How to convert std string to lower case

Question

I want to convert a std  string to lowercase  I am aware of the function tolower    however in the past I have had issues with this function and it is hardly ideal anyway as use with a std  string would require iterating over each character   Is there an alternative which works 100  of the time

User · Answer

This could be another simple version to convert uppercase to lowercase and vice versa. I used VS2017 community version to compile this source code.

#include <iostream>
#include <string>
using namespace std;

int main()
{
    std::string _input = "lowercasetouppercase";
#if 0
    // My idea is to use the ascii value to convert
    char upperA = 'A';
    char lowerA = 'a';

    cout << (int)upperA << endl; // ASCII value of 'A' -> 65
    cout << (int)lowerA << endl; // ASCII value of 'a' -> 97
    // 97-65 = 32; // Difference of ASCII value of upper and lower a
#endif // 0

    cout << "Input String = " << _input.c_str() << endl;
    for (int i = 0; i < _input.length(); ++i)
    {
        _input[i] -= 32; // To convert lower to upper
#if 0
        _input[i] += 32; // To convert upper to lower
#endif // 0
    }
    cout << "Output String = " << _input.c_str() << endl;

    return 0;
}

Note: if there are special characters then need to be handled using condition check.

User · Answer

Try this function    string toLowerCase string str         int str len   str length         string final str    quot  quot        for int i 0  i lt str len  i               char character   str i            if character gt  65  amp  amp  character lt  92                 final str     character 32              else                final str    character                         return final str

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Adapted from Not So Frequently Asked Questions    include  lt algorithm gt   include  lt cctype gt   include  lt string gt   std  string data    Abc   std  transform data begin    data end    data begin           unsigned char c   return std  tolower c         You re really not going to get away without iterating through each character  There s no way to know whether the character is lowercase or uppercase otherwise   If you really hate tolower    here s a specialized ASCII-only alternative that I don t recommend you use   char asciitolower char in        if  in  lt    Z   amp  amp  in  gt    A           return in -   Z  -  z        return in     std  transform data begin    data end    data begin    asciitolower     Be aware that tolower   can only do a per-single-byte-character substitution  which is ill-fitting for many scripts  especially if using a multi-byte-encoding like UTF-8

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C   doesn t have tolower or toupper methods implemented for std  string  but it is available for char  One can easily read each char of string  convert it into required case and put it back into string   A sample code without using any third party library     include lt iostream gt   int main        std  string str   std  string  How IS The Josh        for char  amp ch   str           ch   std  tolower ch             std  cout lt  lt str lt  lt std  endl      return 0      For character based operation on string   For every character in string

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This is a follow-up to Stefan Mai s response  if you d like to place the result of the conversion in another string  you need to pre-allocate its storage space prior to calling std  transform  Since STL stores transformed characters at the destination iterator  incrementing it at each iteration of the loop   the destination string will not be automatically resized  and you risk memory stomping    include  lt string gt   include  lt algorithm gt   include  lt iostream gt   int main  int argc  char  argv        std  string sourceString    Abc     std  string destinationString        Allocate the destination space   destinationString resize sourceString size           Convert the source string to lower case      storing the result in destination string   std  transform sourceString begin                     sourceString end                     destinationString begin                       tolower         Output the result of the conversion   std  cout  lt  lt  sourceString              lt  lt    - gt                 lt  lt  destinationString              lt  lt  std  endl

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I tried std  transform  all i get is abominable stl criptic compilation error that only druids from 200 years ago can understand   cannot convert from to flibidi flabidi flu   this works fine and can be easily tweaked  string LowerCase string s        int dif  a - A       for int i 0 i lt s length   i                  if  s i  gt   A   amp  amp  s i  lt   Z                s i   dif           return s     string UpperCase string s       int dif  a - A       for int i 0 i lt s length   i                  if  s i  gt   a   amp  amp  s i  lt   z                s i - dif           return s

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There is a way to convert upper case to lower WITHOUT doing if tests  and it s pretty straight-forward  The isupper   function macro s use of clocale h should take care of problems relating to your location  but if not  you can always tweak the UtoL   to your heart s content    Given that C s characters are really just 8-bit ints  ignoring the wide character sets for the moment  you can create a 256 byte array holding an alternative set of characters  and in the conversion function use the chars in your string as subscripts into the conversion array    Instead of a 1-for-1 mapping though  give the upper-case array members the BYTE int values for the lower-case characters  You may find islower   and isupper   useful here      The code looks like this      include  lt clocale gt  static char UtoL 256      ---------------------------------------------------------------------------- void InitUtoLMap          for  int i   0  i  lt  sizeof UtoL   i               if  isupper i                 UtoL i     char  i   32               else                  UtoL i    i                       ---------------------------------------------------------------------------- char  LowerStr char  szMyStr        char  p   szMyStr         do conversion in-place so as not to require a destination buffer     while   p              szMyStr must be null-terminated          p   UtoL  p             p              return szMyStr       ---------------------------------------------------------------------------- int main         time t start      char  Lowered  Upper 128       InitUtoLMap        strcpy Upper   Every GOOD boy does FINE          Lowered   LowerStr Upper       return 0      This approach will  at the same time  allow you to remap any other characters you wish to change    This approach has one huge advantage when running on modern processors  there is no need to do branch prediction as there are no if tests comprising branching  This saves the CPU s branch prediction logic for other loops  and tends to prevent pipeline stalls    Some here may recognize this approach as the same one used to convert EBCDIC to ASCII

