switch

If you have a large number of conditionals that depend on the value of a single variable, it is often more convenient to use a switch statement than it is to use a long list of if/else statements. The switch statement has the format

switch( expression ) {
  case value1:
   statement1
   break;
  case value2:
   statement2
   break;
   . . .
  default:
   statement3
}

This is best illustrated by example. The following switch statement converts an integer value n to a text representation of the integer:

switch(n) {
case 0:
  name="zero";
  break;
case 1:
  name="one";
  break;
case 2:
  name="two";
  break;
case 3:
  name="three";
  break;
case 4:
  name="four";
  break;
case 5:
  name="five";
  break;
case 6:
  name="six";
  break;
case 7:
  name="seven";
  break;
case 8:
  name="eight";
  break;
case 9:
  name="nine";
  break;
default:
  name="ERROR: n is not between 0 and 9";
}

This statement first evaluates the expression n and then scans all case statements to find one that matches the value of n. For example, if n is equal to 3, execution of the conditional statements begins at case 3:. In the example, all you want to do for each case is set the value of the variable called name and then jump out of the case statement. You jump out of the statement using the break command. If none of the case statements contains a match for the value of n, execution jumps to default:.

Note that execution of the conditional statements starts at the matching case statement, but does not end with the start of the next case statement. For example, if you remove the break command between the third and fourth cases, as follows:

case 3:
name="three";
case 4:
name="four";
break;

When n is equal to 3, the variable name will be set equal to three and then execution will chain into the next case statement. That is, name will be changed to be equal to four.

You can intentionally use this feature to combine cases as in the following example:

switch(n) {
case 0:
case 2:
case 4:
case 6:
case 8:
  name="even number";
  break;
case 1:
case 3:
case 5:
case 7:
case 9:
  name="odd number";
  break;
default:
  name="ERROR: n is not between 0 and 9";
}

The individual case values can be any data type, not just numbers. All of the following are legal case expressions:

case 3:
case 2.78:
case "number":
case true:
case [1,2,3]:
case 2+3:
case f(x)+g(y):