I have this code in which I need to find all prime numbers from 2 to 1000 and need to print them out in groups of 5 in each line. How can I do that?

#include <stdio.h>

int main() {
    int i, a, count;
    printf("Prime numbers between 2 and 1000 are : \n");
    for (i = 2; i < 1000; i  ) {
        count = 0;
        for (a = 1; a <= i; a  ) {
            if (i % a == 0)
                count  ;
        }
        if (count == 2)
            printf("%d\t", i);
    }
    return 0;
}

CodePudding user response：

You can add a new counter to count the number of prime numbers printed until the current loop. If this counter value is divisable by 5, print a new line.

int main()
{
    int i,a,count;
    printf("Prime numbers between 2 and 1000 are : \n");

    int cnt_prime = 0;  // count the number of prime numbers until this loop

    for (i=2;i<1000;i  )
    {
        count=0;
        for (a=1;a<=i;a  )
        {
            if (i%a==0)
                count  ;
        }
        if (count==2) {
            printf("%d\t", i);
            cnt_prime  ;
            if (cnt_prime % 5 == 0)   // print new line after each five numbers
                printf("\n");
        }
    }
    return 0;
}

There is another faster approach to find the prime numbers in a range. You can read about sieve of eratosthenes from here: https://www.geeksforgeeks.org/sieve-of-eratosthenes/

CodePudding user response：

Maybe not the best answer, but you could add another counter-variable called prime_count and initialize it with value 0. Then each time you print a prime, you increment that variable. After printing a prime you then check wether prime_count is equal to 4. If that's the case you print a newline-character and reset prime_counter to 0. The code could look something like this:

int main()
{
 int i,a,count,prime_count=0;
 printf("Prime numbers between 2 and 1000 are : \n");
 for (i=2;i<1000;i  )
 {
     count=0;
         for (a=1;a<=i;a  )
         {
            if (i%a==0)
                count  ;
         }
     if (count==2)
     {
         printf("%d\t",i);
         prime_count  ;
         if (prime_count == 4)
         {
             printf("\n");
             prime_count = 0;
         }
     }
 }
 return 0;
}

CodePudding user response：

You can check till square root of N to verify prime no need to check till N it makes your code O(sqrt(n)) refer this for more info about the algorithm.You can have a variable called printCounter to check total elements printed on console when it become a multiple of 5 we can print a new line.

int main() {
    int i, a, printCount = 0 ;
    printf("Prime numbers between 2 and 1000 are : \n");
    for (i = 2; i < 1000; i  ) {
        int isPrime = 1;
        for (a = 2; a * a <= i; a  ) {
            if (i % a == 0){
                isPrime = 0;
                break;
            }
        }
        if (isPrime == 1) {
            printf("%d\t", i);
            printCount  ;
        }
        if(printCount%5 == 0){
            printf("\n");
        }
    }
    return 0;
}