User · Answer

Adapted from Not So Frequently Asked Questions    include  lt algorithm gt   include  lt cctype gt   include  lt string gt   std  string data    Abc   std  transform data begin    data end    data begin           unsigned char c   return std  tolower c         You re really not going to get away without iterating through each character  There s no way to know whether the character is lowercase or uppercase otherwise   If you really hate tolower    here s a specialized ASCII-only alternative that I don t recommend you use   char asciitolower char in        if  in  lt    Z   amp  amp  in  gt    A           return in -   Z  -  z        return in     std  transform data begin    data end    data begin    asciitolower     Be aware that tolower   can only do a per-single-byte-character substitution  which is ill-fitting for many scripts  especially if using a multi-byte-encoding like UTF-8

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If the string contains UTF-8 characters outside of the ASCII range  then boost  algorithm  to lower will not convert those  Better use boost  locale  to lower when UTF-8 is involved  See http   www boost org doc libs 1 51 0 libs locale doc html conversions html

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On microsoft platforms you can use the strlwr family of functions  http   msdn microsoft com en-us library hkxwh33z aspx     crt strlwr c    compile with   W3    This program uses  strlwr and  strupr to create    uppercase and lowercase copies of a mixed-case string   include  lt string h gt   include  lt stdio h gt   int main  void        char string 100     The String to End All Strings       char   copy1    strdup  string       make two copies    char   copy2    strdup  string         strlwr  copy1       C4996     strupr  copy2       C4996     printf   Mixed   s n   string       printf   Lower   s n   copy1       printf   Upper   s n   copy2        free  copy1       free  copy2

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std  ctype  tolower   from the standard C   Localization library will correctly do this for you  Here is an example extracted from the tolower reference page   include  lt locale gt   include  lt iostream gt   int main        std  locale  global std  locale  en US utf8       std  wcout imbue std  locale       std  wcout  lt  lt   In US English UTF-8 locale  n     auto amp  f   std  use facet lt std  ctype lt wchar t gt  gt  std  locale       std  wstring str   L HELLo  wORLD      std  wcout  lt  lt   Lowercase form of the string     lt  lt  str  lt  lt     is      f tolower  amp str 0    amp str 0    str size       std  wcout  lt  lt       lt  lt  str  lt  lt     n

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If the string contains UTF-8 characters outside of the ASCII range  then boost  algorithm  to lower will not convert those  Better use boost  locale  to lower when UTF-8 is involved  See http   www boost org doc libs 1 51 0 libs locale doc html conversions html

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Another approach using range based for loop with reference variable  string test    Hello World   for auto amp  c   test       c   tolower c      cout lt  lt test lt  lt endl

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Boost provides a string algorithm for this    include  lt boost algorithm string hpp gt   std  string str    HELLO  WORLD    boost  algorithm  to lower str      modifies str   Or  for non-in-place    include  lt boost algorithm string hpp gt   const std  string str    HELLO  WORLD    const std  string lower str   boost  algorithm  to lower copy str

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Using range-based for loop of C  11 a simpler code would be     include  lt iostream gt           std  cout  include  lt string gt             std  string  include  lt locale gt             std  locale  std  tolower  int main        std  locale loc    std  string str  Test String  n     for auto elem   str      std  cout  lt  lt  std  tolower elem loc

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Use fplus  to lower case   from fplus library   Search to lower case in fplus API Search  Example   fplus  to lower case std  string  ABC       std  string  abc

