《C++primer(第五版)》学习之路-第五章：语句
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5.1 简单语句
1.绝大多数语句以分号结束。空语句只有一个单独的分号。
2.复合语句是指用花括号括起来的语句和声明的序列，复合语句也被称作块。
一个块就是一个作用域，在块中引入的名字只能在块内部以及嵌套在块中的子块里访问。
5.2 语句作用域
1.可以在if,switch,for和while语句的控制结构内定义变量。定义在控制结构当中的变量只在相应语句的内部可见，一旦语句结束，变量也就超出其作用范围了。
5.3 条件语句
1.C++提供了两种按条件执行的语句。
一种是if语句，它根据条件决定控制流
一种是switch语句，它计算一个整型表达式的值，然后根据这个值从几条执行路径中选择一条。
2.if语句的作用是：判断一个指定的条件是否为真，根据判断结果决定是否执行另外一条语句。
3.switch语句首先对括号里的表达式求值，表达式值转换成整数类型，然后与每个case标签的值比较。
case关键字和它对应的值一起被成为case标签。case标签必须是整型常量表达式。
4.如果没有任何一个case标签能匹配上switch表达式的值，程序将执行紧跟在default标签后面的语句。
5.4 迭代语句
1.迭代语句通常称为循环，它重复执行操作直到满足某个条件才停下来。while和for语句在执行循环之前检查条件，do while先执行循环体，然后再检查条件。
2.范围for语句：C++11新标准引入了一种更简单的for语句，这种语句可以遍历容器或者其他序列的所有元素。
范围for语句的语法形式是：
for(declaration: expression)
statement;
5.5 跳转语句
1.C++提供了4种跳转语句：break,continue,goto和return。
2.break语句负责终止离它最近的while、do while、for或switch语句，并从这些语句之后的第一条语句开始继续执行。
3. continue语句终止最近的循环中的当前迭代并立即开始下一次迭代。
4.goto语句的作用是从goto语句无条件跳转到同一函数的另一条语句。
5.6 try语句块和异常处理
1.在C++语言中，异常处理包括：
throw表达式，异常检测部分使用throw表达式来表示它遇到了无法处理的问题。我们说throw引发了异常。
try语句块，异常处理部分使用try语句块处理异常。try语句块以关键字try开始，并以一个或多个catch子句结束。try语句块中代码抛出的异常通常会被某个catch子句处理。因为catch子句“处理”异常，所以它们也被称作异常处理代码。
一套异常类，用于在throw表达式和相关的catch子句之间传递异常的具体信息。
2.程序的异常检测部分使用throw表达式引发一个异常。throw表达式包含关键字throw和紧随其后的一个表达式，其中表达式的类型就是抛出的异常类型。
3.try语句块的通用语法形式是
try{
program-statements;
}catch(exception-declaration){
handler-statements;
}catch(exception-declaration){
handler-statements;
} //...跟在try块之后的是一个或多个catch子句。catch子句包括三部分：关键字catch、括号内一个(可能是未命名的)对象声明(称作异常声明)以及一个块。当选中了某个catch子句处理异常之后，执行与之对应的块。catch一旦完成，程序跳转到try语句块最后一个catch子句之后的那条语句继续执行。
try语句块中的program-statements组成程序的正常逻辑，像其他任何块一样，program-statements可以有包括声明在内的任意C++语句。
4.C++标准库定义了一组类，用于报告标准库函数遇到的问题。这些异常类也可以在用户编写的程序中使用，它们分别定义在四个头文件中：
exception头文件定义了最通常的异常类exception。它只报告异常的发生，不提供额外信息。
stdexcept头文件定义了几中常用的异常类。
new头文件定义了bad_alloc异常类型。
type_info头文件定义了bad_cast异常类型。
5.
