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Switch statements are typically faster than equivalent if-else-if statements (as e.g. descibed in this article) due to compiler optimizations.
How does this optimization actually work? Does anyone have a good explanation?
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A switch statement works much faster than an equivalent if-else ladder. It's because the compiler generates a jump table for a switch during compilation. As a result, during execution, instead of checking which case is satisfied, it only decides which case has to be executed.
Use switch every time you have more than 2 conditions on a single variable, take weekdays for example, if you have a different action for every weekday you should use a switch. Other situations (multiple variables or complex if clauses you should Ifs, but there isn't a rule on where to use each.
Generally switch statements are faster than if else statements. But when there are few cases (less than 5) it is better to with if else statements as there will no significant performance improvement.
A switch statement is significantly faster than an if-else ladder if there are many nested if-else's involved. This is due to the creation of a jump table for switch during compilation. As a result, instead of checking which case is satisfied throughout execution, it just decides which case must be completed.
The compiler can build jump tables where applicable. For example, when you use the reflector to look at the code produced, you will see that for huge switches on strings, the compiler will actually generate code that uses a hash table to dispatch these. The hash table uses the strings as keys and delegates to the case codes as values.
This has asymptotic better runtime than lots of chained if tests and is actually faster even for relatively few strings.
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This is a slight simplification as typically any modern compiler that encounters a if..else if .. sequence that could trivially be converted into a switch statement by a person, the compiler will as well. But just to add extra fun the compiler is not restricted by syntax so can generate "switch" like statements internally that have a mix of ranges, single targets, etc -- and they can (and do) do this for both switch and if..else statements.
Anyhoo, an extension to Konrad's answer is that the compiler may generate a jump table, but that's not necessarily guaranteed (nor desirable). For a variety of reasons jump tables do bad things to the branch predictors on modern processors, and the tables themselves do bad things to cache behaviour, eg.
switch(a) { case 0: ...; break; case 1: ...; break; } If a compiler actually generated a jump table for this it would likely be slower that the alternative if..else if.. style code because of the jump table defeating branch prediction.
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