Why the switch statement cannot be applied on strings

Question

Compiling the following code and got the error of type illegal   int main            Compilation error - switch expression of type illegal     switch std  string  raj              case sda             You cannot use string in either switch or case  Why  Is there any solution that works nicely to support logic similar to switch on strings

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In c   strings are not first class citizens  The string operations are done through standard library  I think  that is the reason  Also  C   uses branch table optimization to optimize the switch case statements  Have a look at the link   http   en wikipedia org wiki Switch statement

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The reason why has to do with the type system   C C   doesn t really support strings as a type   It does support the idea of a constant char array but it doesn t really fully understand the notion of a string     In order to generate the code for a switch statement the compiler must understand what it means for two values to be equal   For items like ints and enums  this is a trivial bit comparison   But how should the compiler compare 2 string values   Case sensitive  insensitive  culture aware  etc      Without a full awareness of a string this cannot be accurately answered     Additionally  C C   switch statements are typically generated as branch tables   It s not nearly as easy to generate a branch table for a string style switch

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in many cases you can avid extra work by pulling the first char from the string and switching on that  may end up having to do a nested switch on charat 1  if your cases start with the same value  anyone reading your code would appreciate a hint though because most would prob just if-else-if

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C   11 update of apparently not  MarmouCorp above but http   www codeguru com cpp cpp cpp mfc article php c4067 Switch-on-Strings-in-C htm  Uses two maps to convert between the strings and the class enum  better than plain enum because its values are scoped inside it  and reverse lookup for nice error messages    The use of static in the codeguru code is possible with compiler support for initializer lists which means VS 2013 plus  gcc 4 8 1 was ok with it  not sure how much farther back it would be compatible        lt summary gt      Enum for String values we want to switch on      lt  summary gt  enum class TestType       SetType      GetType          lt summary gt      Map from strings to enum values      lt  summary gt  std  map lt std  string  TestType gt  MnCTest  s mapStringToTestType            setType   TestType  SetType           getType   TestType  GetType            lt summary gt      Map from enum values to strings      lt  summary gt  std  map lt TestType  std  string gt  MnCTest  s mapTestTypeToString        TestType  SetType   setType          TestType  GetType   getType               std  string someString    setType   TestType testType   s mapStringToTestType someString   switch  testType        case TestType  SetType          break       case TestType  GetType          break       default          LogError  Unknown TestType    s mapTestTypeToString testType

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The problem is that for reasons of optimization the switch statement in C   does not work on anything but primitive types  and you can only compare them with compile time constants   Presumably the reason for the restriction is that the compiler is able to apply some form of optimization compiling the code down to one cmp instruction and a goto where the address is computed based on the value of the argument at runtime  Since branching and and loops don t play nicely with modern CPUs  this can be an important optimization   To go around this  I am afraid you will have to resort to if statements

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C    constexpr hash function   constexpr unsigned int hash const char  s  int off   0                                return  s off    5381    hash s  off 1  33    s off                                                                                                                 switch  hash str     case hash  one        do something case hash  two        do something

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Why not  You can use switch implementation with equivalent syntax and same semantics  The C language does not have objects and strings objects at all  but strings in C is null terminated strings referenced by pointer  The C   language have possibility to make overload functions for objects comparision or checking objects equalities  As C as C   is enough flexible to have such switch for strings for C language and  for objects of any type that support comparaison or check equality for C   language  And modern C  11 allow to have this switch implementation enough effective   Your code will be like this   std  string name    Alice    std  string gender    boy   std  string role   SWITCH name    CASE  Alice     FALL   CASE  Carol     gender    girl   FALL   CASE  Bob       FALL   CASE  Dave      role      participant   BREAK   CASE  Mallory   FALL   CASE  Trudy     role      attacker      BREAK   CASE  Peggy     gender    girl   FALL   CASE  Victor    role      verifier      BREAK   DEFAULT         role      other   END     the role will be   participant     the gender will be   girl    It is possible to use more complicated types for example std  pairs or any structs or classes that support equality operations  or comarisions for quick mode    Features   any type of data which support comparisions or checking equality possibility to build cascading nested switch statemens  possibility to break or fall through case statements possibility to use non constatnt case expressions possible to enable quick static dynamic mode with tree searching  for C  11    Sintax differences with language switch is   uppercase keywords need parentheses for CASE statement semicolon     at end of statements is not allowed colon     at CASE statement is not allowed need one of BREAK or FALL keyword at end of CASE statement   For C  97 language used linear search  For C  11 and more modern possible to use quick mode wuth tree search where return statement in CASE becoming not allowed  The C language implementation exists where char  type and zero-terminated string comparisions is used   Read more about this switch implementation

