5.3.1

5.5
@Mooophy

#include <iostream>
#include <vector>
#include <string>
using std::vector; using std::string; using std::cout; using std::endl; using std::cin;

int main()
{
    vector<string> scores = { "F", "D", "C", "B", "A", "A++" };
    for (int g; cin >> g;)
    {
        string letter;
        if (g < 60)
        {
            letter = scores[0];
        }
        else
        {
            letter = scores[(g - 50) / 10];
            if (g != 100)
                letter += g % 10 > 7 ? "+" : g % 10 < 3 ? "-" : "";
            cout << letter << endl;
        }
    }

    return 0;
}

5.6

#include <iostream>
#include <vector>
#include <string>
using std::vector; using std::string; using std::cout; using std::endl; using std::cin;

int main()
{
    vector<string> scores = { "F", "D", "C", "B", "A", "A++" };

    int grade = 0;
    while (cin >> grade)
    {
        string lettergrade = grade < 60 ? scores[0] : scores[(grade - 50) / 10];
        lettergrade += (grade == 100 || grade < 60) ? "" : (grade % 10 > 7) ? "+" : (grade % 10 < 3) ? "-" : "";
        cout << lettergrade << endl;
    }

    return 0;
}

5.7

(a) if (ival1 != ival2) ival1 = ival2  //缺少一个分号
    else ival1 = ival2 = 0;
(b) if (ival < minval) minval = ival;
    occurs = 1;  //缺少花括号括起来
(c) if (int ival = get_value())
    cout << "ival = " << ival << endl;
    if (!ival)     //要使用else if ？
    cout << "ival = 0\n";
(d) if (ival = 0)   //使用  ==
    ival = get_value();  

5.8
经常会出现 if 比else多的情况

else 一般和离它最近的if 匹配成一对