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tl dr Use the ICU library  If you don t  your conversion routine will break silently on cases you are probably not even aware of existing   First you have to answer a question  What is the encoding of your std  string  Is it ISO-8859-1  Or perhaps ISO-8859-8  Or Windows Codepage 1252  Does whatever you re using to convert upper-to-lowercase know that   Or does it fail miserably for characters over 0x7f   If you are using UTF-8  the only sane choice among the 8-bit encodings  with std  string as container  you are already deceiving yourself if you believe you are still in control of things  You are storing a multibyte character sequence in a container that is not aware of the multibyte concept  and neither are most of the operations you can perform on it  Even something as simple as  substr   could result in invalid  sub-  strings because you split in the middle of a multibyte sequence  As soon as you try something like std  toupper          or std  tolower   S    in any encoding  you are in trouble  Because 1   the standard only ever operates on one character at a time  so it simply cannot turn    into SS as would be correct  And 2   the standard only ever operates on one character at a time  so it cannot decide whether S is in the middle of a word  where s would be correct   or at the end      Another example would be std  tolower   I     which should yield different results depending on the locale -- virtually everywhere you would expect i  but in Turkey i  LATIN SMALL LETTER DOTLESS I  is the correct answer  which  again  is more than one byte in UTF-8 encoding   So  any case conversion that works on a character at a time  or worse  a byte at a time  is broken by design  This includes all the std   variants in existence at this time  Then there is the point that the standard library  for what it is capable of doing  is depending on which locales are supported on the machine your software is running on    and what do you do if your target locale is among the not supported on your client s machine  So what you are really looking for is a string class that is capable of dealing with all this correctly  and that is not any of the std  basic string lt  gt  variants   C  11 note  std  u16string and std  u32string are better  but still not perfect  C  20 brought std  u8string  but all these do is specify the encoding  In many other respects they still remain ignorant of Unicode mechanics  like normalization  collation       While Boost looks nice  API wise  Boost Locale is basically a wrapper around ICU  If Boost is compiled with ICU support    if it isn t  Boost Locale is limited to the locale support compiled for the standard library  And believe me  getting Boost to compile with ICU can be a real pain sometimes   There are no pre-compiled binaries for Windows that include ICU  so you d have to supply them together with your application  and that opens a whole new can of worms     So personally I would recommend getting full Unicode support straight from the horse s mouth and using the ICU library directly   include  lt unicode unistr h gt   include  lt unicode ustream h gt   include  lt unicode locid h gt    include  lt iostream gt   int main                                      quot Odysseus quot         char const   someString   u8 quot    SS  S quot       icu  UnicodeString someUString  someString   quot UTF-8 quot            Setting the locale explicitly here for completeness         Usually you would use the user-specified system locale         which  does  make a difference  see i vs  i above       std  cout  lt  lt  someUString toLower   quot el GR quot     lt  lt   quot  n quot       std  cout  lt  lt  someUString toUpper   quot el GR quot     lt  lt   quot  n quot       return 0     Compile  with G   in this example   g   -Wall example cpp -licuuc -licuio  This gives   d sse    Note that the S lt - gt s conversion in the middle of the word  and the S lt - gt   conversion at the end of the word  No  lt algorithm gt -based solution can give you that

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Boost provides a string algorithm for this    include  lt boost algorithm string hpp gt   std  string str    HELLO  WORLD    boost  algorithm  to lower str      modifies str   Or  for non-in-place    include  lt boost algorithm string hpp gt   const std  string str    HELLO  WORLD    const std  string lower str   boost  algorithm  to lower copy str

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As far as I see Boost libraries are really bad performance-wise  I have tested their unordered map to STL and it was average 3 times slower  best case 2  worst was 10 times   Also this algorithm looks too low   The difference is so big that I am sure whatever addition you will need to do to tolower to make it equal to boost  for your needs  will be way faster than boost   I have done these tests on an Amazon EC2  therefore performance varied during the test but you still get the idea     test Elapsed time  12365milliseconds Elapsed time  1640milliseconds   test Elapsed time  26978milliseconds Elapsed time  1646milliseconds   test Elapsed time  6957milliseconds Elapsed time  1634milliseconds   test Elapsed time  23177milliseconds Elapsed time  2421milliseconds   test Elapsed time  17342milliseconds Elapsed time  14132milliseconds   test Elapsed time  7355milliseconds Elapsed time  1645milliseconds   -O2 made it like this     test Elapsed time  3769milliseconds Elapsed time  565milliseconds   test Elapsed time  3815milliseconds Elapsed time  565milliseconds   test Elapsed time  3643milliseconds Elapsed time  566milliseconds   test Elapsed time  22018milliseconds Elapsed time  566milliseconds   test Elapsed time  3845milliseconds Elapsed time  569milliseconds   Source   string str  bench start    for long long i 0 i lt 1000000 i          str  DSFZKMdskfdsjfsdfJDASFNSDJFXCKVdnjsafnjsdfjdnjasnJDNASFDJDSFSDNJjdsanjfsdnfjJNFSDJFSD       boost  algorithm  to lower str     bench end     bench start    for long long i 0 i lt 1000000 i          str  DSFZKMdskfdsjfsdfJDASFNSDJFXCKVdnjsafnjsdfjdnjasnJDNASFDJDSFSDNJjdsanjfsdnfjJNFSDJFSD       for unsigned short loop 0 loop  lt  str size   loop                  str loop  tolower str loop            bench end      I guess I should to the tests on a dedicated machine but I will be using this EC2 so I do not really need to test it on my machine

