#include <stdio.h>

void main()
{
    int InputNumber = 0; // Create a variable to input a number.
    int i, j, k = 0; // Create variables for looping.
    int PreviousNumber = 0; // Create a variable that will act as numbers adding up to the input number.

    printf("Input number greater than two >> ");
    scanf("%d", InputNumber);

    /** The following 'for' loop is supposed to take every single number between two and the given
        and calculate to see whether it is a prime or not. **/
    for(PreviousNumber = 2; PreviousNumber <= InputNumber; PreviousNumber++)
    {
        for(j = 2; j <= PreviousNumber; j++) // this loop is to bring every number between two and the divisor
        {
            if(PreviousNumber % j == 0 && j != PreviousNumber)
            {
                k++;
            }
        }
        if(k == 0)
        {
            printf("%d ", PreviousNumber); // This prints the number if it's a prime, be it the given or something before it.
        }
    }
}

/*  
    'C' all primes greater than 2,
        up to, and including a 'top'
        number that the user inputs.
        
        The user may ask for another
        'top' number to test repeatedly ...
    
        A careful choice of variable names
        may also assist solving a problem .
*/

#include <stdio.h>

void showPrimesUpTo( int num );

int main()
{
    int c, max; /* 'c' is used as a character 'scratch' pad here */
    for(;;) /* loop for ever ... until break or exit or return */
    {  
        printf("Input 'top prime' number to test (greater than 1) : ");
        scanf("%d", &max);
        while( (c=getchar()) != '\n' ); /* 'flush' stdin buffer */

        if (max < 2)
        {
            printf( "\nIs %d greater than 1 ?\n\n", max  );
            continue; /* from top of the for loop ... */
        }
        
        printf( "\nPrimes are 1 2 ...\n" );
        showPrimesUpTo( max );
        printf("\nMore y/n ? ");
        c=getchar();
        if ( c=='n' || c=='N' ) return 0;
        while( (c=getchar()) != '\n' ); /* 'flush' stdin buffer */
    }
}

void showPrimesUpTo( int topNum )
{      
    /*
        A number IS 'prime' if it IS divisible by ONLY 1 and the
        number ITSELF. Some times we may go like this { 1, 2, 3, 5, 7, 11 ... }
        (So here ... NO need to divide by 1, 2, or self  ...we need ONLY
        to check ODD numbers beyond 2 to see if they divide with NO remainder ?
    */
    
    /* 'j' is used as a counter, 'k' is used as a flag */
    int testNum, i=0, j, k; /* i is used to limit numbers printed on a line */

    /* only NEED to test ODD numbers; All EVEN beyond 2 are NOT prime. */
    for(testNum = 3; testNum <= topNum; testNum += 2)
    {
        /* always 're-set' or 'initial' flags before each new iteration */
        k=0;
        
        /* this loop is to try every ODD number from 3 up to top as a divisor */
        for(j = 3; j < testNum; j += 2)                          
        {
            if(testNum % j == 0)
            {
                /* Set k ... We have a divisor and testNum is NOT a prime. */
                k=1;
                break; /* So can EXIT INNER 'for loop' right NOW. */
            }
        }
        
        /*  When we reach hear, if k==0, then that 'testNum' was a prime. */
        if(k == 0)
        {
            printf("%4d ", testNum);
            if( ++i == 15 ) { i=0; printf("\n"); }
    
        }
        // end of inner 'trial divisor' loop
        
    } // end of outer 'for testNum ...' loop

}