概述
5.1
空语句：只含有一个单独的分号；如果在程序的某个地方，语法上需要一条语句但是逻辑上不需要，此时应该使用空语句。
5.2
块：复合语句，用花括号括起来的语句和声明的序列；如果在程序的某个地方，语法上需要一句语句，但是逻辑上需要多条语句，则应该用复合语句。
5.3
可读性降低了
1 #include <iostream>
2 using namespace std;
3
4 int main()
5 {
6 int val = 0, sum;
7
8 while (val <= 10)
9 sum += val, ++val;
10 }
5.8
悬垂else：if分支多余else分支；
C++规定else与离他最近地尚未匹配地if匹配，从而消除了程序地二义性；
5.9
1 #include <iostream>
2 using namespace std;
3
4 int main()
5 {
6 char ch;
7 int cst;
8 while (cin >> ch)
9 {
10 switch (ch)
11 {
12 case 'a': case'e': case 'i': case 'o': case 'u': ++cst;
13 }
14 }
15 cout << cst << endl;
16 }
~
5.12
解法一：
1 #include <iostream>
2 using namespace std;
3
4 int main()
5 {
6 char cur_char, bef_char = ' ';
7 int cun_ff = 0, cun_fi = 0, cun_fl = 0;
8 while (cin >> std::noskipws >> cur_char)
9 {
10 switch (cur_char)
11 {
12 case 'f':
13 if (bef_char == 'f')
14 ++cun_ff;
15 break;
16 case 'i':
17 if (bef_char == 'f')
18 ++cun_fi;
19 break;
20 case 'l':
21 if (bef_char == 'f')
22 ++cun_fl;
23 break;
24 default :break;
25 }
26 bef_char = cur_char;
27 }
28 cout << endl;
29
30 cout << "the number of ff is : " << cun_ff << endl;
31 cout << "the number of fi is : " << cun_fi << endl;
32 cout << "the number of fl is : " << cun_fl << endl;
33
34 }
35
解法二：
1 #include <iostream>
2
3 using namespace std;
4
5 int main()
6 {
7 unsigned ffCnt = 0, flCnt = 0, fiCnt = 0;
8 char str;
9 int flag = 0;
10 while (cin >> noskipws >> str)
11 {
12 switch (str)
13 {
14 case 'f':
15 if (flag == 1)
16 ++ffCnt;
17 else
18 flag = 1;
19 break;
20 case 'l':
21 if (flag == 1)
22 ++flCnt;
23 flag = 0;
24 break;
25 case 'i':
26 if (flag == 1)
27 ++fiCnt;
28 flag = 0;
29 break;
30 default:
31 flag = 0;
32 break;
33 }
34 }
35 cout << "the number of ff are : " << ffCnt << endl;
36 cout << "the number of fi are : " << fiCnt << endl;
37 cout << "the number of fl are : " << flCnt << endl;
38
5.13
- break;
- 如果在某处一个带有初值的变量位于作用域之外，在另一处该变量位于作用域之内，则从前一处跳转到后一处的行为是非法的；
- 每个case标签只能对应一个值；
- case标签必须是整数型常量表达式；
5.14
1 #include <iostream>
2 #include <string>
3 #include <vector>
4 using namespace std;
5
6 int main()
7 {
8 vector<string> v;
9 string cur_string;
10 //读取若干string对象
11 while (cin >> cur_string)
12 v.push_back(cur_string);
13 cout << endl;
14 //定义存储不重复string对象的vector和相应联系重复出现的次数vector
15 vector<int> cnt_string;
16 vector<string> sig_string;
17 auto temp = v.begin();//不重复string对象的迭代器
18 auto beg = v.begin();
19 //统计相应string对象和连续重复出现的次数
20 while (temp != v.end())
21 {
22 int i = 0;
23 for (beg = temp; beg != v.end(); ++beg)
24 {
25 if (*temp == *beg)
26 ++i;
27 else
28 break;
29 }
30 cnt_string.push_back(i);
31 sig_string.push_back(*temp);
32 temp = beg;
33 }
34 //遍历输出
35 for (auto &c : sig_string)
36 cout << c << ' ';