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Boost provides a string algorithm for this    include  lt boost algorithm string hpp gt   std  string str    HELLO  WORLD    boost  algorithm  to lower str      modifies str   Or  for non-in-place    include  lt boost algorithm string hpp gt   const std  string str    HELLO  WORLD    const std  string lower str   boost  algorithm  to lower copy str

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Since none of the answers mentioned the upcoming Ranges library  which is available in the standard library since C  20  and currently separately available on GitHub as range-v3  I would like to add a way to perform this conversion using it   To modify the string in-place   str    action  transform    unsigned char c   return std  tolower c         To generate a new string   auto new string   original string       view  transform    unsigned char c   return std  tolower c          Don t forget to  include  lt cctype gt  and the required Ranges headers    Note  the use of unsigned char as the argument to the lambda is inspired by cppreference  which states      Like all other functions from  lt cctype gt   the behavior of std  tolower is undefined if the argument s value is neither representable as unsigned char nor equal to EOF  To use these functions safely with plain chars  or signed chars   the argument should first be converted to unsigned char    char my tolower char ch        return static cast lt char gt  std  tolower static cast lt unsigned char gt  ch             Similarly  they should not be directly used with standard algorithms when the iterator s value type is char or signed char  Instead  convert the value to unsigned char first    std  string str tolower std  string s        std  transform s begin    s end    s begin                        static cast lt int    int  gt  std  tolower             wrong                       int c   return std  tolower c                  wrong                       char c   return std  tolower c                 wrong                       unsigned char c   return std  tolower c        correct                          return s

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Code Snippet   include lt bits stdc   h gt  using namespace std    int main          ios  sync with stdio false        string str  String Convert n        for int i 0  i lt str size    i                str i    tolower str i              cout lt  lt str lt  lt endl       return 0

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tl dr Use the ICU library  If you don t  your conversion routine will break silently on cases you are probably not even aware of existing   First you have to answer a question  What is the encoding of your std  string  Is it ISO-8859-1  Or perhaps ISO-8859-8  Or Windows Codepage 1252  Does whatever you re using to convert upper-to-lowercase know that   Or does it fail miserably for characters over 0x7f   If you are using UTF-8  the only sane choice among the 8-bit encodings  with std  string as container  you are already deceiving yourself if you believe you are still in control of things  You are storing a multibyte character sequence in a container that is not aware of the multibyte concept  and neither are most of the operations you can perform on it  Even something as simple as  substr   could result in invalid  sub-  strings because you split in the middle of a multibyte sequence  As soon as you try something like std  toupper          or std  tolower   S    in any encoding  you are in trouble  Because 1   the standard only ever operates on one character at a time  so it simply cannot turn    into SS as would be correct  And 2   the standard only ever operates on one character at a time  so it cannot decide whether S is in the middle of a word  where s would be correct   or at the end      Another example would be std  tolower   I     which should yield different results depending on the locale -- virtually everywhere you would expect i  but in Turkey i  LATIN SMALL LETTER DOTLESS I  is the correct answer  which  again  is more than one byte in UTF-8 encoding   So  any case conversion that works on a character at a time  or worse  a byte at a time  is broken by design  This includes all the std   variants in existence at this time  Then there is the point that the standard library  for what it is capable of doing  is depending on which locales are supported on the machine your software is running on    and what do you do if your target locale is among the not supported on your client s machine  So what you are really looking for is a string class that is capable of dealing with all this correctly  and that is not any of the std  basic string lt  gt  variants   C  11 note  std  u16string and std  u32string are better  but still not perfect  C  20 brought std  u8string  but all these do is specify the encoding  In many other respects they still remain ignorant of Unicode mechanics  like normalization  collation       While Boost looks nice  API wise  Boost Locale is basically a wrapper around ICU  If Boost is compiled with ICU support    if it isn t  Boost Locale is limited to the locale support compiled for the standard library  And believe me  getting Boost to compile with ICU can be a real pain sometimes   There are no pre-compiled binaries for Windows that include ICU  so you d have to supply them together with your application  and that opens a whole new can of worms     So personally I would recommend getting full Unicode support straight from the horse s mouth and using the ICU library directly   include  lt unicode unistr h gt   include  lt unicode ustream h gt   include  lt unicode locid h gt    include  lt iostream gt   int main                                      quot Odysseus quot         char const   someString   u8 quot    SS  S quot       icu  UnicodeString someUString  someString   quot UTF-8 quot            Setting the locale explicitly here for completeness         Usually you would use the user-specified system locale         which  does  make a difference  see i vs  i above       std  cout  lt  lt  someUString toLower   quot el GR quot     lt  lt   quot  n quot       std  cout  lt  lt  someUString toUpper   quot el GR quot     lt  lt   quot  n quot       return 0     Compile  with G   in this example   g   -Wall example cpp -licuuc -licuio  This gives   d sse    Note that the S lt - gt s conversion in the middle of the word  and the S lt - gt   conversion at the end of the word  No  lt algorithm gt -based solution can give you that

