第5章 循环和关系表达式

for循环

表达式和语句

递增运算符和递减运算符：++和--

组合赋值运算符

复合语句（语句块）

逗号运算符

关系运算符：>  >= == <= < !=

while循环

typedef工具

do while循环

字符输入方法get()

文件尾条件

嵌套循环和二维数组

 

5.1 for循环

// forloop.cpp -- introducing the for loop
#include <iostream>
int main()
{
    using namespace std;
    int i;  // create a counter
//   initialize; test ; update
    for (i = 0; i < 5; i++)
        cout << "C++ knows loops.\n";
    cout << "C++ knows when to stop.\n";
    // cin.get();
    return 0;
}

[[e-mail]] chapter_5$ g++ forloop.cpp 
[[e-mail]] chapter_5$ ./a.out 
C++ knows loops.
C++ knows loops.
C++ knows loops.
C++ knows loops.
C++ knows loops.
C++ knows when to stop.

5.1.1 for循环的组成部分

// num_test.cpp -- use numeric test in for loop
#include <iostream>
int main()
{
    using namespace std;
    cout << "Enter the starting countdown value: ";
    int limit;
    cin >> limit;
    int i;
    for (i = limit; i; i--)     // quits when i is 0
        cout << "i = " << i << "\n";
    cout << "Done now that i = " << i << "\n";
    // cin.get();
    // cin.get();
    return 0;
}

[[e-mail]] chapter_5$ g++ num_test.cpp 
[[e-mail]] chapter_5$ ./a.out 
Enter the starting countdown value: 4
i = 4
i = 3
i = 2
i = 1
Done now that i = 0

// express.cpp -- values of expressions
#include <iostream>
int main()
{
    using namespace std;
    int x;

    cout << "The expression x = 100 has the value ";
    cout << (x = 100) << endl;
    cout << "Now x = " << x << endl;
    cout << "The expression x < 3 has the value ";
    cout << (x < 3) << endl;
    cout << "The expression x > 3 has the value ";
    cout << (x > 3) << endl;
    cout.setf(ios_base::boolalpha);   //a newer C++ feature
    cout << "The expression x < 3 has the value ";
    cout << (x < 3) << endl;
    cout << "The expression x > 3 has the value ";
    cout << (x > 3) << endl;
    /// cin.get();
    return 0; 
}

// express.cpp -- values of expressions
#include <iostream>
int main()
{
    using namespace std;
    int x;

    cout << "The expression x = 100 has the value ";
    cout << (x = 100) << endl;
    cout << "Now x = " << x << endl;
    cout << "The expression x < 3 has the value ";
    cout << (x < 3) << endl;
    cout << "The expression x > 3 has the value ";
    cout << (x > 3) << endl;
    cout.setf(ios_base::boolalpha);   //a newer C++ feature
    cout << "The expression x < 3 has the value ";
    cout << (x < 3) << endl;
    cout << "The expression x > 3 has the value ";
    cout << (x > 3) << endl;
    /// cin.get();
    return 0; 
}

5.1.2 回到for循环

// formore.cpp -- more looping with for
#include <iostream>
const int ArSize = 16;      // example of external declaration
int main()
{
    long long factorials[ArSize];
    factorials[1] = factorials[0] = 1LL;
    for (int i = 2; i < ArSize; i++)
        factorials[i] = i * factorials[i-1];
    for (int i = 0; i < ArSize; i++)
        std::cout << i << "! = " << factorials[i] << std::endl;
	// std::cin.get();
    return 0;
}

[[e-mail]] chapter_5$ g++ formore.cpp 
[[e-mail]] chapter_5$ ./a.out 
0! = 1
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880
10! = 3628800
11! = 39916800
12! = 479001600
13! = 6227020800
14! = 87178291200
15! = 1307674368000

5.1.3 修改步长

// bigstep.cpp -- count as directed
#include <iostream>
int main()
{
	using std::cout;    // a using declaration
    using std::cin;
    using std::endl;;
    cout << "Enter an integer: ";
    int by;
    cin >> by;
    cout << "Counting by " << by << "s:\n";
    for (int i = 0; i < 100; i = i + by)
        cout << i << endl;
    // cin.get();
    // cin.get();
    return 0;
}

[[e-mail]] chapter_5$ g++ bigstep.cpp 
[[e-mail]] chapter_5$ ./a.out 
Enter an integer: 12
Counting by 12s:
0
12
24
36
48
60
72
84
96

5.1.4 使用for循环访问字符串

// forstr1.cpp -- using for with a string
#include <iostream>
#include <string>
int main()
{
    using namespace std;
    cout << "Enter a word: ";
    string word;
    cin >> word;

    // display letters in reverse order
    for (int i = word.size() - 1; i >= 0; i--)
        cout << word[i];
    cout << "\nBye.\n";
    // cin.get();
    // cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ forstr1.cpp 
[[e-mail]] chapter_5$ ./a.out 
Enter a word: animal
lamina
Bye.