37 cout << endl;
38 for (auto &c : cnt_string)
39 cout << c << ' ';
40 cout << endl;
41 int max, max_i;
42 max = cnt_string[0];
43 max_i = 0;
44
45 for (decltype(cnt_string.size()) i = 1; i < cnt_string.size(); ++i)
46 {
47 if (max < cnt_string[i])
48 {
49 max = cnt_string[i];
50 max_i = i;
51 }
52 }
53 cout << sig_string[max_i] << ", has been appear continuely " << max << " times." << endl;
54
55
56 return 0;
57 }
5.15
1.if语句必为假；
2.for语句语法错误，缺少init-statement;
3.死循环；
5.16
如果知道循环次数，选择for,否则while;
5.17
1 #include <iostream>
2 #include <vector>
3 using namespace std;
4
5 template <typename T>
6 bool subvector(vector<T> vc0, vector<T> vc1)
7 { for (decltype(vc0.size()) i = 0; i < ((vc0.size() > vc1.size()) ? vc1.size() : vc0.size()); ++i)
8 if (vc0[i] != vc1[i])
9 return false;
10 return true;
11 }
12 int main()
13 {
14 vector<int> vc0{0, 1, 1, 2, 3};
15 vector<int> vc1{0, 1, 1, 2, 3, 5, 8};
16 cout << boolalpha << subvector(vc0, vc1) << endl;
17
18 return 0;
19 }
~
5.18
- 语句块，需要{}；
- ==
- ival块作用域，在函数体外面失效；
5.19
1 #include <iostream>
2 #include <string>
3 using namespace std;
4
5 int main()
6 {
7 do
8 {
9 string s1, s2;
10 cout << "Please enter two string : ";
11 cin >> s1 >> s2;
12 cout << ((s1 > s2) ? s2 : s1) << endl;
13 }while (cin);
14 }
15
5.20
1 #include <iostream>
2 #include <string>
3 using namespace std;
4
5 int main()
6 {
7 string cur_string, bef_string{};
8 bool flag = 1;
9 while (cin >> cur_string)
10 {
11 if (cur_string == bef_string)
12 {
13 cout << cur_string << " appear continuely." << endl;
14 flag = 0;
15 break;
16 }
17 bef_string = cur_string;
18 }
19
20 if (flag)
21 {
22 cout << "Not have anyone words appear continuely" << endl;
23 }
24
25
26 }
5.21
1 #include <iostream>
2 #include <string>
3 #include <cctype>
4 using namespace std;
5
6 int main()
7 {
8 string cur_string, bef_string{};
9 bool flag = 1;
10 while (cin >> cur_string)
11 {
12 if (cur_string == bef_string)
13 {
14 if (isupper(cur_string[0]))
15 {
16 cout << cur_string << " appear continuely." << endl;
17 flag = 0;
18 break;
19 }
20 }
21 bef_string = cur_string;
22 }
23
24 if (flag)
25 {
26 cout << "Not have anyone words appear continuely" << endl;
27 }
28
29
30 }
5.22
continue
5.23-5.25
1 #include <iostream>
2 #include <stdexcept>
3 using namespace std;
4
5 int main()
6 {
7 int it1, it2;
8 while (cin >> it1 >> it2)
9 {
10 try{
11 if (it2 == 0)
12 throw runtime_error("it2 = 0");
13 else
14 cout << it1 / it2 << endl;
15 }
16 catch (runtime_error err){
17 cout << err.what()
18 << "nTry Again? Enter y or n?" << endl;
19 char c;
20 cin >> c;
21 if (!cin || c == 'n')
22 break;
23 }
24
25 }
26 }
最后
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