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std  ctype  tolower   from the standard C   Localization library will correctly do this for you  Here is an example extracted from the tolower reference page   include  lt locale gt   include  lt iostream gt   int main        std  locale  global std  locale  en US utf8       std  wcout imbue std  locale       std  wcout  lt  lt   In US English UTF-8 locale  n     auto amp  f   std  use facet lt std  ctype lt wchar t gt  gt  std  locale       std  wstring str   L HELLo  wORLD      std  wcout  lt  lt   Lowercase form of the string     lt  lt  str  lt  lt     is      f tolower  amp str 0    amp str 0    str size       std  wcout  lt  lt       lt  lt  str  lt  lt     n

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An alternative to Boost is POCO  pocoproject org    POCO provides two variants    The first variant makes a copy without altering the original string  The second variant changes the original string in place    In Place  versions always have  InPlace  in the name     Both versions are demonstrated below    include  Poco String h  using namespace Poco   std  string hello  Stack Overflow         Copies  STACK OVERFLOW   into  newString  without altering  hello   std  string newString toUpper hello        Changes newString in-place to read  stack overflow   toLowerInPlace newString

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Is there an alternative which works 100  of the time    No  There are several questions you need to ask yourself before choosing a lowercasing method    How is the string encoded  plain ASCII  UTF-8  some form of extended ASCII legacy encoding  What do you mean by lower case anyway  Case mapping rules vary between languages  Do you want something that is localised to the users locale  do you want something that behaves consistently on all systems your software runs on  Do you just want to lowercase ASCII characters and pass through everything else   What libraries are available     Once you have answers to those questions you can start looking for a soloution that fits your needs  There is no one size fits all that works for everyone everywhere

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Copy because it was disallowed to improve answer  Thanks SO    string test    Hello World   for auto amp  c   test       c   tolower c         Explanation   for auto amp  c   test  is a range-based for loop of the kind  for  range declaration range expression loop statement    range declaration  auto amp  c  Here the auto specifier is used for for automatic type deduction  So the type gets deducted from the variables initializer  range expression  test The range in this case are the characters of string test     The characters of the string test are available as a reference inside the for loop through identifier c

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I wrote this simple helper function   include  lt locale gt     tolower  string to lower string s                for char  amp c   s          c   tolower c       return s     Usage  string s    quot TEST quot   cout  lt  lt  to lower  quot HELLO WORLD quot       output   quot hello word quot  cout  lt  lt  to lower s      won t change the original variable

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Code Snippet   include lt bits stdc   h gt  using namespace std    int main          ios  sync with stdio false        string str  String Convert n        for int i 0  i lt str size    i                str i    tolower str i              cout lt  lt str lt  lt endl       return 0

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This is a follow-up to Stefan Mai s response  if you d like to place the result of the conversion in another string  you need to pre-allocate its storage space prior to calling std  transform  Since STL stores transformed characters at the destination iterator  incrementing it at each iteration of the loop   the destination string will not be automatically resized  and you risk memory stomping    include  lt string gt   include  lt algorithm gt   include  lt iostream gt   int main  int argc  char  argv        std  string sourceString    Abc     std  string destinationString        Allocate the destination space   destinationString resize sourceString size           Convert the source string to lower case      storing the result in destination string   std  transform sourceString begin                     sourceString end                     destinationString begin                       tolower         Output the result of the conversion   std  cout  lt  lt  sourceString              lt  lt    - gt                 lt  lt  destinationString              lt  lt  std  endl

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Since none of the answers mentioned the upcoming Ranges library  which is available in the standard library since C  20  and currently separately available on GitHub as range-v3  I would like to add a way to perform this conversion using it   To modify the string in-place   str    action  transform    unsigned char c   return std  tolower c         To generate a new string   auto new string   original string       view  transform    unsigned char c   return std  tolower c          Don t forget to  include  lt cctype gt  and the required Ranges headers    Note  the use of unsigned char as the argument to the lambda is inspired by cppreference  which states      Like all other functions from  lt cctype gt   the behavior of std  tolower is undefined if the argument s value is neither representable as unsigned char nor equal to EOF  To use these functions safely with plain chars  or signed chars   the argument should first be converted to unsigned char    char my tolower char ch        return static cast lt char gt  std  tolower static cast lt unsigned char gt  ch             Similarly  they should not be directly used with standard algorithms when the iterator s value type is char or signed char  Instead  convert the value to unsigned char first    std  string str tolower std  string s        std  transform s begin    s end    s begin                        static cast lt int    int  gt  std  tolower             wrong                       int c   return std  tolower c                  wrong                       char c   return std  tolower c                 wrong                       unsigned char c   return std  tolower c        correct                          return s

