prime numbers

[Nazmul]

Hey i was trying to write a program that accepts an user input and displays all the prime factors of that number. But i can'nt get my program working can someone please point out where the mistake is or give me hints i would be highly obliged if anyone does.

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

int primefactor(int a);


int main()
{
   int a;
   printf("Please enter a number: ");
   scanf("%d",&a);
   primefactor (a);
   return 0;
}

int primefactor(int a)
{
   int b,i;
   int prime= 2&&5&&7&&11&&13&&17&&19&&23&&29&&31;
   for(i=1; i<=100; i++)
   {
      b=a/i;
         
      //printf("  %lf \n",b);
      if(a%i==0)
      {
          //printf("Display the factors %d %t %t  %t %d\n",b,i);
      }
      
              }
               if(a%i==prime)
          {
             printf("display the prime factors %d \n",b);
          }
   return 0;
}

[wbhart]

Smells like a comp sci assignment to me.

So here is a hint.

The line

 int prime= 2&&5&&7&&11&&13&&17&&19&&23&&29&&31;

doesn't make a whole lot of sense. Try defining an array of integers, with the primes in it, then search through the array to see if your factor is in there. If so, then print it. The way you've done it, this will mean a second loop to test against each element of your prime array.

Of course this will only print each prime factor in your number, once. So a number like 12 = 2x2x3 will only output 2 and 3. But maybe you can find a way to change your code so it fixes that problem.

Bill.

[project_00]

A simple say is to test whether it is divisible by 2 and then loop, testing if it is divisible by any odd number from 3 to the square root of the number that the user entered.
Alternatively, you can use the sieve of erasthoesene(or sonething) to get all the prime numbers up to the square root of the user's number, then see if these numbers divide the user's number.

[project_00]

Actually it should read "A simple say is to test whether it is divisible by 2 and then loop, testing if it is divisible by any prime number from 3 to the square root of the number that the user entered. "

Which brings up the question how do you know what the prime numbers are? :oops:
So I think should stick to the 2nd method