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Switches only work with integral types  int  char  bool  etc     Why not use a map to pair a string with a number and then use that number with the switch

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You can t use string in switch case Only int  amp  char are allowed  Instead you can try enum for representing the string and use it in the switch case block like   enum MyString raj taj aaj     Use it int the swich case statement

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Late to the party  here s a solution I came up with some time ago  which completely abides to the requested syntax   include  lt uberswitch uberswitch hpp gt   int main         uberswitch  std  string  quot raj quot                  case   quot sda quot              break     notice the parenthesis around the value           Here s the code  https   github com falemagn uberswitch

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To add a variation using the simplest container possible  no need for an ordered map     I wouldn t bother with an enum--just put the container definition immediately before the switch so it ll be easy to see which number represents which case    This does a hashed lookup in the unordered map and uses the associated int to drive the switch statement  Should be quite fast  Note that at is used instead of     as I ve made that container const  Using    can be dangerous--if the string isn t in the map  you ll create a new mapping and may end up with undefined results or a continuously growing map   Note that the at   function will throw an exception if the string isn t in the map  So you may want to test first using count     const static std  unordered map lt std  string int gt  string to case       raj  1        ben  2     switch string to case at  raj        case 1     this is the  raj  case        break    case 2     this is the  ben  case        break        The version with a test for an undefined string follows   const static std  unordered map lt std  string int gt  string to case       raj  1        ben  2        in C  20  you can replace  count with  contains switch string to case count  raj     string to case at  raj     0      case 1     this is the  raj  case        break    case 2     this is the  ben  case        break    case 0    this is for the undefined case

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In C   and C switches only work on integer types  Use an if else ladder instead  C   could obviously have implemented some sort of swich statement for strings - I guess nobody thought it worthwhile  and I agree with them

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As mentioned previously  compilers like to build lookup tables that optimize switch statements to near O 1  timing whenever possible  Combine this with the fact that the C   Language doesn t have a string type - std  string is part of the Standard Library which is not part of the Language per se   I will offer an alternative that you might want to consider  I ve used it in the past to good effect  Instead of switching over the string itself  switch over the result of a hash function that uses the string as input  Your code will be almost as clear as switching over the string if you are using a predetermined set of strings   enum string code       eFred      eBarney      eWilma      eBetty              string code hashit  std  string const amp  inString        if  inString     Fred   return eFred      if  inString     Barney   return eBarney             void foo         switch  hashit stringValue         case eFred                  case eBarney                        There are a bunch of obvious optimizations that pretty much follow what the C compiler would do with a switch statement    funny how that happens

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In C   you can only use a switch statement on int and char

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You can use switch on strings  What you need is table of strings  check every string char   strings 4      quot Banana quot    quot Watermelon quot    quot Apple quot    quot Orange quot     unsigned get case string char  str  char    strings  unsigned n        while n                n--         if strcmp str   strings n      0  return n            return 0     unsigned index   get case string  quot Banana quot   strings  4    switch index        case 1  break   Found string  Banana        default    No string

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More functional workaround to the switch problem   class APIHandlerImpl       define map of  cases  std  map lt string  std  function lt void server   websocketpp  connection hdl  string  gt  gt  in events   public      APIHandlerImpl                    bind handler method in constructor         in events   hello     std  bind  amp APIHandlerImpl  handleHello  this   1   2   3           in events   bye     std  bind  amp APIHandlerImpl  handleBye  this   1   2   3              void onEvent string event     hello   string data                          execute event based on incomming event         in events event  s  hdl  data              void APIHandlerImpl  handleHello server  s  websocketpp  connection hdl hdl  string data                                  void APIHandlerImpl  handleBye server  s  websocketpp  connection hdl hdl  string data

