Warnings: Type Mismatch, functions, parameters? Homework: Guidance, not answers.

Class assignment (requires use of function). Wrote the code. It compiles, runs and gives me the desired result. However, I get warnings... "parameter names (without types) in function declaration, type mismatch with previous implicit declaration, previous declaration of 'is_prime, is_prime was previously implicitly declared to return 'int".

Code:

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#include<stdio.h>

void Is_Prime (num);

int main (void)
{

int num;

printf("Enter a Number to Test if it is Prime: ");
scanf("%d",&num);

is_prime (num);

      system("PAUSE");
      return 0;

} /* End of Main */

void is_prime (int num)
{
int i;
int p = 0;

for(i=1;i<=num;i++)
  {
    if (num%i==0)
      {
        p=p+1;
        }
  }
if(p==2)

printf("%d is a Prime Number\n", num);
else
printf("%d is NOT a Prime Number\n", num);

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I have tried renaming the parameter, but then the program does not work. I reordered the code to put the function first and that eliminated all but one warning, "parameter names (without types) in function declaration".

Guidance or a point in the right direction will be appreciated. Thanks in advance.

the function declaration at the top should name the type not the variable... as in void is_prime (int);
You have mismatched braces in your function... Better indentation rules would help you spot that.
Also note that C source files have to end in a blank line...

Also note the difference of is_prime and Is_prime.

Quote:

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Originally Posted by AndrewHunter

Also note the difference of is_prime and Is_prime.

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Nice catch... I do that one to myself all the time. (Old Pascal habit, I'm afraid... Pascal isn't case sensitive.)

Quote:

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Originally Posted by CommonTater

the function declaration at the top should name the type not the variable... as in void is_prime (int);

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I suggest that you do both, i.e., keep both the type name and the parameter name.

Also, you should make is_prime return a value to indicate if the argument is prime. The printing of output should be done in the main function instead.

Worked. Thanks.

Quote:

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Originally Posted by CommonTater

Nice catch... I do that one to myself all the time. (Old Pascal habit, I'm afraid... Pascal isn't case sensitive.)

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Ah, the implicit declaration warning got my attention. Specifically with the return type.

So... return a 1 (prime) or 0 (not prime) and then have the main function print. What is used in the main to print?

Quote:

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Originally Posted by C-bob

So... return a 1 (prime) or 0 (not prime) and then have the main function print.

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Yes.

Quote:

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Originally Posted by C-bob

What is used in the main to print?

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More or less what you currently use in is_prime. The difference is that by moving out this output code from is_prime, is_prime becomes more reusable (and it then does one thing and does it well).

Well, I would use a printf statement.....

What Laser is referring to is the design of your program. Functions should usually just do one thing, either perform some task such as calculations or user interfacing (input/ouput), or whatever; just one of those things though. It is about proper modularizing your code.

OK. I gave it a shot and it works... but again, with warnings ("comparison between pointer and integers".

Code:

---

#include<stdio.h>

int is_prime (int p_num);


/* start of is_prime function */
int is_prime (int p_num)
{
int i;
int p = 0;

for(i=1;i<=p_num;i++)
  {
    if (p_num%i==0)
      {
        p=p+1;
        }
        }
 if(p==2)
  return 1;
  else
  {
  return 0;
  }
} /* is_prime */


int main (void)
{

int num;

printf("Enter a Number to Test if it is Prime: ");
scanf("%d",&num);

if (is_prime == 1)
{
printf("%d is a Prime Number\n", num);
}
else
{
printf("%d is NOT a Prime Number\n", num);
}
      system("PAUSE");
      return 0;

} /* End of Main */

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To be honest, I am getting there through trial and error. Is this an appropriate approach?

Quote:

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Originally Posted by C-bob

So... return a 1 (prime) or 0 (not prime) and then have the main function print. What is used in the main to print?

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And here's a little trick to make your code more efficient... An even number > 2 can never be a prime number ... 4, 6 ,8 etc. as all even numbers are evenly divisible by 2... One extra line of code to check that could save you a lot of time checking even numbers. You may also want to structure your loops to examine only odd numbers...

Code:

---

/* start of is_prime function */
int is_prime (int p_num)
{
    int i;
    int p = 0;
 if (! (p_num & 1) )
    return 0;

  for(i = 1; i <= p_num; i++)
  {
    if (p_num%i==0)
      {
        p=p+2;
        }
        }
    if(p==2)
      return 1;
    else
      {
    return 0;
  }
} /* is_prime */

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Quote:

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Originally Posted by C-bob

I gave it a shot and it works... but again, with warnings ("comparison between pointer and integers".

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It does not actually work. You need to call the function, e.g., is_prime(num), not just compare the function pointer with 1.

Also, as was mentioned to you earlier, you need to improve your indentation.

Quote:

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Originally Posted by C-bob

OK. I gave it a shot and it works... but again, with warnings ("comparison between pointer and integers".

Code:

---

int main (void)
{

int num;

printf("Enter a Number to Test if it is Prime: ");
scanf("%d",&num);

if (is_prime == 1)
{
printf("%d is a Prime Number\n", num);
}
else
{
printf("%d is NOT a Prime Number\n", num);
}
      system("PAUSE");
      return 0;

} /* End of Main */

---

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You need to take a look at functions and how to use them. How to use functions.

Plus indent your code.

I think this is it (tested multiple numbers and it does work), but it just doesn't seem like the right approach.

If I have wore out my welcome here, just say so. I am new to the forum and getting to know the etiquette.

Code:

---


#include<stdio.h>

int is_prime (int p_num);

int is_prime (int p_num)
{
 int i;
 int p = 0;

 for(i=1;i<=p_num;i++)
 {
    if (p_num%i==0)
    {
      p=p+1;
      }
      }
      if(p==2)
        return 1;
            else
            {
              return 0;
            }
} /* is_prime */

int main (void)
{
  int num;
  int Y;
  int N;

  printf("Enter a Number to Test if it is Prime: ");
  scanf("%d",&num);

  Y = is_prime (num);
  N = is_prime (num);

  if (Y == 1)
  {
    printf("%d is a Prime Number\n", num);
    }
    else
    {
    printf("%d is NOT a Prime Number\n", num);
    }
      system("PAUSE");
      return 0;

} /* End of Main */

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Ok, in C anything nonzero evaluates to true, conversely 0 evaluates to false. So you could:

Code:

---

int main (void){
       
        int num;
       
        printf("Enter a Number to Test if it is Prime: ");
        scanf("%d",&num);

        if(is_prime(num))
                printf("%d is prime", num);
        else
                printf("%d is not prime", num);

        system("PAUSE");
        return(0);
} /* End of Main*/

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Note that N is unused and you are calling is_prime twice for no good reason...

Code:

---

  Y = is_prime (num);
  N = is_prime (num);

  if (Y == 1)
  {
    printf("%d is a Prime Number\n", num);
    }
    else
    {
    printf("%d is NOT a Prime Number\n", num);
    }

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You could have also done this...

Code:

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if (is_prime ( num ))
  printf("%d is a prime number.\n\n", num);
else
  printf("%d is not a prime number.\n\n",num);

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or maybe this...

Code:

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printf ("%d is ",num);
if (!is_prime (num))
  printf("not ");
printf("a prime number\n\n");

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In your is_prime function you could have done this...

Code:

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int is_prime (int p_num)
{ int i;

  if ( ! (p_num & 1) )  // prime numbers are all odd numbers
    return 0;

 for(i = 3; i < p_num; i ++)  // no need to check 1 or 2
  if (p_num%i==0)
    return 0;
 return 1; }

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There are always other ways of doing things...