How do you find prime no in C programming?

Answer 1

program to find C Programming

#include<stdio.h>

#include<conio.h>

void main()

{

int i,a,b;

printf("Enter a number:");

scanf("%d",&a);

if(a==1)

{

printf("It is not a Prime number");

}

else if(a==2a==3)

{

printf("It is a pime number");

}

else

{

b=0;

for(i=2;i<=a/2;i++)

{

if(a%i==0)

{

b=1;

}

}

if(b==1)

{

printf("It is not a prime number");

}

else

{

printf("It is a prime number");

}

}

getch();

}

Answer 2

Use the following C function to determine whether a number is prime or composite (non-prime).

int is_prime (unsigned num)

{

/**********

input: a positive integer

output: 0 (non-prime) or -1 (prime)

**********/

/* variables */

unsigned int max_factor, factor;

/* all positive integers less than 2 are non-prime */

if (num<2)

return 0;

/* the only even prime is 2, all other even numbers are non-prime */

if (!(num&0x1))

{

if (num==2)

{

return -1;

}

else

{

return 0;

}

}

/* at this point, num is definitely odd */

/* the maximum possible factor is the square root of num */

max_factor = (unsigned) sqrt (num) + 1;

/* test all odd factors from 3 to the maximum factor */

for (factor=3; factor

{

/* if num is evenly divisible by factor then num is non-prime */

if ((num%factor)==0)

{

return 0;

}

}

/* num has no factors other than 1 and itself, so num must be prime */

return -1;

}

int is_composite (unsigned num)

{

/**********

input: a positive integer

output: 0 (prime) or -1 (non-prime)

**********/

if (is_prime (num))

return 0;

else

return -1;

}