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本章小结

        循环结构可以让程序处理各类重复操作。for语句是最常用且功能最强的循环语句,如果恰当地使用for语句,可以对控制变量、判断条件、递增/递减操作和循环体等各种要素进行全面的管理。在一些情况下使用 while或do… while语句来代替for语句可以得到更加优化的循环结构。在实际进行程序设计时,有许多问题需要使用选择结构或循环结构的嵌套来解决,这往往会增加程序的复杂程度    

c++提供了3种循环:for循环、while循环和do while循环，如果循环测试条件为true 或非零，则循环将重复执行一组指令:如果测试条件为false或0,则结束箱环。for循环和while循环都是入口条件循环，这意味着程序将在执行循环体中的语句之前检查测试条件。do while循环是出口条件循环，这意味着其将在执行循环体中的语句之后检查条件。    每种循环的句法都要求循环体由一条语句组成。然而，这条语句可以是复合语句，也可以是语句块( 由花括号括起的多条语句）。    关系表达式对两个值进行比较，常被用作循环测试条件。关系表达式是通过使用6种关系运算符之构成的:<、<=、==、>=、>或!=。关系表达式的结果为bool类型，值为true或false。 许多程序都逐字节地读取文木输入或文本文件，istream类提供了多种可完成这种工作的方法。如果ch是一个char变量，则下面的语句将输入中的下一个字符读入到ch中: cin>>ch;    然而，它将忽略空格、换行符和制表符。下面的成员函数调用读取输入中的下一个字符(而不管该字符是什么)并将其存储到ch中:    cin.get(ch);     成员函数调用cin.get()返回下一个输入字符—包括空格、换行符和制表符，因此，可以这样使用它:     ch = cin.get();  cin.get(char)成员函数调用通过返回转换为false的bool值来指出己到达EOF，而cin_get()成员函数调用则通过返回EOF值来指出已到达EOF，EOF是在iostream中定义的。

嵌套循环是循环中的循环，适用于处理二维数组。

程序清单

5.1 了解for的工作

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

5.2 了解for的判断特性

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

5.3 关系表达式

//express.cpp -- values of exprssions#include "stdafx.h"#include 
   
    int main(){	using namespace std;	int x;	cout << "the express 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;	return 0;}
   

5.4 阶乘

//formore.cpp -- more looping with for#include "stdafx.h"#include 
   
    int main(){	using namespace std;	const int ArSize = 16;	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++)		cout << i << "! = " << factorials[i] << endl;	return 0;}
   

5.5 选择步长

// bigstep.cpp -- count as directed#include "stdafx.h"#include 
   
    int main(){	using namespace std;	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;	return 0;}
   

5.6 大于等于

//forstr1.cpp -- using for with a string#include "stdafx.h"#include
   
    #include 
    
     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";	return 0;}
    
   

5.7 递增运算符

//plus_one.cpp -- the increment operator#include "stdafx.h"#include
   
    int main(){	using namespace std;	int a = 20;	int b = 20;	cout << "a   = " << a   << ":   b = " << b << endl;	cout << "a++ = " << a++ << ": ++b = " << ++b << endl;	cout << "a   = " << a   << ":   b = " << b << endl;	return 0;}
   

5.8 代码块

// block.cpp -- use a block statement#include "stdafx.h"#include
   
    int main(){	using namespace std;	cout << "The Amazing Accounto will sum and averge ";	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++)	{		cout << "Value " << i << ": ";		cin >> number;		sum += number;	}	cout << "Five exquisite choices indeed! ";	cout << "They sum to " << sum << endl;	cout << "and average to " << sum / 5 << ".\n";	return 0;}
   

5.9 两次运算

//forstr2.cpp -- reversing an array#include "stdafx.h"#include
   
    #include 
    
     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)	{		temp = word[i];		word[i] = word[j];		word[j] = temp;	}	cout << word << "\nDone!\n";	return 0;}
    
   

5.10 错误

//equal.cpp -- equality vs asignment#include "stdafx.h"#include
   
    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";	cout << "Doing it dangerously wrong:\n";	for (i = 0; quizscores[i] = 20; i++)//不能使用等号赋值相当于循环一直是true，导致一直循环下去		cout << "quiz " << i << " is a 20\n";	return 0;}
   

5.11 strcmp（）

//compstr1.cpp -- comparing string using arrays#include "stdafx.h"#include
   
    #include 
    
     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;	return 0;}
    
   

5.12 使用string

//compstr2.cpp -- comparing strings using arrays#include "stdafx.h"#include
   
    #include 
    
     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;	return 0;}
    
   

5.13 使用while

//while.cpp -- inroducing the while loop#include "stdafx.h"#include
   
    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;	while (name[i] != '\0')	{		cout << name[i] << ": " << int(name[i]) << endl;		i++;	}	return 0;}
   

5.14 使用clock

//waiting.cpp -- using clock() in a time-delay loop#include "stdafx.h"#include
   
    #include 
    
     int main(){	using namespace std;	cout << "Enter the delay time, in second: ";	float secs;	cin >> secs;	clock_t delay = secs * CLOCKS_PER_SEC;	cout << "starting\a\n";	clock_t start = clock();	while (clock() - start < delay);	cout << "done \a\n";	return 0;}
    
   

