Is "else if" faster than "switch() case"?

Asked 2009-04-20T11:07:15.247

437

I'm an ex Pascal guy, currently learning C#. My question is the following:

Is the code below faster than making a switch?

int a = 5;

if (a == 1)
{
    ....
}
else if(a == 2)
{
    ....
}
else if(a == 3)
{
    ....
}
else if(a == 4)
{
    ....
}
else
    ....

And the switch:

int a = 5;

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

    case 2:
        ...
        break;

    case 3:
        ...
        break;

    case 4:
        ...
        break;

    default:
        ...
        break;


}

Which one is faster?

I'm asking, because my program has a similar structure (many, many "else if" statements). Should I turn them into switches?

c# performance if-statement switch-statement

Edit Report

4 Answers

773

For just a few items, the difference is small. If you have many items you should definitely use a switch.

If a switch contains more than five items, it's implemented using a lookup table or a hash list. This means that all items get the same access time, compared to a list of if:s where the last item takes much more time to reach as it has to evaluate every previous condition first.

answered 2009-04-20T11:14:15.947

196

Why do you care?

99.99% of the time, you shouldn't care.

These sorts of micro-optimizations are unlikely to affect the performance of your code.

Also, if you NEEDED to care, then you should be doing performance profiling on your code. In which case finding out the performance difference between a switch case and an if-else block would be trivial.

Edit: For clarity's sake: implement whichever design is clearer and more maintainable. Generally when faced with a huge switch-case or if-else block the solution is to use polymorphism. Find the behavior that's changing and encapsulate it. I've had to deal with huge, ugly switch case code like this before and generally it's not that difficult to simplify. But oh so satisfying.

answered 2009-04-20T11:27:23.840

Short answer: Switch statement is quicker

The if statement you need two comparisons (when running your example code) on average to get to the correct clause.

The switch statement the average number of comparisons will be one regardless of how many different cases you have. The compiler/VM will have made a "lookup table" of possible options at compile time.

Can virtual machines optimize the if statement in a similar way if you run this code often?

answered 2009-04-20T11:14:58.050

Since the switch statement expresses the same intent as your if / else chain but in a more restricted, formal manner, your first guess should be that the compiler will be able to optimize it better, since it can draw more conclusions about the conditions placed on your code (i.e. only one state can possibly be true, the value being compared is a primitive type, etc.) This is a pretty safe general truth when you are comparing two similar language structures for runtime performance.

answered 2009-04-20T19:25:51.163

Is "else if" faster than "switch() case"?

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