entropy.cpp

#include <iostream>
#include <set>
#include <vector>
#include <string>
#include <cmath>

using namespace std;

double entropy(string& st) {
    vector<char> stvec(st.begin(), st.end());
    set<char> alphabet(stvec.begin(), stvec.end());
    vector<double> freqs;
    for (set<char>::iterator c = alphabet.begin(); c != alphabet.end(); ++c) {
        int ctr = 0;
        for (vector<char>::iterator s = stvec.begin(); s != stvec.end(); ++s) {
            if (*s == *c) {
                ++ctr;
            }
        }
        freqs.push_back((double)ctr / (double)stvec.size());
    }
    double ent = 0;
    double ln2 = log(2);
    for (vector<double>::iterator f = freqs.begin(); f != freqs.end(); ++f) {
        ent += *f * log(*f)/ln2;
    }
    ent = -ent;
    return ent;
}

int main(int argc, char** argv) {
    if (argc < 2) {
        cout << "prints the base2 entropy of the argument" << endl;
    }
    string input = string(argv[1]);
    cout << entropy(input) << endl;
}