Case vs If Else If Which is more efficient

Question

Possible Duplicates    is  ldquo else if rdquo  faster than  ldquo switch   case rdquo      What is the relative performance of if else vs  switch in Java        Ive been coding-in-the-run again    when the debugger steps through a case statement it jumps to the item that matches the conditions immediately  however when the same logic is specified using if else it steps through every if statement until it finds the winner  Is the case statement more efficient  or is my debugger just optimizing the step through   don t worry about the syntax errors  i typed this in SO  don t know if it will compile  its the principle i m after  I didn t want to do them as ints cause i vaguely remember something about case using an offset with ints  I use C   but im interested in a general answer across programming languages   switch myObject GetType          case typeof Car             do something         break       case typeof Bike             do something         break       case typeof Unicycle             do something         break       case default          break      VS     Type myType   myObject GetType        if  myType    typeof Car                  do something          else if  myType    typeof Bike                  do something          else if  myType    typeof Unicycle                  do something         else

User · Accepted Answer

It seems that the compiler is better in optimizing a switch-statement than an if-statement.

The compiler doesn't know if the order of evaluating the if-statements is important to you, and can't perform any optimizations there. You could be calling methods in the if-statements, influencing variables. With the switch-statement it knows that all clauses can be evaluated at the same time and can put them in whatever order is most efficient.

Here's a small comparison:
http://www.blackwasp.co.uk/SpeedTestIfElseSwitch.aspx

User · Answer

I believe because cases must be constant values  the switch statement does the equivelent of a goto  so based on the value of the variable it jumps to the right case  whereas in the if then statement it must evaluate each expression

User · Answer

Many programming language optimize the switch statement so that it is much faster than a standard if-else if structure provided the cases are compiler constants  Many languages use a jump table or indexed branch table to optimize switch statements  Wikipedia has a good discussion of the switch statement  Also  here is a discussion of switch optimization in C   One thing to note is that switch statements can be abused and  depending on the case  it may be preferable to use polymorphism instead of switch statements  See here for an example

User · Answer

it can do this for case statements as the values are compiler constants  An explanation in more detail is here http   sequence-points blogspot com 2007 10 why-is-switch-statement-faster-than-if html

User · Answer

i think it s just the debugger making it simple  Note that a case and  if list  are not ultimately the same  There is is a reason why case blocks normally end with  break   The case stmt actually looks something like this when broken down in assembly   if myObject GetType      type of Car     GOTO START CAR else if myObject GetType      type of Bike     GOTO START BIKE  LABEL START CAR   do something car      GOTO END  LABEL START BIKE   do something bike   GOTO END  LABEL END   If you don t have the break  then the case blocks would be missing the  GOTO END  stmts  and in fact if you landed in the  car  case you d actually run both sections    do something car        do something bike   GOTO END

User · Answer

The debugger is making it simpler  because you don t want to step through the actual code that the compiler creates   If the switch contains more than five items  it s implemented using a lookup table or hash table  otherwise it s implemeneted using an if  else   See the closely related question is    else if    faster than    switch   case        Other languages than C  will of course implement it more or less differently  but a switch is generally more efficient

User · Answer

Wikipedia s Switch statement entry is pretty big and actually pretty good  Interesting points    Switches are not inherently fast  It depends on the language  compiler  and specific use  A compiler may optimize switches using jump tables or indexed function pointers  The statement was inspired by some interesting math from Stephen Kleene  and others     For a strange and interesting optimization using a C switch see Duff s Device

[performance] Case vs If Else If: Which is more efficient?

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