关于 C 语言中的 if else 语句

Question

Please explain to me why this code is wrong for the task and below I have explained all the four conditions -[][1]

#include stdio.h
int main()    
{
      int n;
      scanf ("%d", &n);     //taking input
      if (n / 2 != 0)
        {
          printf ("Weird");     //checking first condition
        }
      else if (n % 2 == 0 && 2 <= n <= 5)
        {               //checking second condition
          printf ("Not Weird");
        }
      else if (n % 2 == 0 && 6 <= n <= 20)
        {               //checking third condition
          printf ("Weird");
        }
      else if (n % 2 == 0 && n > 20)
        {               //checking fourth condition
          printf ("Not Weird");
        }
      else
        {
          printf ("Error");
        }
      return 0;
    }

this is the image for the question[1]: https://i.stack.imgur.com/OtY7o.png **

Answer 1

Testing for Odd or Even

n / 2 != 0 does not test whether n is odd. n/2 calculates the quotient that results from dividing n by 2 (rounding any fraction down). So 0/2 is 0, 1/2 is 0, 2/2 is 1, 3/2 is 1, 4/2 is 2, and so on. So n / 2 != 0 is true for all n other than −1, 0, and 1.

To test whether a number is odd, you can use n % 2 != 0 . n%2 calculates the remainder from the division. If it is zero, n is even. If n is not zero, n is odd.

Using Else Efficiently

Once you have tested whether n is odd using n % 2 != 0 , you do not have to test whether it is even in the else clauses. The else expressions and their statements will be evaluated only if the if expression is false, which happens (after the correction above) only when n is even. So we do not need to test again.

Testing For an Interval

In C, 2 <= n <= 5 does not test whether n is between 2 and 5. It is parsed as (2 <= n) <= 5 . This is evaluated by comparing 2 to n , which produces 0 (if false) or 1 (if true). This result, 0 or 1, is then used in … <= 5 . Since 0 and 1 are both less than or equal to 5, the result is always 1 (for true).

To test whether n is greater than or equal to 2 and less than or equal to 5, you must write this out explicitly: 2 <= n and n <= 5 , which we join with the “and” operator, && : 2 <= n && n <= 5 .

Other Issues

The proper form for including stdio.h is #include <stdio.h> , not #include stdio.h .

A proper declaration for main is int main(void) , not int main() .

Corrected Program

A program with these issues corrected is:

#include <stdio.h>

int main(void)
{
    int n;
    scanf("%d", &n);     //taking input
    if (n % 2 != 0)
    {
        printf ("Weird");     //checking first condition
    }
    else if (2 <= n && n <= 5)
    {               //checking second condition
        printf ("Not Weird");
    }
    else if (6 <= n && n <= 20)
    {               //checking third condition
        printf ("Weird");
    }
    else if (n > 20)
    {               //checking fourth condition
        printf ("Not Weird");
    }
    else
    {
        printf ("Error");
    }
    return 0;
}

Answer 2

There is a typo in the line:

if (n / 2 != 0)

The compiler will not complain, but you will get unexpected results at run time.

Here you meant to check if the remainder of division by 2 is not equal to zero (ie: modulus operator), and not the division by 2. This line should be

if (n % 2 != 0)

Second thing: you can't tell C to compare values in ranges like this 2 >= n >= 4 . You will have to split the comparison into 2 comparisons. This line:

else if (n % 2 == 0 && 2 <= n <= 5)

Should be:

else if (n % 2 == 0 && 2 <= n && n <= 5)

You will need to fix all the lines that have this comparison as well.