How can I compare strings in C using a switch statement

Question

In C there is a switch construct which enables one to execute different conditional branches of code based on an test integer value  e g   int a     Read the value of  quot a quot  from some source  e g  user input    switch  a      case 100         Code     break    case 200         Code     break    default         Code     break     How is it possible to obtain the same behavior  i e  avoid the so-called  quot if-else ladder quot   for a string value  i e   a char

User · Answer

If you mean  how to write something similar to this      switch statement switch  string      case  B1           do something     break       more case  xxx  parts        Then the canonical solution in C is to use an if-else ladder   if  strcmp string   B1      0          do something    else if  strcmp string   xxx      0         do something else      more else if clauses    else    default

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To add to Phimueme s answer above  if your string is always two characters  then you can build a 16-bit int out of the two 8-bit characters - and switch on that  to avoid nested switch case statements

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If it is a 2 byte string you can do something like in this concrete example where I switch on ISO639-2 language codes       LANIDX TYPE LanCodeToIdx const char  Lan              if Lan          switch Lan 0               case  A     switch Lan 1                             case  N   return LANIDX AN                          case  R   return LANIDX AR                                                break            case  B     switch Lan 1                             case  E   return LANIDX BE                          case  G   return LANIDX BG                          case  N   return LANIDX BN                          case  R   return LANIDX BR                          case  S   return LANIDX BS                                                break            case  C     switch Lan 1                             case  A   return LANIDX CA                          case  C   return LANIDX CO                          case  S   return LANIDX CS                          case  Y   return LANIDX CY                                                break            case  D     switch Lan 1                             case  A   return LANIDX DA                          case  E   return LANIDX DE                                                break            case  E     switch Lan 1                             case  L   return LANIDX EL                          case  N   return LANIDX EN                          case  O   return LANIDX EO                          case  S   return LANIDX ES                          case  T   return LANIDX ET                          case  U   return LANIDX EU                                                break            case  F     switch Lan 1                             case  A   return LANIDX FA                          case  I   return LANIDX FI                          case  O   return LANIDX FO                          case  R   return LANIDX FR                          case  Y   return LANIDX FY                                                break            case  G     switch Lan 1                             case  A   return LANIDX GA                          case  D   return LANIDX GD                          case  L   return LANIDX GL                          case  V   return LANIDX GV                                                break            case  H     switch Lan 1                             case  E   return LANIDX HE                          case  I   return LANIDX HI                          case  R   return LANIDX HR                          case  U   return LANIDX HU                                                break            case  I     switch Lan 1                             case  S   return LANIDX IS                          case  T   return LANIDX IT                                                break            case  J     switch Lan 1                             case  A   return LANIDX JA                                                break            case  K     switch Lan 1                             case  O   return LANIDX KO                                                break            case  L     switch Lan 1                             case  A   return LANIDX LA                          case  B   return LANIDX LB                          case  I   return LANIDX LI                          case  T   return LANIDX LT                          case  V   return LANIDX LV                                                break            case  M     switch Lan 1                             case  K   return LANIDX MK                          case  T   return LANIDX MT                                                break            case  N     switch Lan 1                             case  L   return LANIDX NL                          case  O   return LANIDX NO                                                break            case  O     switch Lan 1                             case  C   return LANIDX OC                                                break            case  P     switch Lan 1                             case  L   return LANIDX PL                          case  T   return LANIDX PT                                                break            case  R     switch Lan 1                             case  M   return LANIDX RM                          case  O   return LANIDX RO                          case  U   return LANIDX RU                                                break            case  S     switch Lan 1                             case  C   return LANIDX SC                          case  K   return LANIDX SK                          case  L   return LANIDX SL                          case  Q   return LANIDX SQ                          case  R   return LANIDX SR                          case  V   return LANIDX SV                          case  W   return LANIDX SW                                                break            case  T     switch Lan 1                             case  R   return LANIDX TR                                                break            case  U     switch Lan 1                             case  K   return LANIDX UK                          case  N   return LANIDX UN                                                break            case  W     switch Lan 1                             case  A   return LANIDX WA                                                break            case  Z     switch Lan 1                             case  H   return LANIDX ZH                                                break                  return LANIDX UNDEFINED          LANIDX   being constant integers used to index in arrays

User · Answer

If you have many cases and do not want to write a ton of strcmp   calls  you could do something like   switch my hash function the string         case HASH B1                etc           You just have to make sure your hash function has no collisions inside the set of possible values for the string

