log

#include<bits/stdc++.h>

using namespace std;

int main() {
    double a = 10;
    cout << log10(a) << endl;//lg

    cout << log(a) << endl;//ln
    double b = 100;
    cout << log(b)/log(a) << endl;//a为底
    return 0;
}

输出

1
2.30259
2