exception 最常见的问题
runtime_error 运行时错误：仅在运行时才能检测到得错误
range_error 运行时错误：生成的结果超出了有意义的值域范围
overflow_error 运行时错误：计算上溢
underflow_error 运行时错误 ：计算下溢
logic_error 逻辑错误：可在运行前检测到得问题
domain_error 逻辑错误：参数的结果值不存在
invalid_argument 逻辑错误：不合适的参数
length_error 逻辑错误：试图生成 一个超出改类型最大长度的对象
out_of_range 逻辑错误：使用一个超出有效范围的值
PS：部分练习答案
练习5.3
#include
int main()
{
int sum = 0, val = 1;
while (val <= 10)
sum += val, ++val;
std::cout << "Sum of 1 to 10 inclusive is "<< sum << std::endl;
return 0;
} 练习5.4
(a)迭代器没有指向
#include
int main()
{
int sum = 0, val = 1;
while (val <= 10)
sum += val, ++val;
std::cout << "Sum of 1 to 10 inclusive is "<< sum << std::endl;
return 0;
} (b)if不在while循环里面，因此if里的status没有定义
bool status;
while ((status = find(word))) {/* ... */}
if (!status) {/* ... */}练习5.5
#include
#include
#include
using std::vector;
using std::string;
using std::cout;
using std::endl;
using std::cin;
int main()
{
vector scores = {"F", "D", "C", "B", "A", "A++"};
int grade {0};
while (cin >> grade)
{
string lettergrade;
if (grade < 60)
lettergrade = scores[0];
else
{
lettergrade = scores[(grade - 50) / 10];
if (grade != 100)
{
if (grade % 10 > 7)
lettergrade += "+";
else if (grade % 10 < 3)
lettergrade += "-";
}
}
cout << lettergrade << endl;
}
return 0;
} 练习5.6
#include
#include
#include
using std::vector;
using std::string;
using std::cout;
using std::endl;
using std::cin;
int main()
{
vector 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;
} 练习5.7
(a) if (ival1 != ival2) ival1 = ival2;
else ival1 = ival2 = 0;
(b) if (ival < minval)
{
minval = ival;
occurs = 1;
}
(c) int val;
if (ival = get_value())
cout << "ival = " << ival << endl;
if (!ival)
cout << "ival = 0\n";
(d) if (ival == 0)
ival = get_value();练习5.9
#include
using std::cout;
using std::endl;
using std::cin;
int main()
{
unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0;
char ch;
while (cin >> ch)
{
if (ch == 'a')
++aCnt;
else if (ch == 'e')
++eCnt;
else if (ch == 'i')
++iCnt;
else if (ch == 'o')
++oCnt;
else if (ch == 'u')
++uCnt;
}
cout << "Number of vowel a: \t" << aCnt << "\n"
<< "Number of vowel e: \t" << eCnt << "\n"
<< "Number of vowel i: \t" << iCnt << "\n"
<< "Number of vowel o: \t" << oCnt << "\n"
<< "Number of vowel u: \t" << uCnt << endl;
return 0;
} 练习5.10
#include
using std::cin;
using std::cout;
using std::endl;
int main()
{
unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0;
char ch;
while (cin >> ch)
switch (ch)
{
case 'a':
case 'A':
++aCnt;
break;
case 'e':
case 'E':
++eCnt;
break;
case 'i':
case 'I':
++iCnt;
break;
case 'o':
case 'O':
++oCnt;
break;
case 'u':
case 'U':
++uCnt;
break;
}
cout << "Number of vowel a(A): \t" << aCnt << "\n"
<< "Number of vowel e(E): \t" << eCnt << "\n"
<< "Number of vowel i(I): \t" << iCnt << "\n"
<< "Number of vowel o(O): \t" << oCnt << "\n"
<< "Number of vowel u(U): \t" << uCnt << endl;
return 0;
}
练习5.11
#include
using std::cin;
using std::cout;
using std::endl;
int main()
{
unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0, spaceCnt = 0,tabCnt = 0, newLineCnt = 0;
char ch;
while (cin >> std::noskipws >> ch)
switch (ch)
{
case 'a':
case 'A':
++aCnt;
break;
case 'e':
case 'E':
++eCnt;
break;
case 'i':
case 'I':
++iCnt;
