rail-fence-cipher-encrypt.cpp

#include <cmath>
#include <iostream>
using namespace std;

int main()
{
    string s;
    cout << "Plain text: ";
    getline(cin, s);
    for (auto it = s.begin(); it != s.end();) {
        char c = tolower(*it);
        if (c >= 'a' && c <= 'z') {
            *it = c;
            it++;
        } else {
            s.erase(it);
        }
    }

    for (int i = 0, t = ceil(s.size() / 4.0); i != t; ++i) {
        cout << s[4 * i];
    }

    for (int i = 0, t = s.size() / 2; i != t; ++i) {
        cout << s[2 * i + 1];
    }

    for (int i = 0, t = ceil(max((int)s.size() - 2, 0) / 4.0); i != t; ++i) {
        cout << s[4 * i + 2];
    }

    return 0;
}