第一题，信息增益代码 #include <bits/stdc++.h> #include <map> #include <math.h> using namespace std; typedef struct data{     int clas[2]; }data; int n; map<int,data> mp; int classify[2] = {0}; double cpt_entroy(int m[]){     int nu = m[0] + m[1];     double first = 0;     double second = 0;     if(m[0] != 0){         double tmp = 1.0 * m[0] / nu;         first = -1 * tmp * log(tmp) / log(2);     }     if(m[1] != 0){         double tmp = 1.0 * m[1] / nu;         second = -1 * tmp * log(tmp) / log(2);     }     double entroy = 1.0 * nu / n * (first + second);     return entroy; } int main() {     int a, b;     char indep;     cin >> n;     for(int i=0; i<n; ++i){         cin>>a>>indep>>b;         classify[b]++;         if(mp.find(a) == mp.end())             mp[a].clas[b] = 0;         mp[a].clas[b] += 1;     }     double gain_all = cpt_entroy(classify);     double ans = gain_all;     map<int, data>::iterator it;     for(it = mp.begin(); it != mp.end(); ++it) {         double pro = cpt_entroy(it->second.clas);         ans -= pro;     }     int dans = (ans + 0.005) * 100;     double res = double(dans) / 100;     printf("%.2f\n",res);     return 0; }