tolower function 为 C++ 字符串

Question

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.

Answer 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 .

Answer 2

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

Answer 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.

Answer 4

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.

Answer 5

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.

Answer 6

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);

Answer 7

The above answers produced errors. This works perfectly:

std::transform(str.begin(), str.end(), str.begin(),
        [](unsigned char c){ return std::tolower(c); });