Language: C++

#include <algorithm>
#include <bitset>
#include <cmath>
#include <complex>
#include <cstdio>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;

// 最大公約数
ll GCD(ll a, ll b)
{
    if (a < 0)
        a = -a;
    if (b < 0)
        b = -b;
    if (b == 0)
        return a;
    else
        return GCD(b, a % b);
}

// 最小公倍数
ll LCM(ll a, ll b)
{
    ll g = GCD(a, b);
    return a / g * b;
}

int main()
{
    string in;
    cin >> in;
    int ret = 0;
    if (in[0] == "C")
    {
        ret++;
    }
    if (in[1] == "O")
    {
        ret++;
    }
    if (in[2] == "D")
    {
        ret++;
    }
    if (in[3] == "E")
    {
        ret++;
    }
    if (in[4] == "F")
    {
        ret++;
    }
    if (in[5] == "E")
    {
        ret++;
    }
    if (in[6] == "S")
    {
        ret++;
    }
    if (in[7] == "T")
    {
        ret++;
    }
    if (in[8] == "I")
    {
        ret++;
    }
    if (in[9] == "V")
    {
        ret++;
    }
    if (in[10] == "A")
    {
        ret++;
    }
    if (in[11] == "L")
    {
        ret++;
    }
    if (in[12] == "2")
    {
        ret++;
    }
    if (in[13] == "0")
    {
        ret++;
    }
    if (in[14] == "1")
    {
        ret++;
    }
    if (in[15] == "6")
    {
        ret++;
    }
    cout << ret << endl;
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:51:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   51 |     if (in[0] == "C")
a.cc:55:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   55 |     if (in[1] == "O")
a.cc:59:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   59 |     if (in[2] == "D")
a.cc:63:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   63 |     if (in[3] == "E")
a.cc:67:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   67 |     if (in[4] == "F")
a.cc:71:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   71 |     if (in[5] == "E")
a.cc:75:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   75 |     if (in[6] == "S")
a.cc:79:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   79 |     if (in[7] == "T")
a.cc:83:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   83 |     if (in[8] == "I")
a.cc:87:15: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   87 |     if (in[9] == "V")
a.cc:91:16: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   91 |     if (in[10] == "A")
a.cc:95:16: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   95 |     if (in[11] == "L")
a.cc:99:16: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
   99 |     if (in[12] == "2")
a.cc:103:16: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
  103 |     if (in[13] == "0")
a.cc:107:16: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
  107 |     if (in[14] == "1")
a.cc:111:16: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
  111 |     if (in[15] == "6")