5.3.1

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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

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