Ways to eliminate switch in code

Question

What are the ways to eliminate the use of switch in code

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Use a language that doesn t come with a built-in switch statement  Perl 5 comes to mind   Seriously though  why would you want to avoid it  And if you have good reason to avoid it  why not simply avoid it then

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Switch statements can often be replaced by a good OO design   For example  you have an Account class  and are using a switch statement to perform a different calculation based on the type of account   I would suggest that this should be replaced by a number of account classes  representing the different types of account  and all implementing an Account interface   The switch then becomes unnecessary  as you can treat all types of accounts the same and thanks to polymorphism  the appropriate calculation will be run for the account type

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For C     If you are referring to ie an AbstractFactory I think that a registerCreatorFunc     method usually is better than requiring to add a case for each and every  new  statement that is needed  Then letting all classes create and register a creatorFunction     which can be easy implemented with a macro  if I dare to mention   I believe this is a common approach many framework do  I first saw it in ET   and I think many frameworks that require a DECL and IMPL macro uses it

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In JavaScript using Associative array  this     function getItemPricing customer  item        switch  customer type               VIPs are awesome  Give them 50  off          case  VIP               return item price   item quantity   0 50                  Preferred customers are no VIPs  but they still get 25  off          case  Preferred               return item price   item quantity   0 75                  No discount for other customers          case  Regular           case         default              return item price   item quantity            becomes this     function getItemPricing customer  item    var pricing          VIP   function item            return item price   item quantity   0 50              Preferred   function item            if  item price  lt   100 0              return item price   item quantity   0 75              Else         return item price   item quantity              Regular   function item            return item price   item quantity                if  pricing customer type           return pricing customer type  item       else         return pricing Regular item       Courtesy

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Another vote for if else  I m not a huge fan of case or switch statements because there are some people that don t use them  The code is less readable if you use case or switch  Maybe not less readable to you  but to those that have never needed to use the command    The same goes for object factories   If else blocks are a simple construct that everyone gets  There s a few things you can do to make sure that you don t cause problems    Firstly - Don t try and indent if statements more than a couple of times  If you re finding yourself indenting  then you re doing it wrong     if a   1 then       do something else       if a   2 then           do something else      else           if a   3 then               do the last thing          endif      endif    endif   Is really bad - do this instead    if a   1 then     do something endif  if a   2 then     do something else endif  if a   3 then     do something more endif    Optimisation be damned  It doesn t make that much of a difference to the speed of your code    Secondly  I m not averse to breaking out of an If Block as long as there are enough breaks statements scattered through the particular code block to make it obvious  procedure processA a int      if a   1 then         do something        procedure return     endif      if a   2 then         do something else        procedure return     endif      if a   3 then         do something more        procedure return     endif  end procedure   EDIT  On Switch and why I think it s hard to grok    Here s an example of a switch statement     private void doLog LogLevel logLevel  String msg       String prefix     switch  logLevel         case INFO         prefix    INFO          break       case WARN         prefix    WARN          break       case ERROR         prefix    ERROR          break       default         throw new RuntimeException  Oops  forgot to add stuff on new enum constant            System out println String format   s   s   prefix  msg         For me the issue here is that the normal control structures which apply in C like languages have been completely broken  There s a general rule that if you want to place more than one line of code inside a control structure  you use braces or a begin end statement    e g    for i from 1 to 1000  statement1  statement2  if something false then  statement1  statement2  while isOKtoLoop  statement1  statement2    For me  and you can correct me if I m wrong   the Case statement throws this rule out of the window  A conditionally executed block of code is not placed inside a begin end structure  Because of this  I believe that Case is conceptually different enough to not be used    Hope that answers your questions

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Why do you want to  In the hands of a good compiler  a switch statement can be far more efficient than if else blocks  as well as being easier to read   and only the largest switches are likely to be sped up if they re replaced by any sort of indirect-lookup data structure

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Function pointers are one way to replace a huge chunky switch statement  they are especially good in languages where you can capture functions by their names and make stuff with them   Of course  you ought not force switch statements out of your code  and there always is a chance you are doing it all wrong  which results with stupid redundant pieces of code   This is unavoidable sometimes  but a good language should allow you to remove redundancy while staying clean    This is a great divide amp conquer example   Say you have an interpreter of some sort   switch  IP        case OPCODE ADD                      break      case OPCODE NOT ZERO                      break      case OPCODE JUMP                      break      default          fixme  IP       Instead  you can use this   opcode table  IP   IP  vm           in somewhere else  void opcode add byte opcode op  Vm  vm           void opcode not zero byte opcode op  Vm  vm           void opcode jump byte opcode op  Vm  vm           void opcode default byte opcode op  Vm  vm       fixme        OpcodeFuncPtr opcode table 256                  opcode add      opcode not zero      opcode jump      opcode default      opcode default             etc       Note that I don t know how to remove the redundancy of the opcode table in C  Perhaps I should make a question about it

