Ask the prime number algorithm

Ken H

14y2.3k

hi all,
Algorithm code as follows:

#include
#include
bool PrimeNumber(int n);
void main()
{ int a;
printf("%s","please input one integer:");
scanf("%d",&a);
if(PrimeNumber(a)) printf("%d%s\n",a," is a prime number.");
else printf("%d%s\n",a," not prime number.");
}

bool PrimeNumber(int n)
{ bool flag=false;
int k=(int)sqrt(n),i;

for(i=2;i<=k;i++)
{
  if(n%i==0)
      flag=false;
     else
      flag=true;
      break;
}
return flag;
}

Now the question is When a user enter 2 or 3 to show why not prime number.Other numbers can be displayed correctly.

thanks.

2 Replies

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Vulpes14y ago
Accepted answer
The reason is because when n = 2 or n = 3, then k = 1.

Consequently, the body of the for loop is never entered and the function returns the value of flag which is false.

To fix it, I'd recode your function as follows:

bool PrimeNumber(int n)
{
if (n < 2) return false;
int k = (int)sqrt((double)n), i;

for(i = 2; i <= k; i++)
{
if(n % i == 0) return false;
}

return true;
}
Ken H14y ago
hi Vulpes,
very good code
thanks.