switch Versus if-else Ladder
There are some things that you simply cannot do with a switch. These are:
a) A float expression cannot be tested using a switch
b) Cases can never have variable expressions (for example it is wrong to say case a +3 : )
c) Multiple cases cannot use same expressions. Thus the following switch is illegal:
switch ( a )
{
case 3 :
...
case 1 + 2 :
...
}
(a), (b) and (c) 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-elses 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. If on the other hand the conditions in the if-else were simple and
less in number then if-else would work out faster than the lookup mechanism of a switch. Hence a switch with two cases would work slower than an equivalent if-else. Thus, you as a programmer should take a decision which of the two should be used when.
Note :- above information taken from Let's C book