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Switch is not a good way to go as it breaks the Open Close Principal  This is how I do it   public class Animal          public abstract void Speak        public class Dog   Animal      public virtual void Speak               Console WriteLine  Hao Hao            public class Cat   Animal      public virtual void Speak               Console WriteLine  Meauuuu             And here is how to use it  taking your code    foreach  var animal in zoo         echo animal speak        Basically what we are doing is delegating the responsibility to the child class instead of having the parent decide what to do with children   You might also want to read up on  Liskov Substitution Principle

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Switch in itself isn t that bad  but if you have lots of  switch  or  if else  on objects in your methods it may be a sign that your design is a bit  procedural  and that your objects are just value buckets  Move the logic to your objects  invoke a method on your objects and let them decide how to respond instead

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switch  is just a language construct and all language constructs can be thought of as tools to get a job done  As with real tools  some tools are better suited to one task than another  you wouldn t use a sledge hammer to put up a picture hook   The important part is how  getting the job done  is defined  Does it need to be maintainable  does it need to be fast  does it need to scale  does it need to be extendable and so on   At each point in the programming process there are usually a range of constructs and patterns that can be used  a switch  an if-else-if sequence  virtual functions  jump tables  maps with function pointers and so on  With experience a programmer will instinctively know the right tool to use for a given situation   It must be assumed that anyone maintaining or reviewing code is at least as skilled as the original author so that any construct can be safely used

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If the switch is there to distinguish between various kinds of objects  you re probably missing some classes to precisely describe those objects  or some virtual methods

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if-else  I refute the premise that switch is inherently bad though

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Switch-statements are not an antipattern per se  but if you re coding object oriented you should consider if the use of a switch is better solved with polymorphism instead of using a switch statement   With polymorphism  this   foreach  var animal in zoo        switch  typeof animal             case  dog               echo animal bark                break           case  cat               echo animal meow                break            becomes this   foreach  var animal in zoo        echo animal speak

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Switch is not a good way to go as it breaks the Open Close Principal  This is how I do it   public class Animal          public abstract void Speak        public class Dog   Animal      public virtual void Speak               Console WriteLine  Hao Hao            public class Cat   Animal      public virtual void Speak               Console WriteLine  Meauuuu             And here is how to use it  taking your code    foreach  var animal in zoo         echo animal speak        Basically what we are doing is delegating the responsibility to the child class instead of having the parent decide what to do with children   You might also want to read up on  Liskov Substitution Principle

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The most obvious  language independent  answer is to use a series of  if     If the language you are using has function pointers  C  or has functions that are 1st class values  Lua  you may achieve results similar to a  switch  using an array  or a list  of  pointers to  functions   You should be more specific on the language if you want better answers

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I think the best way is to use a good Map  Using a dictionary you can map almost any input to some other value object function   your code would look something psuedo  like this   void InitMap        Map key1    Object Action      Map key2    Object Action     Object Action DoStuff Object key       return Map key

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switch statements would be good to replace if you find yourself adding new states or new behaviour to the statements    int state   String getString        switch  state         case 0      behaviour for state 0            return  zero        case 1      behaviour for state 1            return  one           throw new IllegalStateException       double getDouble         switch  this state         case 0      behaviour for state 0            return 0d       case 1      behaviour for state 1            return 1d          throw new IllegalStateException        Adding new behaviour requires copying the switch  and adding new states means adding another case to every switch statement   In Java  you can only switch a very limited number of primitive types whose values you know at runtime  This presents a problem in and of itself  states are being represented as magic numbers or characters    Pattern matching  and multiple if - else blocks can be used  though really have the same problems when adding new behaviours and new states   The solution which others have suggested as  polymorphism  is an instance of the State pattern   Replace each of the states with its own class  Each behaviour has its own method on the class    IState state   String getString        return state getString       double getDouble        return state getDouble        Each time you add a new state  you have to add a new implementation of the IState interface  In a switch world  you d be adding a case to each switch   Each time you add a new behaviour  you need to add a new method to the IState interface  and each of the implementations  This is the same burden as before  though now the compiler will check that you have implementations of the new behaviour on each pre-existing state   Others have said already  that this may be too heavyweight  so of course there is a point you reach where you move from one to another  Personally  the second time I write a switch is the point at which I refactor

