Case Insensitive String comp in C

Question

I have two postcodes char  that I want to compare  ignoring case  Is there a function to do this   Or do I have to loop through each use the tolower function and then do the comparison   Any idea how this function will react with numbers in the string  Thanks

User · Answer

There is no function that does this in the C standard. Unix systems that comply with POSIX are required to have strcasecmp in the header strings.h; Microsoft systems have stricmp. To be on the portable side, write your own:

int strcicmp(char const *a, char const *b)
{
    for (;; a++, b++) {
        int d = tolower((unsigned char)*a) - tolower((unsigned char)*b);
        if (d != 0 || !*a)
            return d;
    }
}

But note that none of these solutions will work with UTF-8 strings, only ASCII ones.

User · Answer

I m not really a fan of the most-upvoted answer here  in part because it seems like it isn t correct since it should continue if it reads a null terminator in either string--but not both strings at once--and it doesn t do this   so I wrote my own  This is a direct drop-in replacement for strncmp    and has been tested with numerous test cases  as shown below  It is identical to strncmp   except   It is case-insensitive  The behavior is NOT undefined  it is well-defined  if either string is a null ptr  Regular strncmp   has undefined behavior if either string is a null ptr  see  https   en cppreference com w cpp string byte strncmp   It returns INT MIN as a special sentinel error value if either input string is a NULL ptr   LIMITATIONS  Note that this code works on the original 7-bit ASCII character set only  decimal values 0 to 127  inclusive   NOT on unicode characters  such as unicode character encodings UTF-8  the most popular   UTF-16  and UTF-32  Here is the code only  no comments   int strncmpci const char   str1  const char   str2  size t num        int ret code   0      size t chars compared   0       if   str1     str2                ret code   INT MIN          return ret code             while   chars compared  lt  num   amp  amp    str1     str2                 ret code   tolower  int   str1   - tolower  int   str2            if  ret code    0                        break                    chars compared            str1            str2               return ret code     Fully-commented version       brief      Perform a case-insensitive string compare   strncmp    case-insensitive  to see                 if two C-strings are equal       note       1  Identical to  strncmp    except                    1  It is case-insensitive                    2  The behavior is NOT undefined  it is well-defined  if either string is a null                   ptr  Regular  strncmp    has undefined behavior if either string is a null ptr                    see  https   en cppreference com w cpp string byte strncmp                     3  It returns  INT MIN  as a special sentinel value for certain errors                  - Posted as an answer here  https   stackoverflow com a 55293507 4561887                    - Aided inspired  in part  by  strcicmp    here                      https   stackoverflow com a 5820991 4561887       param in   str1        C string 1 to be compared       param in   str2        C string 2 to be compared       param in   num         max number of chars to compare      return     A comparison code  identical to  strncmp     except with the addition                 of  INT MIN  as a special sentinel value                       INT MIN  usually -2147483648 for int32 t integers   Invalid arguments  one or both                          of the input strings is a NULL pointer                    lt 0       The first character that does not match has a lower value in str1 than                          in str2                   0       The contents of both strings are equal                   gt 0       The first character that does not match has a greater value in str1 than                          in str2  int strncmpci const char   str1  const char   str2  size t num        int ret code   0      size t chars compared   0          Check for NULL pointers     if   str1     str2                ret code   INT MIN          return ret code                Continue doing case-insensitive comparisons  one-character-at-a-time  of  str1  to  str2   so        long as 1st  we have not yet compared the requested number of chars  and 2nd  the next char        of at least  one  of the strings is not zero  the null terminator for a C-string   meaning        that string still has more characters in it         Note  you MUST check   chars compared  lt  num   FIRST or else dereferencing  reading   str1  or         str2  via   str1  and   str2   respectively  is undefined behavior if you are reading one or        both of these C-strings outside of their array bounds      while   chars compared  lt  num   amp  amp    str1     str2                 ret code   tolower  int   str1   - tolower  int   str2            if  ret code    0                           The 2 chars just compared don t match             break                    chars compared            str1            str2               return ret code     Test code  Download the entire sample code  with unit tests  from my eRCaGuy hello world repository here   quot strncmpci c quot    this is just a snippet  int main         printf  quot ----------------------- n quot              quot String Comparison Tests n quot              quot ----------------------- n n quot         int num failures expected   0       printf  quot INTENTIONAL UNIT TEST FAILURE to show what a unit test failure looks like  n quot        EXPECT EQUALS strncmpci  quot hey quot    quot HEY quot   3    h  -  H        num failures expected        printf  quot ------ beginning ------ n n quot          const char   str1      const char   str2      size t n          NULL ptr checks     EXPECT EQUALS strncmpci NULL   quot  quot   0   INT MIN       EXPECT EQUALS strncmpci  quot  quot   NULL  0   INT MIN       EXPECT EQUALS strncmpci NULL  NULL  0   INT MIN       EXPECT EQUALS strncmpci NULL   quot  quot   10   