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Simplest way to convert string into loweercase without bothering about std namespace is as follows   1 string with without spaces   include  lt algorithm gt   include  lt iostream gt   include  lt string gt  using namespace std  int main        string str      getline cin str     ------------function to convert string into lowercase---------------     transform str begin    str end    str begin      tolower     --------------------------------------------------------------------     cout lt  lt str      return 0      2 string without spaces   include  lt algorithm gt   include  lt iostream gt   include  lt string gt  using namespace std  int main        string str      cin gt  gt str    ------------function to convert string into lowercase---------------     transform str begin    str end    str begin      tolower     --------------------------------------------------------------------     cout lt  lt str      return 0

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I tried std  transform  all i get is abominable stl criptic compilation error that only druids from 200 years ago can understand   cannot convert from to flibidi flabidi flu   this works fine and can be easily tweaked  string LowerCase string s        int dif  a - A       for int i 0 i lt s length   i                  if  s i  gt   A   amp  amp  s i  lt   Z                s i   dif           return s     string UpperCase string s       int dif  a - A       for int i 0 i lt s length   i                  if  s i  gt   a   amp  amp  s i  lt   z                s i - dif           return s

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Another approach using range based for loop with reference variable  string test    Hello World   for auto amp  c   test       c   tolower c      cout lt  lt test lt  lt endl

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tolower example  C     include  lt iostream gt           std  cout  include  lt string gt             std  string  include  lt locale gt             std  locale  std  tolower  int main        std  locale loc    std  string str  Test String  n     for  std  string  size type i 0  i lt str length      i      std  cout  lt  lt  std  tolower str i  loc     return 0      For more information  http   www cplusplus com reference locale tolower

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I wrote this simple helper function   include  lt locale gt     tolower  string to lower string s                for char  amp c   s          c   tolower c       return s     Usage  string s    quot TEST quot   cout  lt  lt  to lower  quot HELLO WORLD quot       output   quot hello word quot  cout  lt  lt  to lower s      won t change the original variable

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Use fplus  to lower case   from fplus library   Search to lower case in fplus API Search  Example   fplus  to lower case std  string  ABC       std  string  abc

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Copy because it was disallowed to improve answer  Thanks SO    string test    Hello World   for auto amp  c   test       c   tolower c         Explanation   for auto amp  c   test  is a range-based for loop of the kind  for  range declaration range expression loop statement    range declaration  auto amp  c  Here the auto specifier is used for for automatic type deduction  So the type gets deducted from the variables initializer  range expression  test The range in this case are the characters of string test     The characters of the string test are available as a reference inside the for loop through identifier c

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My own template functions which performs upper   lower case    include  lt string gt   include  lt algorithm gt          Lowercases string    template  lt typename T gt  std  basic string lt T gt  lowercase const std  basic string lt T gt  amp  s        std  basic string lt T gt  s2   s      std  transform s2 begin    s2 end    s2 begin    tolower       return std  move s2            Uppercases string    template  lt typename T gt  std  basic string lt T gt  uppercase const std  basic string lt T gt  amp  s        std  basic string lt T gt  s2   s      std  transform s2 begin    s2 end    s2 begin    toupper       return std  move s2

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This could be another simple version to convert uppercase to lowercase and vice versa. I used VS2017 community version to compile this source code.

#include <iostream>
#include <string>
using namespace std;

int main()
{
    std::string _input = "lowercasetouppercase";
#if 0
    // My idea is to use the ascii value to convert
    char upperA = 'A';
    char lowerA = 'a';

    cout << (int)upperA << endl; // ASCII value of 'A' -> 65
    cout << (int)lowerA << endl; // ASCII value of 'a' -> 97
    // 97-65 = 32; // Difference of ASCII value of upper and lower a
#endif // 0

    cout << "Input String = " << _input.c_str() << endl;
    for (int i = 0; i < _input.length(); ++i)
    {
        _input[i] -= 32; // To convert lower to upper
#if 0
        _input[i] += 32; // To convert upper to lower
#endif // 0
    }
    cout << "Output String = " << _input.c_str() << endl;

    return 0;
}

Note: if there are special characters then need to be handled using condition check.

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My own template functions which performs upper   lower case    include  lt string gt   include  lt algorithm gt          Lowercases string    template  lt typename T gt  std  basic string lt T gt  lowercase const std  basic string lt T gt  amp  s        std  basic string lt T gt  s2   s      std  transform s2 begin    s2 end    s2 begin    tolower       return std  move s2            Uppercases string    template  lt typename T gt  std  basic string lt T gt  uppercase const std  basic string lt T gt  amp  s        std  basic string lt T gt  s2   s      std  transform s2 begin    s2 end    s2 begin    toupper       return std  move s2

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tolower example  C     include  lt iostream gt           std  cout  include  lt string gt             std  string  include  lt locale gt             std  locale  std  tolower  int main        std  locale loc    std  string str  Test String  n     for  std  string  size type i 0  i lt str length      i      std  cout  lt  lt  std  tolower str i  loc     return 0      For more information  http   www cplusplus com reference locale tolower

