# C program for prime number

To understand better please read the following post first, it will clear your concept of prime number What is prime number and Why 1 is not a prime number?

```
#include<stdio.h>

int main(){
int n, i,check;
printf("Enter a positive number\n");
scanf("%d",&n);
for(i=2;i<=n/2;i++){

if(n%i==0) {
check = 1;
break;
}
}

if(check==1)
printf("\n%d is not a prime number\n",n);
else
printf("\n%d is a prime number\n",n);

return 0;
}

```

Logic: The above program takes an integer n to check whether it is prime and use two control variables i & check Since every number is divisible by 1 so we started from i = 2. And i will be up to n/2. Why n/2 and why not n Because if we divide n, by any number between n/2 and n it will give us a remainder. For example if we divide 12 by 7(12÷2 < 7) it will remain 5, in case of 9 it will remain 3. That makes if(n%i==0) false every time. So if we take n it just increase time complexity.

Then we divide n with 2 to n/2 if it divide without any remainder, we set check = 1 And show it as not a prime.

### Another one

```
#include<stdio.h>
int main(){
int min, max, i,count;

printf("Enter the first end point\n");
scanf("%d",&min);
printf("Enter the second end point\n");
scanf("%d",&max);
printf("\n\n");
for(i=min;i<=max;i++){
for(count=2;count<=i;count++){
if(i%count==0) break;
}
if(count==i) printf("%d\n",count);
}

return 0;
}

```

Here we also use the same logic to find out the prime number from an interval.