comma in switch case

Question

I am confused with the input and output, can anyone tell me why I get such output.

#include<iostream>
#include<stdlib.h>
using namespace std;

int main()
{
    int ival, oddcnt(0), evencnt(0);
    while (cin >> ival) {
        switch (ival) {
        case 1, 3, 5, 7, 9:
            oddcnt++;
            break;
        case 2, 4, 6, 8, 10:
            evencnt++;
            break;
        }
    }
    cout << "Quantity of odd number:" << oddcnt << "\n"
        << "Quantity of even number:" << evencnt << endl;
    system("pause");
    return 0;
}

This is the result I get: Input:1 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 5 5 6 6 6 7 7 7 7 8 8 9 9 10 10 10 EOF Output:Quantity of odd number:2 Quantity of even number:3

Answer 1

This usage of comma in case label is illegal in standard C++. It is simply grammatically incorrect. A conforming compiler must issue a diagnostic message in response to this code, even if it accepts it somehow.

There might (or might not) be some intricate details involved.

• In the original C++98 language comma operator was prohibited form being used in case label. It was "blocked" by two different parts of language specification:

  Firstly, the grammar of constant-expression does not allow top-level comma operators in constant expressions. Grammatically, a comma operator can only appear in constant expressions enclosed in ().

  Secondly, C++98 specification explicitly prohibited comma operator in constant expressions at any level of nested () (see 5.19/1).

  (This was basically identical to C language requirements.)

• However, C++11 made some changes: it allowed comma operator in constant expressions. Yet, the grammar was left unchanged. This means that comma operator is now allowed in constant expressions, but only when enclosed in (). E.g. starting from C++11 you can use

  case (1, 3, 5, 7, 9):
  

  which is an ordinary application of comma operator. It makes the whole thing equivalent to

  case 9:
  

Meanwhile, your

    case 1, 3, 5, 7, 9:

remains illegal in C++ to this day. If your compiler supports it somehow, it must be a language extension implemented by your compiler. Consult your compiler documentation to figure out what it means.

Judging by the output you got, your compiler sees case 1, 3, 5, 7, 9: as equivalent to case (1, 3, 5, 7, 9): and thus equivalent to case 9:. That explains the output. The program simply counts 9s and 10s in the input. You've got two 9s and three 10s.

See also:
Is the comma operator allowed in a constant-expression in C++11?
Why was the restriction on the comma operator being in a constant expression removed in C++11?

Answer 2

You can try this inside the switch instead:

switch (ival) {
    case 1:
    case 3:
    case 5:
    case 7:
    case 9:
     oddcnt++;
     break;

    case 2:
    case 4:
    case 6:
    case 8:
    case 10:
     evencnt++;
     break;
}

Answer 3

switch (var) {
    case value1: /* ... */ break;
    case value2: /* ... */ break;
    /* ... */
}

Answer 4

 switch (ival) {
        case 1, 3, 5, 7, 9:
            oddcnt++;
            break;
        case 2, 4, 6, 8, 10:
            evencnt++;
            break;
        }

case 1, 3, 5, 7, 9: This type of switch statement leads to syntax error but in your case it did not.

Syntax errors(on compiler gcc version 5.4.0 (GCC)):

error: expected ‘:’ before ‘,’ token  
       case 1, 3, 5, 7, 9:
             ^
error: expected ‘:’ before ‘,’ token
     case 2, 4, 6, 8, 10:
           ^
error: expected primary-expression before ‘,’ token

Nevertheless you can try the following switch case:

switch (ival) {
        case 1:
        case 3:
        case 5:
        case 7:
        case 9:
            oddcnt++;
            break;
        case 2:
        case 4:
        case 6:
        case 8:
        case 10:
            evencnt++;
            break;
        }