Using if statement instead of default of switch case

Question

Is it correct to use an if statement before opening a switch case and avoid using the default keyword? For example I want a program which takes as input the number of a month and tells you its name. This is the code which uses the switch case statement:

#include <stdio.h>
#include <stdlib.h>

main() {
    int month;
    printf("Insert the number of the month and the program will return its name");
    scanf("%i", &month);
    switch (month) {
      case (1):
        printf("The month is January");
        break;
      case (2):
        printf("The month is February");
        break;
      case (3):
        printf("The month is March");
        break;
      case (4):
        printf("The month is April");
        break;
      case (5):
        printf("The month is May");
        break;
      case (6):
        printf("The month is June");
        break;
      case (7):
        printf("The month is July");
        break;
      case (8):
        printf("The month is August");
        break;
      case (9):
        printf("The month is September");
        break;
      case (10):
        printf("The month is October");
        break;
      case (11):
        printf("The month is November");
        break;
      case (12):
        printf("The month is December");
        break;
      default:
        printf("not valid");
    }
    system("pause");
    return 0;
}

Then, I was wondering if I can put the non-validity condition in an if statement rather than in the default keyword. To me it seems correct since I want to verify the value before the program executes the switch case statement. What do you think, would it be correct? If I'm not asking too much, would you please tell me why?

The code with the if statement:

#include <stdio.h>
#include <stdlib.h>

main() {
    int month;
    printf("Insert the number of the month and the program will return its name");
    scanf("%i", &month);
    if (month >= 1 && month <= 12) {
        switch (month) {
          case (1):
            printf("The month is January");
            break;
          case (2):
            printf("The month is February");
            break;
          case (3):
            printf("The month is March");
            break;
          case (4):
            printf("The month is April");
            break;
          case (5):
            printf("The month is May");
            break;
          case (6):
            printf("The month is June");
            break;
          case (7):
            printf("The month is July");
            break;
          case (8):
            printf("The month is August");
            break;
          case (9):
            printf("The month is September");
            break;
          case (10):
            printf("The month is October");
            break;
          case (11):
            printf("The month is November");
            break;
          case (12):
            printf("The month is December");
            break;
          default:;
        }
    } else {
        printf("not valid");
    }
    system("pause");
    return 0;
}

Thank you and sorry for my English but it is not my mother tongue. Let me know if I haven't explained myself clearly.

Answer 1

Why wouldn't it be correct? It is. default is just safer: if you change the cases, but fail to change the if, you'll get into trouble.

Answer 2

Both approaches are valid and equivalent except for a few details:

• The way you define main is completely obsolete. It will not compile with a C99 compiler in strict mode. Use int main(void) or int main(int argc, char *argv[]).
• Test the return value of scanf(). If you type something that cannot be parsed as a number, scanf() will return 0 or even EOF if you close the input stream, and the rest of the code will use an uninitialized value of month.
• It is neither useful nor idiomatic to parenthesize the value in case clauses. Remove them.
• It is safer to always add a break; statement at the end of the last clause in a switch statement, default or not. If you ever add another clause, you wont risk omitting it.
• Remove the default:; clause from the second code, it is useless and surprising.

The reason why the second approach might be more indicated is in case you want to do something completely different if the input is out of range, such as restarting an input operation. The if statement will allow you to separate these situations correctly, whereas just using the default clause might not be so appropriate:

for (;;) {
    int n, month;
    printf("Enter a number between 1 and 12: ");
    n = scanf("%d", &month);
    if (n == EOF) {
        printf("Unexpected end of file\n");
        exit(1);
    }
    if (n != 1) {
        printf("Invalid input\n");
        scanf("%*[^\n]%*c"); /* flush the pending input */
        continue;
    }
    if (month >= 1 && month <= 12) {
        switch (month) {
          case 1: 
            printf("The month is January\n");
            break;
          ...
          case 12: 
            printf("The month is December\n");
            break;
        }
        handle_month(month);  // perform other tasks
        break;
    } else {
        printf("Invalid month number\n");
    }
}

Answer 3

Yes, you can use an if statement as you describe. However, I don't see any benefit from doing so.

Answer 4

The original was better. The compiler will do pretty much the same work although the second will be a bit slower because of branch prediction.

But the main reason is maintainability of the code. When adding or removing one value you need to modify two places with the if statement while only one with the switch.

Answer 5

My personal view, which may not be perfect.

Having default case is preferred as it catches any unaccounted cases. People seems to rely on it, Hence, a warning flag -Wswitch-default in gcc. So, sometimes you might get some compiler warning without default case.

In your particular case, It is fine but it is good practice to use default case.

Answer 6

Actually, I would say that your code violates best practices when to use switch statements. Rather, try this:

if (month >= 1 && month <= 12) {
    char const*const names[] = {
        "January",
        "February",
        ...
        "November",
        "December",
    };
    printf("The month is %s\n", names[month -1]);
} else {
    printf("not valid\n");
}

That said, getting back to your ininital question, mixing flow control structures like if/else with switch/case on the same data is usually bad. The reason for that is that it is unnecessarily complicated, so stick with either the single switch statement like in your initial attempt or use the approach I sketched above.