INT MIN       EXPECT EQUALS strncmpci  quot  quot   NULL  10   INT MIN       EXPECT EQUALS strncmpci NULL  NULL  10   INT MIN        EXPECT EQUALS strncmpci  quot  quot    quot  quot   0   0       EXPECT EQUALS strncmp  quot  quot    quot  quot   0   0        str1    quot  quot       str2    quot  quot       n   0      EXPECT EQUALS strncmpci str1  str2  n   0       EXPECT EQUALS strncmp str1  str2  n   0        str1    quot hey quot       str2    quot HEY quot       n   0      EXPECT EQUALS strncmpci str1  str2  n   0       EXPECT EQUALS strncmp str1  str2  n   0        str1    quot hey quot       str2    quot HEY quot       n   3      EXPECT EQUALS strncmpci str1  str2  n   0       EXPECT EQUALS strncmp str1  str2  n    h  -  H         str1    quot heY quot       str2    quot HeY quot       n   3      EXPECT EQUALS strncmpci str1  str2  n   0       EXPECT EQUALS strncmp str1  str2  n    h  -  H         str1    quot hey quot       str2    quot HEdY quot       n   3      EXPECT EQUALS strncmpci str1  str2  n    y  -  d        EXPECT EQUALS strncmp str1  str2  n    h  -  H         str1    quot heY quot       str2    quot hEYd quot       n   3      EXPECT EQUALS strncmpci str1  str2  n   0       EXPECT EQUALS strncmp str1  str2  n    e  -  E         str1    quot heY quot       str2    quot heyd quot       n   6      EXPECT EQUALS strncmpci str1  str2  n   - d        EXPECT EQUALS strncmp str1  str2  n    Y  -  y         str1    quot hey quot       str2    quot hey quot       n   6      EXPECT EQUALS strncmpci str1  str2  n   0       EXPECT EQUALS strncmp str1  str2  n   0        str1    quot hey quot       str2    quot heyd quot       n   6      EXPECT EQUALS strncmpci str1  str2  n   - d        EXPECT EQUALS strncmp str1  str2  n   - d         str1    quot hey quot       str2    quot heyd quot       n   3      EXPECT EQUALS strncmpci str1  str2  n   0       EXPECT EQUALS strncmp str1  str2  n   0        str1    quot hEY quot       str2    quot heyYOU quot       n   3      EXPECT EQUALS strncmpci str1  str2  n   0       EXPECT EQUALS strncmp str1  str2  n    E  -  e         str1    quot hEY quot       str2    quot heyYOU quot       n   10      EXPECT EQUALS strncmpci str1  str2  n   - y        EXPECT EQUALS strncmp str1  str2  n    E  -  e         str1    quot hEYHowAre quot       str2    quot heyYOU quot       n   10      EXPECT EQUALS strncmpci str1  str2  n    h  -  y        EXPECT EQUALS strncmp str1  str2  n    E  -  e         EXPECT EQUALS strncmpci  quot nice to meet you    quot    quot NICE TO MEET YOU    quot   100   0       EXPECT EQUALS strncmp    quot nice to meet you    quot    quot NICE TO MEET YOU    quot   100    n  -  N        EXPECT EQUALS strncmp    quot nice to meet you    quot    quot nice to meet you    quot   100   0        EXPECT EQUALS strncmpci  quot nice to meet you    quot    quot NICE TO UEET YOU    quot   100    m  -  u        EXPECT EQUALS strncmp    quot nice to meet you    quot    quot nice to uEET YOU    quot   100    m  -  u        EXPECT EQUALS strncmp    quot nice to meet you    quot    quot nice to UEET YOU    quot   100    m  -  U         EXPECT EQUALS strncmpci  quot nice to meet you    quot    quot NICE TO MEET YOU    quot   5   0       EXPECT EQUALS strncmp    quot nice to meet you    quot    quot NICE TO MEET YOU    quot   5    n  -  N         EXPECT EQUALS strncmpci  quot nice to meet you    quot    quot NICE eo UEET YOU    quot   5   0       EXPECT EQUALS strncmp    quot nice to meet you    quot    quot nice eo uEET YOU    quot   5   0        EXPECT EQUALS strncmpci  quot nice to meet you    quot    quot NICE eo UEET YOU    quot   100    t  -  e        EXPECT EQUALS strncmp    quot nice to meet you    quot    quot nice eo uEET YOU    quot   100    t  -  e         EXPECT EQUALS strncmpci  quot nice to meet you    quot    quot nice-eo UEET YOU    quot   5       -  -        EXPECT EQUALS strncmp    quot nice to meet you    quot    quot nice-eo UEET YOU    quot   5       -  -          if  globals error count    num failures expected                printf ANSI COLOR GRN  quot All unit tests passed  quot  ANSI COLOR OFF  quot  n quot              else               printf ANSI COLOR RED  quot FAILED UNIT TESTS  NUMBER OF UNEXPECTED FAILURES    i quot              ANSI COLOR OFF  quot  n quot   globals error count - num failures expected              assert globals error count    num failures expected       return globals error count     Sample output     gcc -Wall -Wextra -Werror -ggdb -std c11 -o   bin tmp strncmpci c  amp  amp    bin tmp ----------------------- String Comparison Tests -----------------------  INTENTIONAL UNIT TEST FAILURE to show what a unit test failure looks like  FAILED at line 250 in function main  strncmpci  quot hey quot    quot HEY quot   3      h  -  H    a  strncmpci  quot hey quot    quot HEY quot   3  is 0   b   h  -  H  is 32  ------ beginning ------  All unit tests passed    References   This question  amp  other answers here served as inspiration and gave some insight  Case Insensitive String comp in C  http   www cplusplus com reference cstring strncmp  https   en wikipedia org wiki ASCII https   en cppreference com w c language operator precedence Undefined Behavior research I did to fix part of my code above  see comments below    Google search for  quot c undefined behavior reading outside array bounds quot  Is accessing a global array outside its bound undefined behavior  https   en cppreference com w cpp language ub - see also the many really great  quot External links quot  at the bottom  1 3  http   blog llvm org 2011 05 what-every-c-programmer-should-know html 2 3  https   blog llvm org 2011 05 what-every-c-programmer-should-know 14 html 3 3  https   blog llvm org 2011 05 what-every-c-programmer-should-know 21 html https   blog regehr org archives 213 https   www geeksforgeeks org accessing-array-bounds-ccpp     Topics to further research   Note  this is C    not C  Lowercase of Unicode character tolower tests c on OnlineGDB  https   onlinegdb com HyZieXcew  TODO   Make a version of this code which also works on Unicode s UTF-8 implementation  character encoding

