CODEWITHSUNDEEP
cprimes
MFM
MFM

Reputation: 13

C: Check for prime with various conditions

For instance, where a number = 0 or 1, it outputs:

"It's neither prime nor composite" //Desired
"0/1 is a prime number."           //Not desired

For another instance, where a number is a negative number (e.g. -1):

"Error! You have entered a negative number." //Desired
"-1 is a prime number."                      //Not desired

How do I go about excluding those not desired outputs? I suspect the for loop causing those numbers(0,1,-1) to return 1, resulting in those outputs to appear. Any suggestions and hints would be great. Thanks!

Here's my implementation:

#include <stdio.h>

/* Checks if a positive number is a prime number.  */
int is_prime(int num) {
  /* Divisible by any integers from 2 to a num's square root */
  for (int j=2; j<=num/2; j++) {
    /* return "false" */
    if (num%2 == 0) {
      return 0;
    }
  }
  /* Otherwise, return "true" */
  return 1;
}

int main() {
  int number;

  printf("Enter a positive integer:\n");
  scanf("%d", &number);

  if (number == 0 || number == 1) {
    printf("It's neither prime nor composite.\n");
  } else if (number < 0) {
    printf("Error! You have entered a negative number.\n");
  }

  is_prime(number);

  if (is_prime(number) == 1) {
    printf("%d is a prime number.\n", number);
  } else if (is_prime(number) == 0) {
    printf("%d is not a prime number.\n", number);
  }

  return 0;
}

Upvotes: 0

Views: 103

Answers (2)

Tom Karzes
Tom Karzes

Reputation: 24052

Just change main as follows:

int main() {
  int number;

  printf("Enter a positive integer:\n");
  scanf("%d", &number);

  if (number == 0 || number == 1) {
    printf("It's neither prime nor composite.\n");
  } else if (number < 0) {
    printf("Error! You have entered a negative number.\n");
  } else if (is_prime(number)) {
    printf("%d is a prime number.\n", number);
  } else {
    printf("%d is not a prime number.\n", number);
  }

  return 0;
}

This way it doesn't call is_prime if one of the special cases is in effect. I also removed some unnecessary calls to is_prime (the first call did nothing at all, and the last call was redundant).

Update: I should point out that this post only addresses the problems in main. When I posted this, I had not looked at is_prime. See coderredoc's post for how to fix it.

Upvotes: 1

user2736738
user2736738

Reputation: 30926

The second line is also desired behavior.

Look at your code...

if (number == 0 || number == 1) {
    printf("It's neither prime nor composite.\n");
    return 0;
  } else if (number < 0) {
    printf("Error! You have entered a negative number.\n");
    return 0;
  }

Because if you don't do this or something similar then it calls is_prime which returns 1 and prints the second line.

For 0,1,-1 the for loop in is_prime method is not run. And in the last line of the method you return 1. As a result printf prints that it is prime.

Error in logic

In your prime detection code..you should use this if (num%j == 0). otherwise it is an error.

int is_prime(int num) {
  /* Divisible by any integers from 2 to a num's square root */
  for (int j=2; j*j<=num; j++) {
    /* return "false" */
    if (num%j == 0) {
      return 0;
    }
  }
  /* Otherwise, return "true" */
  return 1;
}

Few things to look into :-

  1. In your code you have unnecessariily called the is_prime method twice. One of them was not even considering the returned value.

  2. Also sometimes you will have to call a method and whose result will decide which one you need to output. Then instead of if-else you can store the return value and then work on it.

int retVal = func();
if(retVal==1)
   printf("..");
else if( retVal==2)
   printf("..something different..");
else
   printf("..something else..");

When it is just two values or two types you are returning you can do this

if(func()==2)
 // do this
else
 // do that

Upvotes: 2

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