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My preferred method for doing this is via a hash function  borrowed from here    This allows you to utilize the efficiency of a switch statement even when working with char   s    include  stdio h    define LS 5863588  define CD 5863276  define MKDIR 210720772860  define PWD 193502992  const unsigned long hash const char  str        unsigned long hash   5381        int c       while   c    str             hash     hash  lt  lt  5    hash    c      return hash     int main int argc  char  argv          char  p command   argv 1       switch hash p command         case LS          printf  Running ls    n            break      case CD          printf  Running cd    n            break      case MKDIR          printf  Running mkdir    n            break      case PWD          printf  Running pwd    n            break      default          printf   ERROR    s  is not a valid command  n   p command             Of course  this approach requires that the hash values for all possible accepted char   s are calculated in advance   I don t think this is too much of an issue  however  since the switch statement operates on fixed values regardless   A simple program can be made to pass char   s through the hash function and output their results   These results can then be defined via macros as I have done above

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We cannot escape if-else ladder in order to compare a string with others  Even regular switch-case is also an if-else ladder  for integers  internally  We might only want to simulate the switch-case for string  but can never replace if-else ladder  The best of the algorithms for string comparison cannot escape from using strcmp function  Means to compare character by character until an unmatch is found  So using if-else ladder and strcmp are inevitable   DEMO  And here are simplest macros to simulate the switch-case for strings    ifndef SWITCH CASE INIT  define SWITCH CASE INIT      define SWITCH X    for  char    switch p     X  int   switch next   1     switch p       switch p   0    switch next   1           define CASE X            if     switch next          switch next     strcmp   switch p    X           define DEFAULT                  define END             endif   And you can use them as  char  str    def    SWITCH  str      CASE   abc           printf   in abc n            break      CASE   def                   Notice   break   statement missing so the control rolls through subsequent CASE s until DEFAULT          printf  in def n        CASE   ghi           printf   in ghi n        DEFAULT         printf  in DEFAULT n    END   Output   in def in ghi in DEFAULT   Below is nested SWITCH usage   char  str    def   char  str1    xyz    SWITCH  str      CASE   abc           printf   in abc n            break      CASE   def                                           printf  in def n            SWITCH  str1                                lt    Notice  Nested SWITCH             CASE   uvw                   printf  in def   gt  uvw n                    break              CASE   xyz                   printf  in def   gt  xyz n                    break              DEFAULT                 printf  in def   gt  DEFAULT n            END     CASE   ghi           printf   in ghi n        DEFAULT         printf  in DEFAULT n    END   Output   in def in def   gt  xyz in ghi in DEFAULT   Here is reverse string SWITCH  where in you can use a variable  rather than a constant  in CASE clause   char  str2    def   char  str3    ghi    SWITCH   ghi                            lt    Notice  Use of variables and reverse string SWITCH      CASE  str1          printf   in str1 n            break      CASE  str2                               printf   in str2 n            break      CASE  str3                               printf   in str3 n            break      DEFAULT         printf  in DEFAULT n    END   Output   in str3

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This is how you do it  No  not really    include  lt stdio h gt   include  lt string h gt   include  lt assert h gt   include  lt stdint h gt      define p ntohl u    const uint32 t Q 0xFF000000                               uint32 t S  uint32 t  u                                     uint8 t   amp Q  S                                             S lt  lt 24                                                       S lt  lt 8  amp 0x00FF0000                                           S gt  gt 8  amp 0x0000FF00                                           S gt  gt 24  amp 0xFF          main  void        uint32 t s 0x40        assert  unsigned char 1     unsigned char  257        memset s  0  sizeof s        fgets  char  s  sizeof s   stdin        switch  p ntohl s 0              case  open           case  read           case  seek               puts  ok                break          case  rm n 0               puts  not authorized                break          default              puts  unrecognized command                return 0

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This is generally how I do it   void order plane const char  p        switch    p    256     p 1                 case 0x4231      B1                         printf  Yes  order this bomber   It s a blast  n               break                     case 0x5354      ST                          printf  Nah   I just can t see this one  n                break                     default                         printf  Not today   Can I interest you in a crate of SAMs  n

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I have published a header file to perform the switch on the strings in C  It contains a set of macro that hide the call to the strcmp    or similar  in order to mimic a switch-like behaviour  I have tested it only with GCC in Linux  but I m quite sure that it can be adapted to support other environment   EDIT  added the code here  as requested  This is the header file you should include    ifndef   SWITCHS H    define   SWITCHS H     include  lt string h gt   include  lt regex h gt   include  lt stdbool h gt       Begin a switch for the string x     define switchs x          char    sw    x   bool   done   false  bool   cont   false            regex t   regex  regcomp  amp   regex        0   do        Check if the string matches the cases argument  case sensitive      define cases x       if     cont     strcmp     sw  x                                   done   true    cont   true       Check if the string matches the icases argument  case insensitive      define icases x       if     cont     strcasecmp     sw  x                                   done   true    cont   true       Check if the string matches the specified regular expression using regcomp 3      define cases re x flags    regfree    amp   regex    if     cont                                      0    regcomp    amp   regex  x  flags    amp  amp                                  0    regexec    amp   regex    sw  0  NULL  0                                             done   true    cont   true       Default behaviour     define defaults      if      done      cont          Close the switchs     define switchs end   while   0    regfree  amp   regex       endif      SWITCHS H     And this is how you use it   switchs argv 1         cases  foo       cases  bar           printf  foo or bar  case sensitive  n            break       icases  pi           printf  pi or Pi or pI or PI  case insensitive  n            break       cases re   D    0          printf  Something that start with D  case sensitive  n            break       cases re   E    REG ICASE          printf  Something that start with E  case insensitive  n            break       cases  1           printf  1 n               break omitted on purpose      cases  2           printf  2  or 1  n            break       defaults         printf  No match n            break    switchs end

