switch Versus if-else Ladder

There are some things that you simply cannot do with a switch. These are:

  1. A float expression cannot be tested using a switch.
  2. Cases can never have variable expressions (for example it is wrong to say case a +3 : )
  3. Multiple cases cannot use same expressions.

Thus the following switch is illegal:

switch ( a )
  case 3 :
  case 1 + 2 :

(1), (2) and (3) above may lead you to believe that these are obvious disadvantages with a switch, especially since there weren’t any such limitations with if-else. Then why use a switch at all? For speed—switch works faster than an equivalent if-else ladder. How come? This is because the compiler generates a jump table for a switch during compilation. As a result, during execution it simply refers the jump table to decide which case should be executed, rather than actually checking which case is satisfied. As against this, if-elses are slower because they are evaluated at execution time. A switch with 10 cases would work faster than an equivalent if-else ladder. Also, a switch with 2 cases would work slower than if-else ladder.
Why? If the 10th case is satisfied then jump table would be referred and statements for the 10th case would be executed. As against this, in an if-else ladder 10 conditions would be evaluated at execution time, which makes it slow. Note that a lookup in the jump table is faster than evaluation of a condition, especially if the condition is complex.


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