4.7

4.21

#include <iostream>
#include <vector>

using namespace std;
int main()
{
    vector<int> ivec{ 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    for (auto i : ivec) {
        cout << ((i % 2) ? i : i * 2) << " ";
    }
    cout << endl;

    return 0;
}

4.22

#include <iostream>
using namespace std;

int main()
{
    for (unsigned g; cin >> g; )
    {

        auto result = g > 90 ? "high pass" : g < 60 ? "fail" : g < 75 ? "low pass" : "pass";
        cout << result << endl;


        if (g > 90)         cout << "high pass";
        else if (g < 60)    cout << "fail";
        else if (g < 75)    cout << "low pass";
        else                cout << "pass";
        cout << endl;
    }

    return 0;
}

if更容易理解 ，但是用条件运算符，代码会简洁很多，倾向于使用条件运算符

4.23

string s = "word";
string pl = s + s[s.size() - 1] == 's' ? "" : "s" ;

改正：

string pl = s + (s[s.size() - 1] == 's' ? "" : "s") ;

当你不确定优先级的时候，请使用括号来明确表达你的意图

4.24
boring