User · Answer

I ve found built-in such method named from  which contains additional string functions to the standard header     Here s the relevant signatures     int  strcasecmp const char    const char     int  strncasecmp const char    const char    size t     I also found it s synonym in xnu kernel  osfmk device subrs c  and it s implemented in the following code  so you wouldn t expect to have any change of behavior in number compared to the original strcmp function     tolower unsigned char ch        if  ch  gt    A   amp  amp  ch  lt    Z           ch    a     ch -  A        return ch      int strcasecmp const char  s1  const char  s2        const unsigned char  us1    const u char   s1                           us2    const u char   s2       while  tolower  us1     tolower  us2             if   us1        0               return  0       return  tolower  us1  - tolower  --us2

User · Answer

Take a look at strcasecmp   in strings h

User · Answer

static int ignoreCaseComp  const char  str1  const char  str2  int length        int k      for  k   0  k  lt  length  k                   if   str1 k    32      str2 k    32               break             if  k    length          return 1      return 0      Reference

User · Answer

I would use stricmp    It compares two strings without regard to case   Note that  in some cases  converting the string to lower case can be faster

User · Answer

int strcmpInsensitive char  a  char  b        return strcmp lowerCaseWord a   lowerCaseWord b       char  lowerCaseWord char  a        char  b new char strlen a        for  int i   0  i  lt  strlen a   i                  b i    tolower a i                 return b      good luck  Edit-lowerCaseWord function get a char  variable with  and return the lower case value of this char   For example   AbCdE  for value of char   will return  abcde    Basically what it does is to take the two char  variables  after being transferred to lower case  and make use the strcmp function on them   For example- if we call the strcmpInsensitive function for values of  AbCdE   and  ABCDE   it will first return both values in lower case   abcde    and then do strcmp function on them

