Wrong output for prime numbers [closed] - c

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#include <stdio.h>
int main()
{
int i, n, c, p;
printf("enter\n");
scanf("%d", n);
c = find(n);
if (c == 1)
{
printf("no. is not prime");
}
else
{
printf("no. is prime");
}
}
find(int n)
{
int i = 2, p;
while (i < n)
{
p = n % i;
printf("value of p%d", p);
if (p == 0)
{
return 1;
}
i = i + 1;
}
return 2;
}
....................................
Above program giving me 'not a prime number' output for all inputs...also the value of p is always zero and this shouldn't be the case...
Please help...badly stuck...

Answer 1

Your scanf() call must take the address of n. Furthermore you primality test fails for numbers smaller than 2. It is also better to return non-zero for true, zero otherwise, so that the value can directly be tested with if. And you should find a better name than find.
Try something like this:
#define TRUE 1
#define FALSE 0
int is_prime (int n)
{
int i;
if (n < 2)
return FALSE;
for (i = 2; i < n; i++) {
if (n % i == 0) {
return FALSE;
}
}
return TRUE;
}
int main()
{
int n;
printf ("enter number: ");
scanf ("%d", &n);
if (is_prime (n)) {
printf ("number is prime.\n");
}
else {
printf("number is not prime.\n");
}
return 0;
}
Various improvements are possible but I wanted to stay as close to your code as possible.

Answer 2

This looks like a student exercise so let me start by suggesting that the debugger is your friend. :)
Having said that, you may want to review the Sieve of Eratosthenes and leverage Wikipedia for a source of some good test content.
As already suggested, there are loads of potential improvements... I'd modify your "find" function to be more clear as follows:
bool IsPrime(unsigned int n)
{
unsigned int nCounter = 2;
while (n % nCounter++);
return (nCounter > n);
}
Prime's can't be negative and since you're asking a "TRUE/FALSE" question, the name and return type should enforce that contract.

Answer 3

Several issues:
scanf("%d", n); should be scanf("%d", &n); - you need to pass the address of n so scanf can update it (note that you risk a runtime error,
since the value of n most likely isn't a valid address value);
Implicit typing of functions such as find(int n) {...} is no longer supported as of the C99 standard, and it was never good practice to begin with. You should (and for C99 and later, must) provide a type specifier along with the function name in both function declarations and function definitions - int find( int n ) {...};
Similar to 2, a function declaration must be visible before a function is called; the simplest way to accomplish this is to move the function definition above the definition for main. If you don't want to do that, then you need to add the declaration int find(int n); somewhere before find is called.
Note that you can speed up the primality test in a couple of ways. First, you can skip testing against even factors; if a number is divisible by a multiple of 2, then it's divisible by 2, and you would have already checked for that. Secondly, you don't need to test all factors up to n - 1; you only need to test factors up to the square root of n. You can put that all together like so:
if ( n < 2 )
return 0; // zero indicates false
if ( n == 2 )
return 1; // non-zero indicates true
int result = n % 2;
for ( int i = 3; result && i * i <= n; i += 2 ) // loops as long as result
result = n % i; // is non-zero, only tests
// against odd numbers up to
return result; // sqrt(n)