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Here s a macro technique if you want something simple    define STRTOLOWER x  std  transform  x begin    x end    x begin      tolower   define STRTOUPPER x  std  transform  x begin    x end    x begin      toupper   define STRTOUCFIRST x  std  transform  x begin    x begin   1  x begin       toupper   std  transform  x begin   1  x end      x begin   1   tolower    However  note that  AndreasSpindler s comment on this answer still is an important consideration  however  if you re working on something that isn t just ASCII characters

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On microsoft platforms you can use the strlwr family of functions  http   msdn microsoft com en-us library hkxwh33z aspx     crt strlwr c    compile with   W3    This program uses  strlwr and  strupr to create    uppercase and lowercase copies of a mixed-case string   include  lt string h gt   include  lt stdio h gt   int main  void        char string 100     The String to End All Strings       char   copy1    strdup  string       make two copies    char   copy2    strdup  string         strlwr  copy1       C4996     strupr  copy2       C4996     printf   Mixed   s n   string       printf   Lower   s n   copy1       printf   Upper   s n   copy2        free  copy1       free  copy2

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Using range-based for loop of C  11 a simpler code would be     include  lt iostream gt           std  cout  include  lt string gt             std  string  include  lt locale gt             std  locale  std  tolower  int main        std  locale loc    std  string str  Test String  n     for auto elem   str      std  cout  lt  lt  std  tolower elem loc

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Here s a macro technique if you want something simple    define STRTOLOWER x  std  transform  x begin    x end    x begin      tolower   define STRTOUPPER x  std  transform  x begin    x end    x begin      toupper   define STRTOUCFIRST x  std  transform  x begin    x begin   1  x begin       toupper   std  transform  x begin   1  x end      x begin   1   tolower    However  note that  AndreasSpindler s comment on this answer still is an important consideration  however  if you re working on something that isn t just ASCII characters

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C   doesn t have tolower or toupper methods implemented for std  string  but it is available for char  One can easily read each char of string  convert it into required case and put it back into string   A sample code without using any third party library     include lt iostream gt   int main        std  string str   std  string  How IS The Josh        for char  amp ch   str           ch   std  tolower ch             std  cout lt  lt str lt  lt std  endl      return 0      For character based operation on string   For every character in string

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An alternative to Boost is POCO  pocoproject org    POCO provides two variants    The first variant makes a copy without altering the original string  The second variant changes the original string in place    In Place  versions always have  InPlace  in the name     Both versions are demonstrated below    include  Poco String h  using namespace Poco   std  string hello  Stack Overflow         Copies  STACK OVERFLOW   into  newString  without altering  hello   std  string newString toUpper hello        Changes newString in-place to read  stack overflow   toLowerInPlace newString

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Boost provides a string algorithm for this    include  lt boost algorithm string hpp gt   std  string str    HELLO  WORLD    boost  algorithm  to lower str      modifies str   Or  for non-in-place    include  lt boost algorithm string hpp gt   const std  string str    HELLO  WORLD    const std  string lower str   boost  algorithm  to lower copy str

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Try this function    string toLowerCase string str         int str len   str length         string final str    quot  quot        for int i 0  i lt str len  i               char character   str i            if character gt  65  amp  amp  character lt  92                 final str     character 32              else                final str    character                         return final str

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There is a way to convert upper case to lower WITHOUT doing if tests  and it s pretty straight-forward  The isupper   function macro s use of clocale h should take care of problems relating to your location  but if not  you can always tweak the UtoL   to your heart s content    Given that C s characters are really just 8-bit ints  ignoring the wide character sets for the moment  you can create a 256 byte array holding an alternative set of characters  and in the conversion function use the chars in your string as subscripts into the conversion array    Instead of a 1-for-1 mapping though  give the upper-case array members the BYTE int values for the lower-case characters  You may find islower   and isupper   useful here      The code looks like this      include  lt clocale gt  static char UtoL 256      ---------------------------------------------------------------------------- void InitUtoLMap          for  int i   0  i  lt  sizeof UtoL   i               if  isupper i                 UtoL i     char  i   32               else                  UtoL i    i                       ---------------------------------------------------------------------------- char  LowerStr char  szMyStr        char  p   szMyStr         do conversion in-place so as not to require a destination buffer     while   p              szMyStr must be null-terminated          p   UtoL  p             p              return szMyStr       ---------------------------------------------------------------------------- int main         time t start      char  Lowered  Upper 128       InitUtoLMap        strcpy Upper   Every GOOD boy does FINE          Lowered   LowerStr Upper       return 0      This approach will  at the same time  allow you to remap any other characters you wish to change    This approach has one huge advantage when running on modern processors  there is no need to do branch prediction as there are no if tests comprising branching  This saves the CPU s branch prediction logic for other loops  and tends to prevent pipeline stalls    Some here may recognize this approach as the same one used to convert EBCDIC to ASCII