5.1.5 递增运算符（++）和递减运算符（--）

// plus_one.cpp -- the increment operator
#include <iostream>
int main()
{
    using std::cout;
    int a = 20;
    int b = 20;

    cout << "a   = " << a << ":   b = " << b << "\n";
    cout << "a++ = " << a++ << ": ++b = " << ++b << "\n";
    cout << "a   = " << a << ":   b = " << b << "\n";
    // std::cin.get();
	return 0;
}

[[e-mail]] chapter_5$ g++ plus_one.cpp 
[[e-mail]] chapter_5$ ./a.out 
a   = 20:   b = 20
a++ = 20: ++b = 21
a   = 21:   b = 21

int y = ++x; //change x, then assign to y
int y = x++; //assign to y, then change z

5.1.6 副作用和顺序点

语句中的分号是一个顺序点

 

5.1.7 前缀格式和后缀格式

5.1.8 递增/递减运算符和指针

5.1.9 组合赋值运算符

5.1.10 复合语句（语句块）

// block.cpp -- use a block statement
#include <iostream>
int main()
{
    using namespace std;
    cout << "The Amazing Accounto will sum and average ";
    cout << "five numbers for you.\n";
    cout << "Please enter five values:\n";
    double number;
    double sum = 0.0;
    for (int i = 1; i <= 5; i++)
    {                                   // block starts here
        cout << "Value " << i << ": ";
        cin >> number;
        sum += number;
    }                                   // block ends here
    cout << "Five exquisite choices indeed! ";
    cout << "They sum to " << sum << endl;
    cout << "and average to " << sum / 5 << ".\n";
    cout << "The Amazing Accounto bids you adieu!\n";
    // cin.get();
    // cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ block.cpp 
[[e-mail]] chapter_5$ ./a.out 
The Amazing Accounto will sum and average five numbers for you.
Please enter five values:
Value 1: 1942
Value 2: 1948
Value 3: 1957
Value 4: 1974
Value 5: 1980
Five exquisite choices indeed! They sum to 9801
and average to 1960.2.
The Amazing Accounto bids you adieu!

5.1.11 其他语法技巧--逗号运算符

// forstr2.cpp -- reversing an array
#include <iostream>
#include <string>
int main()
{
    using namespace std;
    cout << "Enter a word: ";
    string word;
    cin >> word;

    // physically modify string object
    char temp;
    int i, j;
    for (j = 0, i = word.size() - 1; j < i; --i, ++j)
    {                       // start block
        temp = word[i];
        word[i] = word[j];
        word[j] = temp;
    }                       // end block
    cout << word << "\nDone\n";
    // cin.get();
    // cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ forstr2.cpp 
[[e-mail]] chapter_5$ ./a.out 
Enter a word: stressed
desserts
Done

5.1.12 关系表达式

5.1.13 赋值、比较和可能犯的错误

// equal.cpp -- equality vs assignment
#include <iostream>
int main()
{
    using namespace std;
    int quizscores[10] =
        { 20, 20, 20, 20, 20, 19, 20, 18, 20, 20};

    cout << "Doing it right:\n";
    int i;
    for (i = 0; quizscores[i] == 20; i++)
        cout << "quiz " << i << " is a 20\n";
// Warning: you may prefer reading about this program
// to actually running it.
    cout << "Doing it dangerously wrong:\n";
    for (i = 0; quizscores[i] = 20; i++)
        cout << "quiz " << i << " is a 20\n";
	// cin.get();
    return 0;
}

[[e-mail]] chapter_5$ g++ equal.cpp 
equal.cpp:16:31: warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
    for (i = 0; quizscores[i] = 20; i++)
                ~~~~~~~~~~~~~~^~~~
equal.cpp:16:31: note: place parentheses around the assignment to silence this warning
    for (i = 0; quizscores[i] = 20; i++)
                              ^
                (                 )
equal.cpp:16:31: note: use '==' to turn this assignment into an equality comparison
    for (i = 0; quizscores[i] = 20; i++)
                              ^
                              ==
1 warning generated.

