Dave's Brain

Browse - programming tips - stl case insensitive map

Date: 2007nov27
Language: C++

Q.  How can I make a STL map with a case insensitive string for key?

A.  There are several ways.

1.  You can make your own string-like class where the compares
are not case sensitive.

2.  You can use traits to make std::string case insensitive.

3.  You can supply a 3rd parameter for comparing to the <map> template
as we show here...

Source: http://www.gammon.com.au/forum/bbshowpost.php?bbsubject_id=2902

// disable warnings about long names
#ifdef WIN32
  #pragma warning( disable : 4786)
#endif

#include <string>
#include <map>
#include <iostream>

using namespace std;

// case-independent (ci) string less_than
// returns true if s1 < s2
struct ci_less : binary_function<string, string, bool>
{

	// case-independent (ci) compare_less binary function
	struct nocase_compare : public binary_function<unsigned char,unsigned
char,bool> 
	{
		bool operator() (const unsigned char& c1, const unsigned char& c2) const 
		{ return tolower (c1) < tolower (c2); }
	};

	bool operator() (const string & s1, const string & s2) const
	{
		return lexicographical_compare 
			(s1.begin (), s1.end (),   // source range
			s2.begin (), s2.end (),   // dest range
			nocase_compare ());  // comparison
	}
}; // end of ci_less

// OR a simpler (and probably faster) version (by dave):
struct ci_less
{
  bool operator() (const string & s1, const string & s2) const
  {
	return stricmp(s1.c_str(), s2.c_str()) < 0;
  }
};


typedef map<string, int, ci_less> age_map;

int main()
{
	// make a map of people
	age_map people;

	// add items to list
	people ["Nick"] = 28;
	people ["John"] = 14;
	people ["Mary"] = 88;

	// find someone by key
	cout << "Finding person 'nick' ..." << endl;

	age_map::const_iterator i = people.find ("nick");

	if (i == people.end ())
		cout << "Not found." << endl;
	else
		cout << "Found age = " << i->second << endl;

	return 0;
} // end of main

Output

Finding person 'nick' ...
Found age = 28

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