Question

Check the following code:

string toLowerCase(const string& str) {
    string res(str);
    int i;

    for (i = 0; i < (int) res.size(); i++)
        res[i] = (char) tolower(res[i]);

    return res;
}

class LeagueComparator
{
public:
    bool operator()(const string& s1, const string& s2)
    {
        return toLowerCase(s1) < toLowerCase(s2);
    }
};

int main()
{
    set<string, LeagueComparator> leagues;
    set<string, LeagueComparator>::iterator iter;

    leagues.insert("BLeague");
    leagues.insert("aLeague");    // leagues = {"aLeague", "BLeague"}
    leagues.insert("ALeague");

    for (iter = leagues.begin(); iter != leagues.end(); iter++)
        cout << *iter << endl;

    return 0;
}

The output is:

aLeague
BLeague

which is shocking to me. I thought (and expecting) the output would be:

aLeague
ALeague
BLeague

Before the execution of leagues.insert("ALeague");, the leagues contains "aLeague" and "BLeague". My question is, while executing leagues.insert("ALeague"); why the machine treats "ALeague" == "aleague"? According to my understanding, there is no element "ALeague" in leagues. So "ALeague" should be inserted into leagues. The comparator should determine where to put "ALeague".

Thanks in advance.

PS: Please don’t hit me for using C style cast. 😛 I’m too lazy to type static_cast.

Answer 1

Your comparator, thanks to the toLowerCase, says that "aLeague" == "ALeague". Since (according to your comparator) "aLeague" < "ALeague" == false and "ALeague" < "aLeague" == false, they must be equivalent. And inserting an equivalent element into a set doesn’t do anything.