Trim a string in C

Question

Briefly    I m after the equivalent of  NET s String Trim in C using the win32 and standard C api  compiling with MSVC2008 so I have access to all the C   stuff if needed  but I am just trying to trim a char      Given that there is strchr  strtok  and all manner of other string functions  surely there should be a trim function  or one that can be repurposed     Thanks

User · Answer

Surprised to see such implementations  I usually do trim like this   char  trim char  s        char  ptr      if   s          return NULL       handle NULL string     if    s          return s          handle empty string     for  ptr   s   strlen s  - 1   ptr  gt   s   amp  amp  isspace  ptr   --ptr       ptr 1      0       return s      It is fast and reliable - serves me many years

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void ltrim char str PATH MAX             int i   0  j   0          char buf PATH MAX           strcpy buf  str           for  str i         i              for  str i       0  i   j                    buf j    str i           buf j      0           strcpy str  buf

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Here s my implementation  behaving like the built-in string functions in libc  that is  it expects a c-string  it modifies it and returns it to the caller    It trims leading spaces  amp  shifts the remaining chars to the left  as it parses the string from left to right  It then marks a new end of string and starts parsing it backwards  replacing trailing spaces with   0 s until it finds either a non-space char or the start of the string  I believe those are the minimum possible iterations for this particular task      ----------------------------------------------------------------------------    trim leading  amp  trailing spaces from string s  return modified string s     alg     - skip leading spaces  via cp1    - shift remaining  cp1 s to the left  via cp2    - mark a new end of string    - replace trailing spaces with   0   via cp2    - return the trimmed s    char  s trim char  s        char  cp1                                  for parsing the whole s     char  cp2                                  for shifting  amp  padding         skip leading spaces  shift remaining chars     for  cp1 s  isspace  cp1   cp1             skip leading spaces  via cp1               for  cp2 s   cp1  cp1    cp2               shift left remaining chars  via cp2          cp2    cp1       cp2--   0                                 mark new end of string for s         replace trailing spaces with   0      while   cp2  gt  s  amp  amp  isspace  cp2             cp2--   0                             pad with   0 s      return s

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This made me want to write my own - I didn t like the ones that had been provided  Seems to me there should be 3 functions   char  ltrim char  s        while isspace  s   s        return s     char  rtrim char  s        char  back   s   strlen s       while isspace  --back          back 1      0       return s     char  trim char  s        return rtrim ltrim s

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static inline void ut trim char   str       char   start   str     char   end   start   strlen str       while  --end  gt   start         trim right          if   isspace  end            break              end      0       while  isspace  start         trim left          start        if  start    str              there is a string          memmove str  start  end - start   1

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void inPlaceStrTrim char  str        int k   0      int i   0      for  i 0  str i       0              if  isspace str i                     we have got a space                k   i              for  int j i  j lt strlen str -1  j                      str j    str j 1                             str strlen str -1      0               i   k     start the loop again where we ended             else               i

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There is no standard library function to do this  but it s not too hard to roll your own  There is an existing question on SO about doing this that was answered with source code

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I like it when the return value always equals the argument  This way  if the string array has been allocated with malloc    it can safely be free   again      Remove leading whitespaces    char  ltrim char  const s            size t len          char  cur           if s     s                    len   strlen s                   cur   s                   while  cur    isspace  cur                             cur  --len                   if s    cur                          memmove s  cur  len   1                       return s        Remove trailing whitespaces    char  rtrim char  const s            size t len          char  cur           if s     s                    len   strlen s                   cur   s   len - 1                   while cur    s    isspace  cur                           --cur  --len                   cur isspace  cur    0   1      0                      return s        Remove leading and trailing whitespaces    char  trim char  const s            rtrim s       order matters         ltrim s            return s

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You can use the standard isspace   function in ctype h to achieve this  Simply compare the beginning and end characters of your character array until both ends no longer have spaces   quot spaces quot  include          0x20   space  SPC    t    0x09   horizontal tab  TAB    n    0x0a   newline  LF    v    0x0b   vertical tab  VT    f    0x0c   feed  FF    r    0x0d   carriage return  CR   although there is no function which will do all of the work for you  you will have to roll your own solution to compare each side of the given character array repeatedly until no spaces remain  Edit  Since you have access to C    Boost has a trim implementation waiting for you to make your life a lot easier

