Why does this program print "nine" twice?

Question

Live version: http://cpp.sh/953y6

The code:

#include <iostream>
#include <cstdio>
using namespace std;

int main() {
    // Complete the code.
    int num1 = 8, num2 = 11;
    for(int n = num1; n <= num2; n++){

        if(n <= 9){
            switch(n){
                case 1: cout << "one\n";
                case 2: cout << "two\n";
                case 3: cout << "three\n";
                case 4: cout << "four\n";
                case 5: cout << "five\n";
                case 6: cout << "six\n";
                case 7: cout << "seven\n";
                case 8: cout << "eight\n";
                case 9: cout << "nine\n";
            }

      }
      else if(n % 2 == 0){ //even
            cout << "even\n";
      }
      else if(n > 9 && n %2 == 1){ //odd
            cout << "odd\n";
      }

   }

    return 0;
}

The numbers 8 through 11 are looped through on the for-loop. if(n <= 9) should only be triggered twice, when n is 8 and when n is 9. Instead, the output is:

eight
nine
nine
even
odd

Why?

Answer 1

Because you don't have break and it's fall-through case.

case 8: cout << "eight\n";  // <-- need break here
case 9: cout << "nine\n";   // otherwise it's fall-through to here even input is 8

• Good practice is to always break after each case of switch.
• In case you need a fall-through intentionally then make an explicit comment about that fall-through.

Answer 2

Because you dont have a break statement in your switch case :

if(n <= 9){
        switch(n){
            case 1: cout << "one\n";
            case 2: cout << "two\n";
            case 3: cout << "three\n";
            case 4: cout << "four\n";
            case 5: cout << "five\n";
            case 6: cout << "six\n";
            case 7: cout << "seven\n";
            case 8: cout << "eight\n";
            case 9: cout << "nine\n";
        }

  }

When case 8 gets called it first prints eight and then falls through to case 9 and prints nine. Then case 9 gets called when n is 9 and nine gets printed again. Add break statements after case like :

if(n <= 9){
        switch(n){
            ...
            case 8: cout << "eight\n";
                    break;
            case 9: cout << "nine\n"; //last case, dont really need a break
        }

}

Its a good idea to put break after every case in switch, unless fall through is intentional.

Answer 3

In order to avoid such problem in future, you should understand the use-case of not having an implementation-defined implicit break;, at the end of each case : statement.

Consider the below example to see the benefit of "fall-through":

switch (day) {  
    case MONDAY:
    case TUESDAY:
    case WEDNESDAY:
    case THURSDAY:
    case FRIDAY:
        cout << "Weekday";  //same for all five mentioned days of the week.
        break;   // << yes, you need a explicit "break" statement to stop this fall through
    case SATURDAY:
    case SUNDAY:
        cout << "yeah! it's a Weekend";  //same for Sat and Sun.
        break;  //not necessary, but a good practice.
}

Answer 4

Basic rule of switch case is if you are not put break after case it will print further also. So here you didn't put break at the

Case 8 : cout << "eight\n"
//Put break here
Case 9: cout << "nine\n"

So for single print nine you must have to put break after case 8.

Answer 5

Your code should look like this

int main() {
    // Complete the code.
    int num1 = 8, num2 = 11;
    for(int n = num1; n <= num2; n++){

        if(n <= 9){
            switch(n){
                case 1: cout << "one\n";
                break;
                case 2: cout << "two\n";
                break;
                case 3: cout << "three\n";
                break;
                case 4: cout << "four\n";
                break;
                case 5: cout << "five\n";
                break;
                case 6: cout << "six\n";
                break;
                case 7: cout << "seven\n";
                break;
                case 8: cout << "eight\n";
                break;
                case 9: cout << "nine\n";
                break;
            }
      }
      else if(n % 2 == 0){ //even
            cout << "even\n";
      }
      else if(n > 9 && n %2 == 1){ //odd
            cout << "odd\n";
      }

   }
    return 0;
}