switch case vs if else

Question

I was wondering if there was any difference in the way the following code was compiled into assembly. I've heard that switch-case is more efficient than if else, but in this example I am not quite sure if that would be the case.

if(x==1){
...
}else if(x==2){
...
}else{
...
}

and

switch(x){
 case 1:
 ...
 break;

 case 2:
 ...
 break;

 default:
 ...
}

Have you considered getting the compiler to emit assembler and compare the resulting code? — JUST MY correct OPINION, May 06 '11 at 10:39
For me swich-case should be avoid and use a [hash](http://hardforo.com/thread-531.html) (associative array) instead. It's cleaner. — boctulus, Jun 01 '16 at 19:09
Found a good discussion here: http://stackoverflow.com/questions/1028437/why-switch-case-and-not-if-else-if — Helene, Nov 10 '16 at 23:58

score 13 · Answer 1 · answered Oct 12 '08 at 18:24

A compiler will sometimes turn a switch into a jump-table, if the entries are contiguous (or nearly so). Or it could theoretically use a binary search to find the case instead of a linear series of tests, which would be faster if you had a large number of cases.

On the other hand, there's nothing stopping the compiler from doing the same optimisations on the same code converted into if/else.

So on a good compiler, switch can be faster in some cases. On a very good compiler, they'd be the same.

score 3 · Answer 2 · answered Oct 12 '08 at 18:18

In this specific case, the switch can be turned into a jump table. The if statement (if you write your = as == :-P) could still do the same thing if the compiler could tell that x isn't changing between the if clauses (which is usually the case, unless x is volatile or something).

score 3 · Answer 3 · answered Oct 13 '08 at 03:45

3

Note too that the if/else construct can be more efficient if you know certain cases are more likely than others.

answered Oct 13 '08 at 03:45

Moishe Lettvin

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But switch cases are ordered, as well, no? – user359996 Oct 18 '10 at 10:56
1

I don't know if all compilers guarantee the ordering. Also the compiler may create a jump table -- when this is more or less efficient than if/else is left as an exercise for the reader. – Moishe Lettvin Oct 18 '10 at 13:39
9

Shouldn't this be posted as a comment? – FluorescentGreen5 Oct 01 '16 at 08:56

switch case vs if else

3 Answers3

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