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Not the best way but it works  char  Trim char  str        int len   strlen str       char  buff   new char len       int i   0      memset buff 0 len sizeof char        do          if isspace  str   continue          buff i     str    i        while     str       0        return buff

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How about this    It only requires one iteration over the string  doesn t use strlen  which iterates over the string   When the function returns you get a pointer to the start of the trimmed string which is null terminated  The string is trimmed of spaces from the left  until the first character is found   The string is also trimmed of all trailing spaces after the last nonspace character   char  trim char  input        char  start   input      while  isSpace  start       trim left         start               char  ptr   start      char  end   start      while   ptr        0       trim right         if   isSpace  ptr       only move end pointer if char isn t a space             end   ptr                        end     0     terminate the trimmed string with a null     return start     bool isSpace char c        switch  c            case              case   n           case   t           case   f           case   r               return true              break          default              return false              break

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iMode 0 ALL  1 Left  2 Right   char  Trim char  szStr const char ch  int iMode        if  szStr    NULL          return NULL      char szTmp 1024 10      0x00        strcpy szTmp  szStr       int iLen   strlen szTmp       char  pStart   szTmp      char  pEnd   szTmp iLen      int i      for i   0 i  lt  iLen i             if  szTmp i     ch  amp  amp  pStart    szTmp i  amp  amp  iMode    2                pStart          if  szTmp iLen-i-1     ch  amp  amp  pEnd    szTmp iLen-i  amp  amp  iMode    1                --pEnd      0             strcpy szStr  pStart       return szStr

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include  stdafx h   include  lt string h gt   include  lt ctype h gt   char  trim char  input     int  tmain int argc   TCHAR  argv          char sz1      MQRFH         char sz2     MQRFH       char sz3      MQR FH       char sz4    MQRFH         char sz5    MQRFH       char sz6    M       char sz7    M        char sz8     M       char sz9           char sz10                    printf  sz1   s   d n  trim sz1   strlen sz1        printf  sz2   s   d n  trim sz2   strlen sz2        printf  sz3   s   d n  trim sz3   strlen sz3        printf  sz4   s   d n  trim sz4   strlen sz4        printf  sz5   s   d n  trim sz5   strlen sz5        printf  sz6   s   d n  trim sz6   strlen sz6        printf  sz7   s   d n  trim sz7   strlen sz7        printf  sz8   s   d n  trim sz8   strlen sz8        printf  sz9   s   d n  trim sz9   strlen sz9        printf  sz10   s   d n  trim sz10   strlen sz10         return 0     char  ltrim char  s              while isspace  s   s             return s        char  rtrim char  s              char  back      int len   strlen s        if len    0          return s         back   s   len           while isspace  --back               back 1      0            return s        char  trim char  s              return rtrim ltrim s             Output   sz1  MQRFH  9 sz2  MQRFH  6 sz3  MQR FH  8 sz4  MQRFH  7 sz5  MQRFH  5 sz6  M  1 sz7  M  2 sz8  M  2 sz9    0 sz10    8

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Easiest thing to do is a simple loop   I m going to assume that you want the trimmed string returned in place   char   strTrim char   s       int ix  jx      int len       char   buf      len   strlen s       possibly should use strnlen        buf    char    malloc strlen s  1        for ix 0  jx 0  ix  lt  len  ix            if  isspace s ix              buf jx      s ix        buf jx      0       strncpy s  buf  jx       always looks as far as the null  but who cares         free buf                 no good leak goes unpunished        return s                 modifies s in place  and  returns it for swank         This gets rid of embedded blanks too  if String Trim doesn t then it needs a bit more logic

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Function to remove white spaces on both sides of a string i e trim     void trim  char  s        int i       while  isspace   s   s         skip left side white spaces     for  i   strlen  s  - 1   isspace  s i     i--         skip right side white spaces     s i   1      0       printf    s n   s

[c] Trim a string in C

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