Case insensitive std::set of strings - c++

Question

How do you have a case insensitive insertion Or search of a string in std::set?
For example-
std::set<std::string> s;
s.insert("Hello");
s.insert("HELLO"); //not allowed, string already exists.

Answer 1

You need to define a custom comparator:
struct InsensitiveCompare {
bool operator() (const std::string& a, const std::string& b) const {
return strcasecmp(a.c_str(), b.c_str()) < 0;
}
};
std::set<std::string, InsensitiveCompare> s;
You may try stricmp or strcoll if strcasecmp is not available.

Answer 2

std::set offers the possibility of providing your own comparer (as do most std containers). You can then perform any type of comparison you like. Full example is available here

Answer 3

This is a generic solution that also works with other string types than std::string (tested with std::wstring, std::string_view, char const*). Basically anything that defines a range of characters should work.
The key point here is to use boost::as_literal that allows us to treat null-terminated character arrays, character pointers and ranges uniformly in the comparator.
Generic code ("iset.h"):
#pragma once
#include <set>
#include <algorithm>
#include <boost/algorithm/string.hpp>
#include <boost/range/as_literal.hpp>
// Case-insensitive generic string comparator.
struct range_iless
{
template< typename InputRange1, typename InputRange2 >
bool operator()( InputRange1 const& r1, InputRange2 const& r2 ) const
{
// include the standard begin() and end() aswell as any custom overloads for ADL
using std::begin; using std::end;
// Treat null-terminated character arrays, character pointers and ranges uniformly.
// This just creates cheap iterator ranges (it doesn't copy container arguments)!
auto ir1 = boost::as_literal( r1 );
auto ir2 = boost::as_literal( r2 );
// Compare case-insensitively.
return std::lexicographical_compare(
begin( ir1 ), end( ir1 ),
begin( ir2 ), end( ir2 ),
boost::is_iless{} );
}
};
// Case-insensitive set for any Key that consists of a range of characters.
template< class Key, class Allocator = std::allocator<Key> >
using iset = std::set< Key, range_iless, Allocator >;
Usage example ("main.cpp"):
#include "iset.h" // above header file
#include <iostream>
#include <string>
#include <string_view>
// Output range to stream.
template< typename InputRange, typename Stream, typename CharT >
void write_to( Stream& s, InputRange const& r, CharT const* sep )
{
for( auto const& elem : r )
s << elem << sep;
s << std::endl;
}
int main()
{
iset< std::string > s1{ "Hello", "HELLO", "world" };
iset< std::wstring > s2{ L"Hello", L"HELLO", L"world" };
iset< char const* > s3{ "Hello", "HELLO", "world" };
iset< std::string_view > s4{ "Hello", "HELLO", "world" };
write_to( std::cout, s1, " " );
write_to( std::wcout, s2, L" " );
write_to( std::cout, s3, " " );
write_to( std::cout, s4, " " );
}
Live Demo at Coliru

Answer 4

From what I have read this is more portable than stricmp() because stricmp() is not in fact part of the std library, but only implemented by most compiler vendors. As a result below is my solution to just roll your own.
#include <string>
#include <cctype>
#include <iostream>
#include <set>
struct caseInsensitiveLess
{
bool operator()(const std::string& x, const std::string& y)
{
unsigned int xs ( x.size() );
unsigned int ys ( y.size() );
unsigned int bound ( 0 );
if ( xs < ys )
bound = xs;
else
bound = ys;
{
unsigned int i = 0;
for (auto it1 = x.begin(), it2 = y.begin(); i < bound; ++i, ++it1, ++it2)
{
if (tolower(*it1) < tolower(*it2))
return true;
if (tolower(*it2) < tolower(*it1))
return false;
}
}
return false;
}
};
int main()
{
std::set<std::string, caseInsensitiveLess> ss1;
std::set<std::string> ss2;
ss1.insert("This is the first string");
ss1.insert("THIS IS THE FIRST STRING");
ss1.insert("THIS IS THE SECOND STRING");
ss1.insert("This IS THE SECOND STRING");
ss1.insert("This IS THE Third");
ss2.insert("this is the first string");
ss2.insert("this is the first string");
ss2.insert("this is the second string");
ss2.insert("this is the second string");
ss2.insert("this is the third");
for ( auto& i: ss1 )
std::cout << i << std::endl;
std::cout << std::endl;
for ( auto& i: ss2 )
std::cout << i << std::endl;
}
Output with case insensitive set and regular set showing the same
ordering:
This is the first string
THIS IS THE SECOND STRING
This IS THE Third
this is the first string
this is the second string
this is the third