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That s because C   turns switches into jump tables  It performs a trivial operation on the input data and jumps to the proper address without comparing  Since a string is not a number  but an array of numbers  C   cannot create a jump table from it   movf    INDEX W       move the index value into the W  working  register from memory addwf   PCL F         add it to the program counter  each PIC instruction is one byte                       so there is no need to perform any multiplication                         Most architectures will transform the index in some way before                        adding it to the program counter  table                     the branch table begins here with this label     goto    index zero    each of these goto instructions is an unconditional branch     goto    index one     of code     goto    index two     goto    index three  index zero       code is added here to perform whatever action is required when INDEX   zero     return  index one        code from wikipedia https   en wikipedia org wiki Branch table

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std  map   C  11 lambdas pattern without enums  unordered map for the potential amortized O 1    What is the best way to use a HashMap in C      include  lt functional gt   include  lt iostream gt   include  lt string gt   include  lt unordered map gt   include  lt vector gt   int main         int result      const std  unordered map lt std  string std  function lt void   gt  gt  m            one       amp      result   1                two       amp      result   2                three     amp      result   3                 const auto end   m end        std  vector lt std  string gt  strings  one    two    three    foobar        for  const auto amp  s   strings            auto it   m find s           if  it    end                it- gt second              else               result   -1                    std  cout  lt  lt  s  lt  lt       lt  lt  result  lt  lt  std  endl            Output   one 1 two 2 three 3 foobar -1   Usage inside methods with static  To use this pattern efficiently inside classes  initialize the lambda map statically  or else you pay O n  every time to build it from scratch   Here we can get away with the    initialization of a static method variable  Static variables in class methods   but we could also use the methods described at  static constructors in C    I need to initialize private static objects  It was necessary to transform the lambda context capture   amp   into an argument  or that would have been undefined  const static auto lambda used with capture by reference  Example that produces the same output as above    include  lt functional gt   include  lt iostream gt   include  lt string gt   include  lt unordered map gt   include  lt vector gt   class RangeSwitch   public      void method std  string key  int  amp result            static const std  unordered map lt std  string std  function lt void int amp   gt  gt  m                one        int amp  result   result   1                    two        int amp  result   result   2                    three      int amp  result   result   3                         static const auto end   m end            auto it   m find key           if  it    end                it- gt second result             else               result   -1                      int main         RangeSwitch rangeSwitch      int result      std  vector lt std  string gt  strings  one    two    three    foobar        for  const auto amp  s   strings            rangeSwitch method s  result           std  cout  lt  lt  s  lt  lt       lt  lt  result  lt  lt  std  endl

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cout  lt  lt    nEnter word to select your choice n        cout  lt  lt   ex to exit program  0  n            cout  lt  lt   m     to set month 1  n       cout  lt  lt   y     to set year 2  n       cout  lt  lt   rm     to return the month 4  n       cout  lt  lt   ry     to return year 5  n       cout  lt  lt   pc     to print the calendar for a month 6  n       cout  lt  lt   fdc      to print the first day of the month 1  n       cin  gt  gt  c      cout  lt  lt  endl      a   c compare  ex    c compare  m    c compare  y     c compare  rm   c compare  ry     c compare  pc     c compare  fdc     7   6    5    4   3   2   1   0      switch  a                case 0              return 1           case 1                       m                       cout  lt  lt   enter month n               cin  gt  gt  c              cout  lt  lt  endl              myCalendar setMonth c               break                    case 2              cout  lt  lt   Enter year yyyy  n               cin  gt  gt  y              cout  lt  lt  endl              myCalendar setYear y               break          case 3               myCalendar getMonth                break          case 4              myCalendar getYear            case 5              cout  lt  lt   Enter month and year n               cin  gt  gt  c  gt  gt  y              cout  lt  lt  endl              myCalendar almanaq c y               break          case 6              break

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I think the reason is that in C strings are not primitive types  as tomjen said  think in a string as a char array  so you can not do things like   switch  char             switch  int

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