I want to get first 10 prime numbers in an array and print it in C. My program is not working for some unknown reasons

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The task is to get the first 10 prime numbers in an array and print it. I don't know why my program is not doing it.

#include <stdio.h>
#include <math.h>

int main()
{
    int flag = 0;
    int arr[10];
    arr[0] = 2;
    int j = 1;
    int num;
    int k;

    for(k = 0,num = 3; k<10; k++,num++) {
        for (int i = 2; i <= sqrt(num); i++) {
            if((num % i) == 0)
                flag = 1;
                break;

        }
        if(flag == 0)
            arr[j++] = num;

    }
    printf("First 10 Prime Numbers: ");
    for (j = 0; j<10; j++)
        printf("%d ", arr[j]);
    return 0;
}
primes nested-loops for-loop c
1 Answers

For starters you need to initialize the variable flag in each iteration of the outer for loop before the inner for loop as for example

for(k = 0,num = 3; k<10; k++,num++) {
    flag = 1;
    for (int i = 2; i <= sqrt(num); i++) {
    //...

However it is much better to set the flag in the case when the number is a prime number. For example

for(k = 0,num = 3; k<10; k++,num++) {
    flag = 1;
    for (int i = 2; flag && i <= sqrt(num); i++) {
        if((num % i) == 0) flag = 0;
    }
    if(flag == 1)
        arr[j++] = num;

}

The outer for loop is executed exactly 10 times independent on how many prime numbers were found.

So rewrite the loops at least like

for ( num = 3; j < 10; num += 2 ) {
    flag = 1;
    for (int i = 3; flag && i <= sqrt(num); i += 2) {
        if((num % i) == 0) flag = 0;
    }
    if(flag == 1)
        arr[j++] = num;

}

Here is a demonstration program. I renamed flag to prime.

#include <stdio.h>

int main( void )
{
    enum { N = 10 };
    int arr[N];
    size_t j = 0;
    arr[j++] = 2;

    for ( int num = 3; j < N; num += 2 ) 
    {
        int prime = 1;
        for ( int i = 3; prime && i <= num / i; i += 2 ) 
        {
            if (( num % i ) == 0) prime = 0;
        }

        if ( prime ) arr[j++] = num;
    }

    printf( "First %d Prime Numbers: ", N );
    for (j = 0; j < N; j++) printf( "%d ", arr[j] );
    putchar( '\n' );
}

The program output is

First 10 Prime Numbers: 2 3 5 7 11 13 17 19 23 29
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