5.1.14 C-风格字符串的比较

word == "mate" 引号括起的字符串常量是地址
C++将C-风格视为地址
strcmp函数，可以是指针，字符串常量或字符数组名

// compstr1.cpp -- comparing strings using arrays
#include <iostream>
#include <cstring>     // prototype for strcmp()
int main()
{
    using namespace std;
    char word[5] = "?ate";

    for (char ch = 'a'; strcmp(word, "mate"); ch++)
    {
        cout << word << endl;
        word[0] = ch;
    }
    cout << "After loop ends, word is " << word << endl;
    // cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ compstr1.cpp 
[[e-mail]] chapter_5$ ./a.out 
?ate
aate
bate
cate
date
eate
fate
gate
hate
iate
jate
kate
late
After loop ends, word is mate

5.1.15 比较string类字符串

string类重载运算符！=的方式：至少有一个操作数为string对象，另一个操作数可以是string类对象，也可以是C_风格字符串。

// compstr2.cpp -- comparing strings using arrays
#include <iostream>
#include <string>     // string class
int main()
{
    using namespace std;
    string word = "?ate";

    for (char ch = 'a'; word != "mate"; ch++)
    {
        cout << word << endl;
        word[0] = ch;
    }
    cout << "After loop ends, word is " << word << endl;
    // cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ compstr2.cpp 
[[e-mail]] chapter_5$ ./a.out 
?ate
aate
bate
cate
date
eate
fate
gate
hate
iate
jate
kate
late
After loop ends, word is mate

5.2 while循环

// while.cpp -- introducing the while loop
#include <iostream>
const int ArSize = 20;
int main()
{
    using namespace std;
    char name[ArSize];

    cout << "Your first name, please: ";
    cin >> name;
    cout << "Here is your name, verticalized and ASCIIized:\n";
    int i = 0;                  // start at beginning of string
    while (name[i] != '\0')     // process to end of string
    {
        cout << name[i] << ": " << int(name[i]) << endl;
        i++;                    // don't forget this step
    }
    // cin.get();
    // cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ while.cpp 
[[e-mail]] chapter_5$ ./a.out 
Your first name, please: Muffy
Here is your name, verticalized and ASCIIized:
u: 117
f: 102
f: 102
y: 121

5.2.1 for与while

5.2.2 等待一段时间：编写延时循环

// waiting.cpp -- using clock() in a time-delay loop
#include <iostream>
#include <ctime> // describes clock() function, clock_t type
int main()
{
    using namespace std;
    cout << "Enter the delay time, in seconds: ";
    float secs;
    cin >> secs;
    clock_t delay = secs * CLOCKS_PER_SEC;  // convert to clock ticks
    cout << "starting\a\n";
    clock_t start = clock();
    while (clock() - start < delay )        // wait until time elapses
        ;                                   // note the semicolon
    cout << "done \a\n";
    // cin.get();
    // cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ ./a.out 
Enter the delay time, in seconds: 5
starting
done 

5.3 do while循环

// dowhile.cpp -- exit-condition loop
#include <iostream>
int main()
{
    using namespace std;
    int n;

    cout << "Enter numbers in the range 1-10 to find ";
    cout << "my favorite number\n";
    do
    {
        cin >> n;       // execute body
    } while (n != 7);   // then test
    cout << "Yes, 7 is my favorite.\n" ;
    // cin.get();
    // cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ dowhile.cpp 
[[e-mail]] chapter_5$ ./a.out 
Enter numbers in the range 1-10 to find my favorite number
9
5
7
Yes, 7 is my favorite.

5.4 基于范围的for循环（C++11）

5.5 循环和文本输入

5.5.1 使用原始的cin进行输入

// textin1.cpp -- reading chars with a while loop
#include <iostream>
int main()
{
    using namespace std;
    char ch;
    int count = 0;      // use basic input
    cout << "Enter characters; enter # to quit:\n";
    cin >> ch;          // get a character
    while (ch != '#')   // test the character
    {
        cout << ch;     // echo the character
        ++count;        // count the character
        cin >> ch;      // get the next character
    }
    cout << endl << count << " characters read\n";
// get rid of rest of line
     // while (cin.get() != '\n')
        // ;
//cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ textin1.cpp 
[[e-mail]] chapter_5$ ./a.out 
Enter characters; enter # to quit:
see ken run#really fast
seekenrun
9 characters read

cin将忽略空格和换行符，因此输入的空格没有回显，被包括在计数内
进一步，发送给cin的输入被缓冲，用户只有按下回车键后，他输入的内容才会被发送给程序。
这就是运行程序时，可以在#后面输入字符的原因，按下回车键后，整个字符序列将被发送给程序，但程序在遇到#后结束对输入的处理。

