bench.cpp

#include <cctype>
#include <cstdio>
#include <string>
#include <algorithm>

std::string clean_string1(std::string w) {
    w.erase(std::remove_if(w.begin(), w.end(), [](char &c) { return !isalpha(c); }), w.end());
    std::transform(w.begin(), w.end(), w.begin(), tolower);

    return w;
}

std::string clean_string2(std::string w) {
    w.erase(std::remove_if(w.begin(), w.end(), [](char &c) -> bool {
        if (!isalpha(c)) {
            return true;
        }
        if (!islower(c)) {
            c = tolower(c);
        }

        return false;
    }), w.end());

    return w;
}

std::string clean_string3(std::string w) {
    auto it = w.begin();
    for (auto jt = w.begin(); jt != w.end(); ++jt) {
        if (!isalpha(*jt)) { continue; }
        *it++ = tolower(*jt);
    }
    w.erase(it, w.end());
    return w;
}

std::string clean_string4(std::string w) {
    int i = 0;
    for (int j = 0; j < (int)w.size(); ++j) {
        if (!isalpha(w[j])) { continue; }
        w[i++] = tolower(w[j]);
    }
    w.erase(w.begin() + i, w.end());
    return w;
}

int main() {
    const int N = 50;
    const int LOOPS = 200000;

    std::string chunk("abcdefghijklmnopqrstuvwxyz");
    std::string s;
    for (int i = 0; i < N; ++i) {
        s += chunk;
    }

    for (int i = 0; i < LOOPS; ++i) {
        std::string x(s);
        x = clean_string3(x);
    }
}