tolower function for C++ strings

问题:

Is there an inbuilt function to convert C++ string from upper case letters to lowercase letters ? If not converting it to cstring and using tolower on each char is the only option ?

Thank you very much in advance.

回答1:

If boost is an option:

#include <boost/algorithm/string.hpp>    

std::string str = "wHatEver";
boost::to_lower(str);

Otherwise, you may use std::transform:

std::string str = "wHatEver";
std::transform(str.begin(), str.end(), str.begin(), ::tolower);

You can also use another function if you have some custom locale-aware tolower.

回答2:

std::transform(myString.begin(), myString.end(), myString.begin(), std::tolower);

回答3:

Like ereOn says: std::transform(str.begin(), str.end(), str.begin(), std::tolower );

Or via for_each: std::for_each(str.begin(), str.end(), std::tolower );

Transform is probably better of the two.

回答4:

There is no built-in function to do this, and doing it is surprisingly complicated, because of locales et al. If tolower does what you need, it may be your best bet.

回答5:

For this problem you can use the STL's transform method to solve it:

std::string str = "simple";
std::transform(str.begin(), str.end(), str.begin(), std::tolower);

回答6:

I have an implementation I found it faster than std::transform , Compiled in g++ -03 Fedora 18. my example converts std::string

performance time in seconds :
transform took         : 11 s
my implementation took : 2 s
Test data size = 26*15*9999999 chars

inline void tolowerPtr(char *p) ;

inline void tolowerStr(std::string& s)
{char* c=const_cast<char*>(s.c_str());
size_t l = s.size();
  for(char* c2=c;c2<c+l;c2++)tolowerPtr(c2); 
};

inline void tolowerPtr(char *p) 
{
switch(*p)
{
  case 'A':*p='a'; return;
  case 'B':*p='b'; return;
  case 'C':*p='c'; return;
  case 'D':*p='d'; return;
  case 'E':*p='e'; return;
  case 'F':*p='f'; return;
  case 'G':*p='g'; return;
  case 'H':*p='h'; return;
  case 'I':*p='i'; return;
  case 'J':*p='j'; return;
  case 'K':*p='k'; return;
  case 'L':*p='l'; return;
  case 'M':*p='m'; return;
  case 'N':*p='n'; return;
  case 'O':*p='o'; return;
  case 'P':*p='p'; return;
  case 'Q':*p='q'; return;
  case 'R':*p='r'; return;
  case 'S':*p='s'; return;
  case 'T':*p='t'; return;
  case 'U':*p='u'; return;
  case 'V':*p='v'; return;
  case 'W':*p='w'; return;
  case 'X':*p='x'; return;
  case 'Y':*p='y'; return;
  case 'Z':*p='z'; return;
};
return ;
}

void testtransform( std::string& word )
{
std::string word2=word; 
time_t t;
time_t t2;
time(&t);
std::cout << "testtransform: start " << "\n";
int i=0;
for(;i<9999999;i++) 
{    word2=word;
    std::transform(word2.begin(), word2.end(), word2.begin(), ::tolower);
}
time(&t2);
std::cout << word2 << "\n";
std::cout << "testtransform: end " << i << ":"<< t2-t << "\n";
}

void testmytolower( std::string& word )
{
std::string word2=word; 
time_t t;
time_t t2;
time(&t);
std::cout << "testmytolower: start " << "\n";
int i=0;
for(;i<9999999;i++)
{   word2=word;
    cstralgo::tolowerStr(word2);
}
time(&t2);
std::cout << word2 << "\n";
std::cout << "testmytolower: end " << i << ":"<< t2-t << "\n";
}

int main(int argc, char* argv[])
{
   std::string word ="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
   word =word+word+word+word+word+word+word+word+word+word+word+word+word+word+word;
   testtransform( word);
   testmytolower( word);
   return 0;
}

I will be glad to know if performance can be improved further.