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There is no way to do this in C   There are a lot of different approaches   Typically the simplest is to define a set of constants that represent your strings and do a look up by string on to get the constant    define BADKEY -1  define A1 1  define A2 2  define B1 3  define B2 4  typedef struct   char  key  int val    t symstruct   static t symstruct lookuptable              A1   A1       A2   A2       B1   B1       B2   B2        define NKEYS  sizeof lookuptable  sizeof t symstruct    int keyfromstring char  key        int i      for  i 0  i  lt  NKEYS  i              t symstruct  sym   lookuptable i           if  strcmp sym- gt key  key     0              return sym- gt val            return BADKEY               switch  keyfromstring somestring     case A1            break  case A2            break  case B1            break  case B2            break  case BADKEY     handle failed lookup        There are  of course  more efficient ways to do this   If you keep your keys sorted  you can use a binary search   You could use a hashtable too   These things change your performance at the expense of maintenance

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There is a way to perform the string search faster  Assumptions  since we are talking about a switch statement  I can assume that the values won t be changing during runtime    The idea is to use the C stdlib s qsort and bsearch   I ll be working on xtofl s code   struct stringcase   char  string  void   func  void       void funcB1    void funcAzA     struct stringcase cases            B1   funcB1        AzA   funcAzA       struct stringcase work cases    NULL  int work cases cnt   0      prepare the data for searching void prepare          allocate the work cases and copy cases values from it to work cases   qsort  cases  i  sizeof  struct stringcase    stringcase cmp          comparator function int stringcase cmp  const void  p1  const void  p2       return strcasecmp    struct stringcase  p1 - gt string    struct stringcase  p2 - gt string         perform the switching void myswitch  char  token       struct stringcase val    val string token    void  strptr   bsearch   amp val  work cases  work cases cnt  sizeof  struct stringcase   stringcase cmp      if  strptr        struct stringcase  foundVal    struct stringcase  strptr        foundVal- gt func         return OK        return NOT FOUND

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Hi this is the easy and fast way if you have this case     QUICK Mode   int concated  char ABC 4     int a 1 b 4 c 2               char   Initializing ABC lt -sprintf ABC   d d d  a b c              without space between  d d d printf   s  ABC                              value as char   is  142 concated atoi ABC                            result is 142 as int  not 1 4 2  separeted     now use switch case on 142 as an integer and all possible cases    EXPLAINED Mode   for example   i have many menus  each choice on the 1st menu takes u to the 2nd menu  the same thing with the 2nd menu and the 3rd menu but the Options are diferent so you know that the user has choised finnaly  exemple    menu 1   1     menu 2   4    menu 3  2      the choice is 142  other cases   111 141 131 122     sollution  store the first 1st in a 2nd in b 3rd on c    a 1  b 4  c 2   char ABC 4       ABC lt -sprintf ABC   d d d  a b c                  without space between  d d d  printf   s  ABC                                  value as char   142          now you want to recover your value 142  from char   to int as  int value 142   concated atoi ABC                                result is 142 as int  not 1 4 2  separeted

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I think the best way to do this is separate the  recognition  from the functionality   struct stringcase   char  string  void   func  void       void funcB1    void funcAzA     stringcase cases            B1   funcB1        AzA   funcAzA       void myswitch  char  token       for  stringcases  pCase   cases        pCase    cases   sizeof  cases     sizeof  cases 0           pCase             if  0    strcmp  pCase- gt string  token                pCase- gt func            break

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Assuming little endianness and sizeof char     1  you could do that  something like this was suggested by MikeBrom    char  txt    B1   int tst     int  txt  if   tst  amp  0x00FFFFFF      1B       printf  B1  n      It could be generalized for BE case

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Function pointers are a great way to do this  e g   result   switchFunction someStringKey     result is an optional return value     this calls a function that you have set by string key  one function per case    setSwitchFunction  foo   fooFunc   setSwitchFunction  bar   barFunc     Use a pre-existing hashmap table dictionary implementation such as khash  return that pointer to a function inside of switchFunction    and execute it  or just return it from switchFunction   and execute it yourself   If the map implementation doesn t store that  just use a uint64 t instead that you cast accordingly to a pointer

[c] How can I compare strings in C using a `switch` statement?

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