5.5.2 使用cin.get(char)进行补救

// textin2.cpp -- using cin.get(char)
#include <iostream>
int main()
{
    using namespace std;
    char ch;
    int count = 0;

    cout << "Enter characters; enter # to quit:\n";
    cin.get(ch);        // use the cin.get(ch) function
    while (ch != '#')
    {
        cout << ch;
        ++count;
        cin.get(ch);    // use it again
    }
    cout << endl << count << " characters read\n";
// get rid of rest of line
    // while (cin.get() != '\n')
    //    ;
    //cin.get();
    return 0; 
}

[[e-mail]] chapter_5$ g++ textin2.cpp 
[[e-mail]] chapter_5$ ./a.out 
Enter characters; enter # to quit:
Did you use a #2 pencil?
Did you use a 
14 characters read

5.5.3 使用哪一个cin.get()

char name[ArSize];
cout << "Enter your name:\n";
cin.get(name, ArsSize).get();

cin.get(name, ArSize)；
cin.get();


char ch;
cin.get(ch);

5.5.4 文件尾条件

如果输入来自文件，可以使用功能更更大的技术--检测文件尾（EOF）
首先很多操作系统，都支持重定向，允许用文件替换键盘输入。
< 重定向运算符
操作系统支持用键盘来模拟文件尾条件 Ctrl + D（Unix）Crtl + Z（Windows）

// textin3.cpp -- reading chars to end of file
#include <iostream>
int main()
{
    using namespace std;
    char ch;
    int count = 0;
    cin.get(ch);        // attempt to read a char
    while (cin.fail() == false)  // test for EOF
#while (!cin.fail()) 
    {
        cout << ch;     // echo character
        ++count;
        cin.get(ch);    // attempt to read another char
    }
    cout << endl << count << " characters read\n";
    return 0; 
}

[[e-mail]] chapter_5$ g++ textin3.cpp 
[[e-mail]] chapter_5$ ./a.out 
The green bird sings in the winter.
The green bird sings in the winter.
^Z
[1]+  Stopped                 ./a.out

5.5.5 另一个cin.get()版本

// textin4.cpp -- reading chars with cin.get()
#include <iostream>
int main(void)
{
    using namespace std;
    int ch;                         // should be int, not char
    int count = 0;

    while ((ch = cin.get()) != EOF) // test for end-of-file
    {
        cout.put(char(ch));
        ++count;
    }
    cout << endl << count << " characters read\n";
	return 0; 
}

[[e-mail]] chapter_5$ g++ textin4.cpp 
[[e-mail]] chapter_5$ ./a.out 
The sullen mackerel sulks in the shadowy shallows
The sullen mackerel sulks in the shadowy shallows
^Z
[2]+  Stopped                 ./a.out

5.6 嵌套循环和二维数组

5.6.1 初始化二维数组

5.6.2 使用二维数组

// nested.cpp -- nested loops and 2-D array
#include <iostream>
const int Cities = 5;
const int Years = 4;
int main()
{
    using namespace std;
    const char * cities[Cities] =   // array of pointers
    {                               // to 5 strings
        "Gribble City",
        "Gribbletown",
        "New Gribble",
        "San Gribble",
        "Gribble Vista"
    };

    int maxtemps[Years][Cities] =   // 2-D array
    {
        {96, 100, 87, 101, 105},   // values for maxtemps[0]
        {96, 98, 91, 107, 104},   // values for maxtemps[1]
        {97, 101, 93, 108, 107}, // values for maxtemps[2]
        {98, 103, 95, 109, 108}   // values for maxtemps[3]
    };

    cout << "Maximum temperatures for 2008 - 2011\n\n";
    for (int city = 0; city < Cities; ++city)
    {
        cout << cities[city] << ":\t";
        for (int year = 0; year < Years; ++year)
            cout << maxtemps[year][city] << "\t";
        cout << endl;
    }
	// cin.get();
    return 0;
}

[[e-mail]] chapter_5$ g++ nested.cpp 
[[e-mail]] chapter_5$ ./a.out 
Maximum temperatures for 2008 - 2011

Gribble City:   96      96      97      98
Gribbletown:    100     98      101     103
New Gribble:    87      91      93      95
San Gribble:    101     107     108     109
Gribble Vista:  105     104     107     108

 

5.7 总结