;
double PI;

printf("Please enter the terms of series that should be included> ");
scanf("%d", &x);

for (int i=1; i<x; i+=2)
if (i%4==1)
sum=sum+1./(double)i;
else
sum=sum-1./(double)i;

PI= 4*sum;
printf("The sum is %f\n", sum);
printf("Approximate value of PI is %f\n", PI);

return 0;
}

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Re: approximate the value of PI

Sep 4th, 2008

I would change the sum to a double as well.

The int i needs to be declared outside the for loop I believe is it C99 mode or something.

And to increase the print precision you could do %.9f

Last edited by iamthwee; Sep 4th, 2008 at 8:36 am.

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Re: approximate the value of PI

Sep 4th, 2008

I think the int declared inside the loops is also fine. I ran this program and everything works accordingly. Btw, what are the differences between float and double?

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Re: approximate the value of PI

Sep 4th, 2008

Well I compiled the following program and got the following errors:

foo.c

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include <stdio.h>
#include <string.h>
 
int main(void)
{
 
   for ( int i = 0; i < 10; i++ ) /*int inside loop*/
   {
 
   }
   return 0;
}
#include <stdio.h>
#include <string.h>

int main(void)
{
   
   for ( int i = 0; i < 10; i++ ) /*int inside loop*/
   {
   
   }
   return 0;
}

output

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 C Syntax (Toggle Plain Text) 
user@ubuntu804desktop:~$ gcc -Wall foo.c
foo.c: In function ‘main’:
foo.c:7: error: ‘for’ loop initial declaration used outside C99 mode
user@ubuntu804desktop:~$ gcc -Wall foo.c
foo.c: In function ‘main’:
foo.c:7: error: ‘for’ loop initial declaration used outside C99 mode

I believe you can suppress this error if you compile with the necessary flags. -sdt=c99

> double or float?
http://www.gidforums.com/t-2934.html

Last edited by iamthwee; Sep 4th, 2008 at 11:08 am.

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Re: approximate the value of PI

#10

Sep 4th, 2008

If you want to be cryptic...

 Help with Code Tags 
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for (i = 0, y = 1.0, sum = 0.0, sign = 1.0 ; 
     i < n; 
     ++i, sum += sign*(1/y), sign *= -1.0, y += 2.0);
for (i = 0, y = 1.0, sum = 0.0, sign = 1.0 ; 
     i < n; 
     ++i, sum += sign*(1/y), sign *= -1.0, y += 2.0);

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