User · Answer

Simple solution   int str case ins cmp const char  a  const char  b      int rc     while  1        rc   tolower  unsigned char  a  - tolower  unsigned char  b       if  rc      a          break               a        b         return rc

User · Answer

Additional pitfalls to watch out for when doing case insensitive compares   Comparing as lower or as upper case   common enough issue  Both below will return 0 with strcicmpL  quot A quot    quot a quot   and strcicmpU  quot A quot    quot a quot    Yet strcicmpL  quot A quot    quot   quot   and strcicmpU  quot A quot    quot   quot   can return different signed results as     is often between the upper and lower case letters  This affects the sort order when used with qsort                strcicmp    Non-standard library C functions like the commonly available  stricmp   or strcasecmp   tend to be well defined and favor comparing via lowercase   Yet variations exist  int strcicmpL char const  a  char const  b      while   b        int d   tolower  a  - tolower  b       if  d            return d             a        b           return tolower  a      int strcicmpU char const  a  char const  b      while   b        int d   toupper  a  - toupper  b       if  d            return d             a        b           return toupper  a       char can have a negative value   not rare  touppper int  and tolower int  are specified for unsigned char values and the negative EOF    Further  strcmp   returns results as if each char was converted to unsigned char  regardless if char is signed or unsigned  tolower  a      Potential UB tolower  unsigned char   a      Correct  Almost - see following    char can have a negative value and not 2 s complement   rare  The above does not handle -0 nor other negative values properly as the bit pattern should be interpreted as unsigned char   To properly handle all integer encodings  change the pointer type first     tolower  unsigned char   a   tolower   const unsigned char   a      Correct   Locale  less common  Although character sets using ASCII code  0-127  are ubiquitous  the remainder codes tend to have locale specific issues   So strcasecmp  quot  xE4 quot    quot a quot   might return a 0 on one system and non-zero on another   Unicode  the way of the future  If a solution needs to handle more than ASCII consider a unicode strcicmp     As C lib does not provide such a function  a pre-coded function from some alternate library is recommended   Writing your own  unicode strcicmp   is a daunting task   Do all letters map one lower to one upper   pedantic   A-Z  maps one-to-one with  a-z   yet various locales map various lower case chracters to one upper and visa-versa   Further  some uppercase characters may lack a lower case equivalent and again  visa-versa  This obliges code to covert through both tolower   and tolower    int d   tolower toupper  a   - tolower toupper  b     Again  potential different results for sorting if code did tolower toupper  a   vs  toupper tolower  a     Portability  B  Nadolson recommends to avoid rolling your own strcicmp   and this is reasonable  except when code needs high equivalent portable functionality  Below is an approach that even performed faster than some system provided functions  It does a single compare per loop rather than two by using 2 different tables that differ with   0    Your results may vary  static unsigned char low1 UCHAR MAX   1        0  1  2  3              a    b    c        z                 ABC    Z             a    b    c        z                 abc    z       static unsigned char low2 UCHAR MAX   1         v--- Not zero  but A which matches none in  low1       A   1  2  3              a    b    c        z                    a    b    c        z               int strcicmp ch char const  a  char const  b         compare using tables that differ slightly    while  low1   const unsigned char   a     low2   const unsigned char   b         a        b             Either strings differ or null character detected       Perform subtraction using same table    return  low1   const unsigned char   a  - low1   const unsigned char   b

User · Answer

As others have stated  there is no portable function that works on all systems  You can partially circumvent this with simple ifdef    include  lt stdio h gt    ifdef  WIN32  include  lt string h gt   define strcasecmp  stricmp  else    assuming POSIX or BSD compliant system  include  lt strings h gt   endif  int main         printf   d   strcasecmp  teSt    TEst

User · Answer

You can get an idea  how to implement an efficient one  if you don t have any in the library  from here  It use a table for all 256 chars     in that table for all chars  except letters - used its ascii codes    for upper case letter codes - the table list codes of lower cased symbols     then we just need to traverse a strings and compare our table cells for a given chars   const char  cm   charmap           us1    const char   s1           us2    const char   s2  while  cm  us1     cm  us2         if   us1        0           return  0   return  cm  us1  - cm  --us2

[c] Case Insensitive String comp in C

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