The switch Statements in C++

1. the switch-case Statement in C++
2. Rules and Properties in Using Switch Case in C++
3. Difference Between switch Statements and if-else Statements in C++

In this article, we will learn about switch statements in C++ and their special properties.

the switch-case Statement in C++

The switch-case evaluates the expression, based on its value, and is tested against a list of constant values present in case statements to perform different actions based on different cases.

Like if-else, switch statements are control flow statements because they can alter a program’s general flow because it allows an expression or a variable to control the flow via a multi-way branching.

Syntax:

switch(variable / expression)
{
case constant-expression 1:
  statements;

case constant-expression 2:
  statements;
}

Example code:

#include <iostream>
using namespace std;

int main() {
  int day = 3;
  switch (day) {
    case 1:
      cout << "Monday\n";

    case 2:
      cout << "Tuesday\n";

    case 3:
      cout << "Wednesday\n";

    case 4:
      cout << "Thursday\n";

    case 5:
      cout << "Friday\n";

    case 6:
      cout << "Saturday\n";

    case 7:
      cout << "Sunday\n";
  }
}

Output:

Wednesday
Thursday
Friday
Saturday
Sunday

The value of variable day = 3 is evaluated and matched with the respective case statement that is case 3, and the statement it contains that is "Wednesday" is printed, but still we get the remaining strings because once the control goes to case 3, everything from there till the end of the switch block, all the statements are printed/evaluated.

That’s why the statements from cases 4,5,6,7 are printed.

To avoid this, we have to modify our switch-case structure.

Syntax - Modified switch-case:

switch(variable / expression)
{
case constant-expression 1:
  statements;
  break; (optional)

case constant-expression 2:
  statements;
  break; (optional)

default:  //default statment optional
  statements;
  break;
}

The break keyword is utilized to stop the execution of the switch block and come out of it. Once the condition/case is matched, we evaluate the statements, break the flow, and come out of the switch block.

The default keyword specifies statements executed when no case matches.

Example code:

#include <iostream>
using namespace std;

int main() {
  int day = 3;
  switch (day) {
    case 1:
      cout << "Monday\n";
      break;
    case 2:
      cout << "Tuesday\n";
      break;

    case 3:
      cout << "Wednesday\n";
      break;
    case 4:
      cout << "Thursday\n";
      break;

    case 5:
      cout << "Friday\n";
      break;

    case 6:
      cout << "Saturday\n";
      break;

    case 7:
      cout << "Sunday\n";
      break;

    default:
      cout << "Wrong option entereted\n";
      break;
  }
}

Output:

Wednesday

In this next example, we can also switch with char. We’ll see an example code that checks whether a character is a vowel or a consonant.

Example code:

#include <iostream>
using namespace std;

int main() {
  char x = 'u';

  switch (x) {
    case 'a':
      cout << "Vowel";
      break;
    case 'e':
      cout << "Vowel";
      break;
    case 'i':
      cout << "Vowel";
      break;
    case 'o':
      cout << "Vowel";
      break;
    case 'u':
      cout << "Vowel";
      break;

    default:
      cout << "Consonant";
      break;
  }
}

Output:

Vowel

Rules and Properties in Using Switch Case in C++

1. The expression used inside the switch statement should be a constant value. Else it will be considered invalid.

   Here we can see that constant and variable expressions provided they are assigned with fixed values, can be used.

   switch (1 + 2 + 3 * 4 + 5)
     switch (a + b + c * d)
   

2. default and break are optional. Even if the switch-case doesn’t have them, it will run without any problem.

3. Nesting of switch statements is valid means we have can switch statements inside another one. Though it’s not a good programming practice, we can still utilize them.

   #include <iostream>
   using namespace std;
   
   int main() {
     int ID = 1234;
     int password = 000;
   
     switch (ID) {
       case 1234:
         cout << "checking info......\n";
   
         switch (password) {
           case 000:
             cout << "Login successful\n";
             break;
           default:
             cout << "Password don't match please check it\n";
             break;
         }
   
         break;
   
       default:
         printf("incorrect ID");
         break;
     }
   }
   

   Output:

   checking info......
   Login successful
   

4. The expression used inside the switch statement must be an integral type meaning it should be int, char, or enum. Else we get a compile error.

   #include <iostream>
   using namespace std;
   
   int main() {
     float x = 12.34;
   
     switch (x) {
       case 1.1:
         cout << "Yes";
         break;
       case 12.34:
         cout << "NO";
         break;
     }
   }
   

   Output:

   [Error] switch quantity not an integer
   

5. The default block can be inside the switch but only executed when no cases are matched.

   #include <iostream>
   using namespace std;
   
   int main() {
     int x = 45;
     switch (x) {
       default:
         cout << "It's not 1 or 2";
         break;
       case 1:
         cout << "It's a 1";
         break;
       case 2:
         cout << "It's a 2";
         break;
     }
   }
   

   Output:

   It's not 1 or 2
   

6. The statements written above the case are never executed. Once the switch statement is evaluated, the control shifts to either the matching case or the default block if present.

   #include <iostream>
   using namespace std;
   
   int main() {
     int x = 45;
     switch (x) {
       cout << "Tony is Iron Man\n";
       case 1:
         cout << "It's a 1";
         break;
       case 2:
         cout << "It's a 2";
         break;
   
       default:
         cout << "It's not 1 or 2";
         break;
     }
   }
   

   Output:

   It's not 1 or 2
   

7. Duplicate case labels are not allowed. If given, we will get a compile error.

   #include <iostream>
   using namespace std;
   
   int main() {
     int x = 45;
     switch (x) {
       case 45:
         cout << "Iron Man";
         break;
   
       case 40 + 5:
         cout << "tony stark";
         break;
   
       default:
         cout << "It's not 1 or 2";
         break;
     }
   }
   

   Output:

   [Error] duplicate case value
   

8. The case labels should also be constant. Else, we get a compile error.

   #include <iostream>
   using namespace std;
   
   int main() {
     int x = 2;
     int arr[] = {100, 200, 300};
     switch (x) {
       case arr[0]:
         cout << "Iron Man";
         break;
   
       case arr[1]:
         cout << "tony stark";
         break;
   
       case arr[2]:
         cout << "It's not 1 or 2";
         break;
     }
   }
   

   Output:

   [Error] the value of 'arr' is not usable in a constant expression
   

9. We can group all the case statements performing the same task.

   #include <iostream>
   using namespace std;
   
   int main() {
     char x = 'A';
   
     switch (x) {
       case 'a':
       case 'e':
       case 'i':
       case 'o':
       case 'u':
       case 'A':
       case 'E':
       case 'I':
       case 'O':
       case 'U':
         cout << "Vowel";
         break;
   
       default:
         cout << "consonant";
     }
   }
   

Difference Between switch Statements and if-else Statements in C++

When we have many if-else statements, the compiler must check all of them until a valid match is found. While in switch-case statements are used if we only want a certain block of code to run, if a certain condition is met.

The following example checks a character if it is an alphabet or not.

Example code:

char x = '+';

if (x == 'A')
....

    else if (x == 'B')...

    else if (x == 'C')...

    ....else cout
    << "Not an alphabet"

Since + is not an alphabet, it takes much longer to reach the last because it must first examine every previous condition. If implemented using switch-case, it uses a lookup table or a hash list when the number of cases/conditions is very large.

This means all items get the same access time. So, when we have many cases/conditions, the switch is preferred over if-else as it is faster.