C Exercises: Attractive numbers up to 100

C Programming Challenges: Exercise-33 with Solution

A number is called an attractive number if the number of its prime factors (whether distinct or not) is also a prime number.
Example:
The number 30, whose prime decomposition is 2 × 3 × 5, is an attractive number because the number of its prime factors (3) is also a prime number.
The number 100, whose prime decomposition is 2 × 2 x 5 × 5, is not an attractive number because the number of its prime factors (4) is not a prime number.
Write a C program to generate attractive numbers up to 100.

C Code:

#include <stdio.h>
#define MAX 100


int PrimeOrNot(int n1)
{
    int i=2;
    while(i<=n1/2)
    {
         if(n1%i==0)
             return 0;
         else
             i++;
    }
    return 1;
}

int no_of_prime_factors(int n) {
    int ctr = 0, nn = 2;
    if (n == 1) return 0;
    if (PrimeOrNot(n)) return 1;
    while (1) {
        if (!(n % nn)) {
            ctr++;
            n /= nn;
            if (n == 1) return ctr;
            if (PrimeOrNot(n)) nn = n;
        } 
        else if (nn >= 3) nn += 2;
        else nn = 3;
    }
}

int main() {    
    int i, n, ctr = 0;
    printf("The attractive numbers up to and including %d are:\n\n", MAX);
    for (i = 1; i <= MAX; ++i) {
        n = no_of_prime_factors(i);
        if (PrimeOrNot(n)) {
            printf("%4d", i);
            if (!(++ctr % 10)) printf("\n");
        }
    }
    printf("\n");
    return 0;  
}

Sample Output:

The attractive numbers up to and including 100 are:

   1   2   3   4   5   6   7   8   9  10
  11  12  13  14  15  17  18  19  20  21
  22  23  25  26  27  28  29  30  31  32
  33  34  35  37  38  39  41  42  43  44
  45  46  47  48  49  50  51  52  53  55
  57  58  59  61  62  63  65  66  67  68
  69  70  71  72  73  74  75  76  77  78
  79  80  82  83  85  86  87  89  91  92
  93  94  95  97  98  99

Flowchart:

C Programming Code Editor:

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