break;
case 'o':
case 'O':
++oCnt;
break;
case 'u':
case 'U':
++uCnt;
break;
case ' ':
++spaceCnt;
break;
case '\t':
++tabCnt;
break;
case '\n':
++newLineCnt;
break;
}
cout << "Number of vowel a(A): \t" << aCnt << "\n"
<< "Number of vowel e(E): \t" << eCnt << "\n"
<< "Number of vowel i(I): \t" << iCnt << "\n"
<< "Number of vowel o(O): \t" << oCnt << "\n"
<< "Number of vowel u(U): \t" << uCnt << "\n"
<< "Number of space: \t" << spaceCnt << "\n"
<< "Number of tab char: \t" << tabCnt << "\n"
<< "Number of new line: \t" << newLineCnt << endl;
return 0;
} 练习5.12
#include
using std::cin;
using std::cout;
using std::endl;
int main()
{
unsigned aCnt = 0, eCnt = 0, iCnt = 0, oCnt = 0, uCnt = 0, spaceCnt = 0,tabCnt = 0, newLineCnt = 0, ffCnt = 0, flCnt = 0, fiCnt = 0;
char ch, prech = '\0';
while (cin >> std::noskipws >> ch) {
switch (ch) {
case 'a':
case 'A':
++aCnt;
break;
case 'e':
case 'E':
++eCnt;
break;
case 'i':
if (prech == 'f') ++fiCnt;
case 'I':
++iCnt;
break;
case 'o':
case 'O':
++oCnt;
break;
case 'u':
case 'U':
++uCnt;
break;
case ' ':
++spaceCnt;
break;
case '\t':
++tabCnt;
break;
case '\n':
++newLineCnt;
break;
case 'f':
if (prech == 'f') ++ffCnt;
break;
case 'l':
if (prech == 'f') ++flCnt;
break;
}
prech = ch;
}
cout << "Number of vowel a(A): \t" << aCnt << "\n"
<< "Number of vowel e(E): \t" << eCnt << "\n"
<< "Number of vowel i(I): \t" << iCnt << "\n"
<< "Number of vowel o(O): \t" << oCnt << "\n"
<< "Number of vowel u(U): \t" << uCnt << "\n"
<< "Number of space: \t" << spaceCnt << "\n"
<< "Number of tab char: \t" << tabCnt << "\n"
<< "Number of new line: \t" << newLineCnt << "\n"
<< "Number of ff: \t" << ffCnt << "\n"
<< "Number of fl: \t" << flCnt << "\n"
<< "Number of fi: \t" << fiCnt << endl;
return 0;
} 练习5.13
(a) //缺少break
unsigned aCnt = 0, eCnt = 0, iouCnt = 0;
char ch = next_text();
switch (ch) {
case 'a': aCnt++; break;
case 'e': eCnt++; break;
default : iouCnt++; break;
}
(b) //ix应该定义在外部
unsigned index = some_value();
int ix;
switch (index) {
case 1:
ix = get_value();
ivec[ ix ] = index;
break;
default:
ix = static_cast(ivec.size())-1;
ivec[ ix ] = index;
}
(c) //case语法错误
unsigned evenCnt = 0, oddCnt = 0;
int digit = get_num() % 10;
switch (digit) {
case 1: case 3: case 5: case 7: case 9:
oddcnt++;
break;
case 2: case 4: case 6: case 8: case 0:
evencnt++;
break;
}
(d) //case标签必须是常数
const unsigned ival=512, jval=1024, kval=4096;
unsigned bufsize;
unsigned swt = get_bufCnt();
switch(swt) {
case ival:
bufsize = ival * sizeof(int);
break;
case jval:
bufsize = jval * sizeof(int);
break;
case kval:
bufsize = kval * sizeof(int);
break;
} 练习5.14
#include
#include
using std::cout;
using std::cin;
using std::endl;
using std::string;
int main()
{
string str,max_str,pre_str;
int max_cnt = 0;
int count = 0;
max_str = pre_str = "";
while(cin>>str)
{
if(str==pre_str)
++count;
else
count = 1;
if(count>max_cnt)
{
max_str = str;
max_cnt = count;
}
pre_str = str;
}
if (max_str == "")
cout << "There's no duplicated string." << endl;
else
cout << "the word " << max_str << " occurred " << max_cnt << " times. " << endl;
return 0;
} 练习5.15
(a) int ix;
for (ix = 0; ix != sz; ++ix) { /* ... */ }
if (ix != sz)
// . . .
