CITS2002 Systems Programming  
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Switch statements

When the same (integer) expression is compared against a number of distinct values, it's preferred to evaluate the expression once, and compare it with possible values:

Cascading if..else..if.. statements: The equivalent switch statement: Less-common features of the switch statement:

if(expression == value1) {
  // more statements;
  .....
}
else if(expression == value2) {
  // more statements;
  .....
}
else {
  // more statements;
  .....
}







switch (expression) {          
  case value1 :
  // more statements;
    .....
    break;

  case value2 :
    // more statements;
    .....
    break;

  default :
    // more statements;
    .....
    break;
}



switch (expression) {
  case value1 :
  case value2 :
    // handle either value1 or value2
    .....
    break;

  case value3 :
    // more statements;
    .....
    // no 'break' statement, drop through

  default :
    // more statements;
    .....
    break;

}

  • Typically the 'expression' is simply an identifier, but it may be arbitrarily complex - such as an arithmetic expression, or a function call.
  • The datatype of the 'expression' must be an integer (which includes characters, Booleans, and enumerated types), but it cannot be a real or floating-point datatype.
  • The break statement at the end of each case indicates that we have finished with the 'current' value, and control-flow leaves the switch statement.
    Without a break statement, control-flow continues "downwards", flowing into the next case branch (even though the expression does not have that case's value!).
  • switch statements with 'dense' values (none, or few integer missing) provide good opportunities for optimised code.
  • There is no need to introduce a new block, with curly-brackets, unless you need to define new local variables for specific case branches.

 


CITS2002 Systems Programming, Lecture 2, p11, 24th July 2024.