``````/* Function to find prime number if so return 1 else return 0 */
#include<stdio.h>
#include<math.h>
int isprime( int num )
{
int i  ;
int sq = ( int ) sqrt ( num );
/* here check should be done for num = 0 and 1 other wise 0   and 1 are printed as primes */
/*  if ( num <= 1) return 0;  */
for (  i = 2 ; i <= sq ; i++ )
{
if ( num % i == 0 )
{
break ;
}
}
if ( i <= sq )
return  0;
else
return  1 ;
}``````
``````int main( void )
{
int num , ch;
while ( 1 )
{
printf("Enter a Num : ");
scanf("%d", &num);
if ( isprime( num ) )
printf("Num ( %d ) is prime \n" , num);
else
printf("Num ( %d ) is not prime \n", num);

printf("Press  < Enter > to continue \n");
if ( (  ch = getchar() )  != '\n' )
break ;
}
return 0;
}``````

in many of the previous posts i dint observe the check for 0 and 1.
please let me know whether this method is correct or not.

## Recommended Answers

> please let me know whether this method is correct or not.
You mean you can't tell?

Does it, or does it not output a list of prime numbers?

I'm sure it could be more efficient, but first you need to make sure it produces the right answers.

Yet Another Version of Findind a Prime number:

``````#define CHK_NUM_IS_2_3_5( n )  ( n == 2 || n == 3 || n == 5 )
#define REM_2( n )  ( n % 2 != 0 )
#define REM_3( n )  ( n % 3 != 0 )
#define …``````

## All 6 Replies

> please let me know whether this method is correct or not.
You mean you can't tell?

Does it, or does it not output a list of prime numbers?

I'm sure it could be more efficient, but first you need to make sure it produces the right answers.

> please let me know whether this method is correct or not.
You mean you can't tell?

Does it, or does it not output a list of prime numbers?

I'm sure it could be more efficient, but first you need to make sure it produces the right answers.

Yet Another Version of Findind a Prime number:

``````#define CHK_NUM_IS_2_3_5( n )  ( n == 2 || n == 3 || n == 5 )
#define REM_2( n )  ( n % 2 != 0 )
#define REM_3( n )  ( n % 3 != 0 )
#define REM_5( n )  ( n % 5 != 0 )
#define CHK_REM_2_3_5( n ) ( REM_2( n ) && REM_3( n ) && (REM_5( n ) )

#include<stdio.h>

int main()
{
int num ;

REPEAT :
printf( "Enter a Num: " ) ;
scanf( "%d", &num ) ;
if( num <= 1 )
printf(" ( %d ) is not PRIME \n " , num );
else if  ( CHK_NUM_IS_2_3_5 ( num ) )
printf(" ( %d ) is PRIME \n " , num );
else if (CHK_REM_2_3_5 ( num ) )
printf(" ( %d ) is PRIME \n " , num );
else
printf(" ( %d ) is not PRIME \n " , num );
printf(" Press < Enter > to continue..\n ");
getchar();
if ( getchar() < 0 )
goto END;
goto REPEAT ;
END :
return 0;
}``````

i Hope this is the Efficient Version of All.
Any other ways, Plz Welcome.

Yet Another Version of Findind a Prime number:

``````#define CHK_NUM_IS_2_3_5( n )  ( n == 2 || n == 3 || n == 5 )
#define REM_2( n )  ( n % 2 != 0 )
#define REM_3( n )  ( n % 3 != 0 )
#define REM_5( n )  ( n % 5 != 0 )
#define CHK_REM_2_3_5( n ) ( REM_2( n ) && REM_3( n ) && (REM_5( n ) )

#include<stdio.h>

int main()
{
int num ;

REPEAT :
printf( "Enter a Num: " ) ;
scanf( "%d", &num ) ;
if( num <= 1 )
printf(" ( %d ) is not PRIME \n " , num );
else if  ( CHK_NUM_IS_2_3_5 ( num ) )
printf(" ( %d ) is PRIME \n " , num );
else if (CHK_REM_2_3_5 ( num ) )
printf(" ( %d ) is PRIME \n " , num );
else
printf(" ( %d ) is not PRIME \n " , num );
printf(" Press < Enter > to continue..\n ");
getchar();
if ( getchar() < 0 )
goto END;
goto REPEAT ;
END :
return 0;
}``````

i Hope this is the Efficient Version of All.
Any other ways, Plz Welcome.

I think your program will flag 49 as a prime number. 49 cannot be divided by 2, 3 or 5.

Yet Another Version of Findind a Prime number:

``snipped``

i Hope this is the Efficient Version of All.
Any other ways, Plz Welcome.

I hope this is a joke.

Please, do not take this code seriously... :icon_rolleyes:

``#define CHK_REM_2_3_5( n ) ( REM_2( n ) && REM_3( n ) && (REM_5( n ) )``

@Gaiety
I think there are more divisors other than 2, 3 & 5

Please clear your concept of prime numbers

``#define CHK_REM_2_3_5( n ) ( REM_2( n ) && REM_3( n ) && (REM_5( n ) )``

@Gaiety
I think there are more divisors other than 2, 3 & 5

Please clear your concept of prime numbers

Thanks for all replies. i dint take that as a serious program .

i just expected that would be.

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