(b) int ix;
for (; ix != sz; ++ix) { /* ... */ }
(c) for (int ix = 0; ix != sz; ++ix) { /*...*/ }练习5.17
#include
#include
using std::cout;
using std::endl;
using std::vector;
int main()
{
vector vec1 {0, 1, 1, 2};
vector vec2 {0, 1, 1, 2, 3, 5, 8};
auto size = vec1.size() < vec2.size() ? vec1.size() : vec2.size();
for (decltype(vec1.size()) i = 0; i != size; ++i)
{
if (vec1[i] != vec2[i])
{
cout << "false" << endl;
break;
}
if (i == size - 1)
cout << "true" << endl;
}
return 0;
} 练习5.18
(a) do {
int v1, v2;
cout << "Please enter two numbers to sum:" ;
if (cin >> v1 >> v2)
cout << "Sum is: " << v1 + v2 << endl;
}while (cin);
(b) int ival;
do {
// . . .
} while (ival = get_response());
(c) int ival = get_response();
do {
ival = get_response();
} while (ival); 练习5.19
#include
#include
using std::cout;
using std::cin;
using std::endl;
using std::string;
int main()
{
string rsp;
do
{
cout << "Input two strings: ";
string str1, str2;
cin >> str1 >> str2;
cout << (str1 <= str2 ? str1 : str2) << " is less than the other. "<< "\n\n"<< "More? Enter yes or no: ";
cin >> rsp;
}
while (!rsp.empty() && rsp[0] == 'y');
return 0;
} 练习5.20
#include
#include
using std::cout;
using std::cin;
using std::endl;
using std::string;
int main()
{
string str, pre;
while (cin >> str)
if (str == pre)
break;
else
pre = str;
if (cin.eof())
cout << "no word was repeated." << endl;
else
cout << read << " occurs twice in succession." << endl;
return 0;
} 练习5.21
#include
using std::cin;
using std::cout;
using std::endl;
#include
using std::string;
int main()
{
string curr, prev;
bool no_twice = false;
while (cin >> curr)
{
if (!isupper(curr[0]))
continue;
else if (prev == curr)
{
cout << curr << " occurs twice in succession." << endl;
no_twice = true;
break;
}
else
prev = curr;
}
if (!no_twice) cout << "no word was repeated." << endl;
} 练习5.22
for (int sz = get_size(); sz <=0; sz = get_size())
; 练习5.23
#include
int main(void)
{
int a, b;
std::cin >> a >> b;
std::cout << static_cast(a) / b << std::endl;
return 0;
} 练习5.24
#include
#include
int main(void)
{
int a, b;
std::cin >> a >> b;
if (b == 0) throw std::runtime_error("divisor is 0");
std::cout << static_cast(a) / b << std::endl;
return 0;
} 练习5.25
#include
#include
using std::cin;
using std::cout;
using std::endl;
using std::runtime_error;
int main(void)
{
int a, b;
cout << "Input two integers: ";
while (cin >> a >> b)
{
try
{
if (b == 0) throw runtime_error("divisor is 0");
cout << static_cast(a) / b << endl;
cout << "Input two integers: ";
}
catch (runtime_error err)
{
cout << err.what() << "\n" << "Try again.\nInput two integers: ";
}
}
return 0;
}