STL String to lower case

Question

Hi,

I want to convert an STL 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 string would require iterating through each character.

Is there an alternative which works 100% of the time?

Answer 1

There is a Boost string algorithm for this:

#include <boost/algorithm/string.hpp>    

std::string str = "HELLO, WORLD!";
boost::to_lower(str);

Answer 2

From http://notfaq.wordpress.com/2007/08/04/cc-convert-string-to-upperlower-case/:

#include <algorithm>
#include <string> 

std::string data = “Abc”; 
std::transform(data.begin(), data.end(), data.begin(), ::tolower);

You're really not going to get away with not 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 an alternative:

char easytolower(char in){
  if(in<='Z' && in>='A')
    return in-('Z'-'z');
  return in;
} 

std::transform(data.begin(), data.end(), data.begin(), easytolower);

Answer 3

You may also want to use the C++ locale support for this. You gain genericity.

#include <functional>
#include <locale>

std::string a = "ABC";

std::transform(a.begin(), a.end(), a.begin(), 
    std::bind2nd(std::ptr_fun(&std::tolower<char>), std::locale("")));

If you have a range of writable characters, you can also do

std::use_facet< std::ctype<char> >(std::locale("")).tolower(&a[0], &a[0] + a.size());

locale("") will construct a locale object representing your preferred locale.

I recommend reading Stroustrup's Appendix D: Locales, which is freely available, and is a great insight into locales in C++.

Answer 4

Note: tolower() doesn't work 100% of the time.

Lowercase/uppercase operations only apply to characters, and std::string is essentially an array of bytes, not characters. Plain tolower is nice for ASCII string, but it will not lowercase a latin-1 or utf-8 string correctly. You must know string's encoding and probably decode it before you can lowercase its characters.

Answer 5

You can't use std::bind2nd(std::ptr_fun(&std::tolower), std::locale()). Use boost::bind(&std::tolower, _1, std::locale()) instead.