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As far as I see Boost libraries are really bad performance-wise  I have tested their unordered map to STL and it was average 3 times slower  best case 2  worst was 10 times   Also this algorithm looks too low   The difference is so big that I am sure whatever addition you will need to do to tolower to make it equal to boost  for your needs  will be way faster than boost   I have done these tests on an Amazon EC2  therefore performance varied during the test but you still get the idea     test Elapsed time  12365milliseconds Elapsed time  1640milliseconds   test Elapsed time  26978milliseconds Elapsed time  1646milliseconds   test Elapsed time  6957milliseconds Elapsed time  1634milliseconds   test Elapsed time  23177milliseconds Elapsed time  2421milliseconds   test Elapsed time  17342milliseconds Elapsed time  14132milliseconds   test Elapsed time  7355milliseconds Elapsed time  1645milliseconds   -O2 made it like this     test Elapsed time  3769milliseconds Elapsed time  565milliseconds   test Elapsed time  3815milliseconds Elapsed time  565milliseconds   test Elapsed time  3643milliseconds Elapsed time  566milliseconds   test Elapsed time  22018milliseconds Elapsed time  566milliseconds   test Elapsed time  3845milliseconds Elapsed time  569milliseconds   Source   string str  bench start    for long long i 0 i lt 1000000 i          str  DSFZKMdskfdsjfsdfJDASFNSDJFXCKVdnjsafnjsdfjdnjasnJDNASFDJDSFSDNJjdsanjfsdnfjJNFSDJFSD       boost  algorithm  to lower str     bench end     bench start    for long long i 0 i lt 1000000 i          str  DSFZKMdskfdsjfsdfJDASFNSDJFXCKVdnjsafnjsdfjdnjasnJDNASFDJDSFSDNJjdsanjfsdnfjJNFSDJFSD       for unsigned short loop 0 loop  lt  str size   loop                  str loop  tolower str loop            bench end      I guess I should to the tests on a dedicated machine but I will be using this EC2 so I do not really need to test it on my machine

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Adapted from Not So Frequently Asked Questions    include  lt algorithm gt   include  lt cctype gt   include  lt string gt   std  string data    Abc   std  transform data begin    data end    data begin           unsigned char c   return std  tolower c         You re really not going to get away without iterating through each character  There s no way to know whether the character is lowercase or uppercase otherwise   If you really hate tolower    here s a specialized ASCII-only alternative that I don t recommend you use   char asciitolower char in        if  in  lt    Z   amp  amp  in  gt    A           return in -   Z  -  z        return in     std  transform data begin    data end    data begin    asciitolower     Be aware that tolower   can only do a per-single-byte-character substitution  which is ill-fitting for many scripts  especially if using a multi-byte-encoding like UTF-8

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Adapted from Not So Frequently Asked Questions    include  lt algorithm gt   include  lt cctype gt   include  lt string gt   std  string data    Abc   std  transform data begin    data end    data begin           unsigned char c   return std  tolower c         You re really not going to get away without iterating through each character  There s no way to know whether the character is lowercase or uppercase otherwise   If you really hate tolower    here s a specialized ASCII-only alternative that I don t recommend you use   char asciitolower char in        if  in  lt    Z   amp  amp  in  gt    A           return in -   Z  -  z        return in     std  transform data begin    data end    data begin    asciitolower     Be aware that tolower   can only do a per-single-byte-character substitution  which is ill-fitting for many scripts  especially if using a multi-byte-encoding like UTF-8

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Is there an alternative which works 100  of the time    No  There are several questions you need to ask yourself before choosing a lowercasing method    How is the string encoded  plain ASCII  UTF-8  some form of extended ASCII legacy encoding  What do you mean by lower case anyway  Case mapping rules vary between languages  Do you want something that is localised to the users locale  do you want something that behaves consistently on all systems your software runs on  Do you just want to lowercase ASCII characters and pass through everything else   What libraries are available     Once you have answers to those questions you can start looking for a soloution that fits your needs  There is no one size fits all that works for everyone everywhere

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Simplest way to convert string into loweercase without bothering about std namespace is as follows   1 string with without spaces   include  lt algorithm gt   include  lt iostream gt   include  lt string gt  using namespace std  int main        string str      getline cin str     ------------function to convert string into lowercase---------------     transform str begin    str end    str begin      tolower     --------------------------------------------------------------------     cout lt  lt str      return 0      2 string without spaces   include  lt algorithm gt   include  lt iostream gt   include  lt string gt  using namespace std  int main        string str      cin gt  gt str    ------------function to convert string into lowercase---------------     transform str begin    str end    str begin      tolower     --------------------------------------------------------------------     cout lt  lt str      return 0

[c++] How to convert std::string to lower case?

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