《C++primer(第五版)》学习之路-第五章：语句

【 声明：版权所有，转载请标明出处，请勿用于商业用途。  联系信箱：[e-mail]】

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;} //...

try语句块一开始是关键字try，随后紧跟着一个块，这个块就像大多数时候那样是花括号括起来的语句序列。

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.<stdexcept>定义的异常类

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 <iostream>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 <iostream>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) {/* ... */}

事实上这个if是没有必要的，因为一旦while结束吗，我们可以知道此事status必然为false

练习5.5

#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;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 <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;}

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 <iostream>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 <iostream>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 <iostream>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 <iostream>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<int>(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 <iostream>#include <string>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;elsecount = 1;if(count>max_cnt){max_str = str;max_cnt = count;}pre_str = str;}if (max_str == "")cout << "There's no duplicated string." << endl;elsecout << "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 <iostream>#include <vector>using std::cout;using std::endl;using std::vector;int main(){vector<int> vec1 {0, 1, 1, 2};vector<int> 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 <iostream>#include <string>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 <iostream>#include <string>using std::cout;using std::cin;using std::endl;using std::string;int main(){string str, pre;while (cin >> str)if (str == pre)break;elsepre = str;if (cin.eof())cout << "no word was repeated." << endl;elsecout << read << " occurs twice in succession." << endl;return 0;}

练习5.21

#include <iostream>using std::cin;using std::cout;using std::endl;#include <string>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;}elseprev = 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 <iostream>int main(void){int a, b;std::cin >> a >> b;std::cout << static_cast<double>(a) / b << std::endl;return 0;}

练习5.24

#include <iostream>#include <stdexcept>int main(void){    int a, b;    std::cin >> a >> b;    if (b == 0) throw std::runtime_error("divisor is 0");    std::cout << static_cast<double>(a) / b << std::endl;    return 0;}

练习5.25

#include <iostream>#include <stdexcept>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<double>(a) / b << endl;cout << "Input two integers: ";}catch (runtime_error err){cout << err.what() << "\n" << "Try again.\nInput two integers: ";}}return 0;}