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Depends why you want to replace it   Many interpreters use  computed gotos  instead of switch statements for opcode execution   What I miss about C C   switch is the Pascal  in  and ranges   I also wish I could switch on strings   But these  while trivial for a compiler to eat  are hard work when done using structures and iterators and things   So  on the contrary  there are plenty of things I wish I could replace with a switch  if only C s switch   was more flexible

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In a procedural language  like C  then switch will be better than any of the alternatives   In an object-oriented language  then there are almost always other alternatives available that better utilise the object structure  particularly polymorphism   The problem with switch statements arises when several very similar switch blocks occur at multiple places in the application  and support for a new value needs to be added  It is pretty common for a developer to forget to add support for the new value to one of the switch blocks scattered around the application   With polymorphism  then a new class replaces the new value  and the new behaviour is added as part of adding the new class  Behaviour at these switch points is then either inherited from the superclass  overridden to provide new behaviour  or implemented to avoid a compiler error when the super method is abstract   Where there is no obvious polymorphism going on  it can be well worth implementing the Strategy pattern   But if your alternative is a big IF     THEN     ELSE block  then forget it

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I think that what you are looking for is the Strategy Pattern     This can be implemented in a number of ways  which have been mentionned in other answers to this question  such as     A map of values -  functions Polymorphism    the sub-type of an object will decide how it handles a specific process     First class functions

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Everybody loves HUGE if else blocks  So easy to read  I am curious as to why you would want to remove switch statements  though  If you need a switch statement  you probably need a switch statement  Seriously though  I d say it depends on what the code s doing  If all the switch is doing is calling functions  say  you could pass function pointers  Whether it s a better solution is debatable   Language is an important factor here also  I think

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See the Switch Statements Smell      Typically  similar switch statements are scattered throughout a program  If you add or remove a clause in one switch  you often have to find and repair the others too    Both Refactoring and Refactoring to Patterns have approaches to resolve this   If your  pseudo  code looks like   class RequestHandler        public void handleRequest int action            switch action                case LOGIN                  doLogin                    break              case LOGOUT                  doLogout                    break              case QUERY                 doQuery                   break                      This code violates the Open Closed Principle and is fragile to every new type of action code that comes along  To remedy this you could introduce a  Command  object   interface Command       public void execute       class LoginCommand implements Command       public void execute                do what doLogin   used to do          class RequestHandler       private Map lt Integer  Command gt  commandMap     injected in  or obtained from a factory     public void handleRequest int action            Command command   commandMap get action           command execute              If your  pseudo  code looks like   class House       private int state       public void enter             switch  state                case INSIDE                  throw new Exception  Cannot enter  Already inside                case OUTSIDE                   state   INSIDE                                        break                       public void exit             switch  state                case INSIDE                  state   OUTSIDE                                      break              case OUTSIDE                  throw new Exception  Cannot leave  Already outside                      Then you could introduce a  State  object      Throw exceptions unless the behavior is overriden by subclasses abstract class HouseState       public HouseState enter             throw new Exception  Cannot enter              public HouseState leave             throw new Exception  Cannot leave             class Inside extends HouseState       public HouseState leave             return new Outside             class Outside extends HouseState       public HouseState enter             return new Inside             class House       private HouseState state      public void enter             this state   this state enter              public void leave             this state   this state leave              Hope this helps

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A switch is a pattern  whether implemented with a switch statement  if else chain  lookup table  oop polymorphism  pattern matching or something else   Do you want to eliminate the use of the  switch statement  or the  switch pattern   The first one can be eliminated  the second one  only if another pattern algorithm can be used  and most of the time that is not possible or it s not a better approach to do so   If you want to eliminate the switch statement from code  the first question to ask is where does it make sense to eliminate the switch statement and use some other technique  Unfortunately the answer to this question is domain specific    And remember that compilers can do various optimizations to switch statements  So for example if you want to do message processing efficiently  a switch statement is pretty much the way to go  But on the other hand running business rules based on a switch statement is probably not the best way to go and the application should be rearchitected   Here are some alternatives to switch statement     lookup table polymorphism  pattern matching  especially used in functional programming  C   templates

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Well  for one  I didn t know using switch was an anti pattern   Secondly  switch can always be replaced with if   else if statements

[design-patterns] Ways to eliminate switch in code

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