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C Program To Generate Prime Numbers.

A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself. Remember two is the only even and also the smallest prime number. First few prime numbers are 2, 3, 5, 7, 11, 13, 17....etc. Prime numbers have many applications in computer science and mathematics. Any number greater than 1 can be factorized into prime numbers; for example 540 = 22*33*51. This fact makes primes very significant to communications.

Most modern computer cryptography works by using the prime factors of large numbers. The large number that was used to encrypt a file can be publicly known and available, because the encryption works so only the prime factors of that large number can be used to decrypt it again. Though finding those factors is technically only a matter of time, it’s a matter of so much time that we say it cannot be done. A modern super-computer could chew on a 256-bit factorization problem for longer than the current age of the universe, and still not get the answer…




There are various methods of generating prime numbers in C Programming language which are mentioned below:

Method 01:
#include<stdio.h>

int main() {
int n, i = 2, count, c;

printf("Total Number Of Prime Numbers Required: ");
scanf("%d",&n);

if (n >= 1) {
printf("\nFirst %d Prime Numbers Are: \n",n);
printf("\n");
}

for (count = 2; count <= n; ) {
for (c = 2; c <= i-1; c++) {
if (i%c == 0)
break;
}

if (c == i) {
printf("%d\t",i);
count++;
}
i++;
}
printf("\n");
return 0;
}

Method 02:
#include<stdio.h>
int check_prime(int num);

int main(){
int n1,n2,i,flag;

printf("Enter Two Numbers (Intervals): ");
scanf("%d %d",&n1, &n2);
printf("\n\nPrime Numbers Between %d and %d Are: \n", n1, n2);

for(i=2;i<n2;++i) {
flag=check_prime(i);

if(flag==0)
printf("%d\t",i);
}
return 0;
}

/*  User-defined function to check prime number  */
int check_prime(int num) {

int j,flag=0;
for(j=2;j<=num/2;++j) {

if(num%j==0){
flag=1;
break;
}}
return flag;
}

Method 3:
#include<stdio.h>

void main() {
int num,i=1,j,count;

printf("\nEnter a number (max range): ");
scanf("%d",&num);

printf("\nPrime Numbers Between 1 and %d:\n\n",num);

while(i<=num) {
count=0; // initializing count to 0 for each iteration

for(j=1;j<=i;j++) {
if(i%j==0) //checking whether num is divisible by j
count++;
}

if(count==2) //if num is divisible by 2 numbers, then it is prime
printf("%d\t",i);
i++;
}
getch();
}
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