5.15 使用do while

//dowhile.cpp -- exit-condition loop#include "stdafx.h"#include
   
    int main(){	using namespace std;	int n;	cout << "Enter number in the range 1-10 to find ";	cout << "my favorite number\n";	do 	{		cin >> n;	} while (n != 7);	cout << "Yes, 7 is my favorite.\n";	return 0;}
   

5.16 总字符数

// textin1.cpp -- reading chars with a while loop#include "stdafx.h"#include
   
    int main(){	using namespace std;	char ch;	int count = 0;	cout << "Enter characters; enter # to quit\n";	cin >> ch;	while (ch != '#')	{		cout << ch;		++count;		cin >> ch;	}	cout << endl << count << " characters read\n";	return 0;}
   

5.17 

// textin2.cpp -- using cin.get(char)#include "stdafx.h"#include
   
    int main(){	using namespace std;	char ch;	int count = 0;	cout << "Enter characters; enter # to quit\n";	cin.get(ch);	while (ch != '#')	{		cout << ch;		++count;		cin.get(ch);	}	cout << endl << count << " characters read\n";	return 0;}
   

5.18 copy

// textin3.cpp -- reading chars to end of file#include "stdafx.h"#include
   
    int main(){	using namespace std;	char ch;	int count = 0;	cin.get(ch);	while (cin.fail() == false)	{		cout << ch;		++count;		cin.get(ch);	}	cout << endl << count << " characters read\n";	return 0;}
   

5.19 更简洁

// textin4.cpp -- reading chars with cin.get()#include "stdafx.h"#include
   
    int main(){	using namespace std;	char ch;	int count = 0;	while ((ch = cin.get()) != EOF)	{		cout.put(char(ch));		++count;	}	cout << endl << count << " characters read\n";	return 0;}
   

5.20 二维数组

//nested.cpp -- nested loops and 2-D array#include "stdafx.h"#include 
   
    const int Cities = 5;const int Years = 4;int main(){	using namespace std;	const char * cities[Cities] =	{		"Gribble City",		"Gribble town",		"New Gribble",		"San Gribble",		"Gribble Vista"	};	int maxtemps[Years][Cities] = 	{		{96,100,87,101,105},		{96,98,91,107,104},		{97,101,93,108,107},		{98,103,95,109,108}	};	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;	}	return 0;}
   

课后编程习题答案

//question.cpp

#include "stdafx.h" #include <iostream> #include<array> #include <string> #include <cstring> using namespace std; //question 1 /*int main() { cout << "Please enter two integers: "; int num1, num2; cin >> num1 >> num2; int sum = 0; for (int temp = num1; temp <= num2; ++temp) sum += temp; cout << "The sum from " << num1 << " to " << num2 << " is " << sum << endl; return 0; }*/ //question 2 /*int main() { using namespace std; array<long double, 101>ad = { 0 }; ad[1] = ad[0] = 1L; for (int i = 2; i < 101; i++) ad[i] = i * ad[i - 1]; for (int i = 0; i < 101; i++) cout << i << "! = " << ad[i] << endl; return 0; }*/ //question 3 /*int main() { cout << "Please enter an integer: "; int sum = 0, num; while ((cin >> num) && num != 0) { sum += num; cout << "So far, the sum is " << sum << endl; cout << "Please enter an integer: "; } return 0; }*/ //question 4 /*int main() { using namespace std; double sum1, sum2; sum1 = sum2 = 0.0; int year = 0; while (sum2 <= sum1) { ++year; sum1 += 10; sum2 = (100 + sum2)*0.05 + sum2; } cout << "After " << year << " years" << endl; cout << "Now，Cleo's value is " << sum1  << "，Daphne's value is "  << sum2 << endl; return 0; }*/ //question 5 /*int main() { const int MONTHS = 12; const char* months[MONTHS]=  { "January  ",  "February ",  "March    ",  "April    ",  "May      ",  "June     ",  "July     ",  "August   ",  "September",  "October  ",  "November ",  "December "  }; int sales[MONTHS], sum = 0; for (int i = 0; i < MONTHS; i++) { cout << "please enter the sales in " << months[i] << " : " ; cin >> sales[i]; sum += sales[i]; } cout << "the all sales of this years is: " << sum << endl; return 0; }*/ //question 6 /*int main() { const int MONTHS = 12; const char* months[MONTHS] = {   "January  ", "February ", "March    ", "April    ", "May      ", "June     ", "July     ", "August   ", "September", "October  ", "November ", "December " }; const char* years[3] = { "first","second","third" }; int year_sale[3], sum = 0, sales[3][MONTHS]; for (int i = 0; i < 3; i++) { int temp = 0; cout  << "the sales of every month in " << years[i] << " year is  " << endl; for (int j = 0; j < MONTHS; j++) { cout << "please enter the sale of " << months[j] << " : "; cin >> sales[i][j]; temp += sales[i][j]; } year_sale[i] = temp; sum += year_sale[i]; } for (int i = 0; i < 3; i++) cout << "the sales of " << years[i] << " year are: " << year_sale[i] << endl; cout << "the all sales of three years are : " << sum << endl; return 0; }*/ //question 6.1 /*int main() { const int Months = 12, Years = 3; const char* months[Months] = {   "January  ", "February ", "March    ", "April    ", "May      ", "June     ", "July     ", "August   ", "September", "October  ", "November ", "December " }; int sale[Years][Months] = { 0 }; for (int i = 0; i < Years; i++) { int sum = 0; for (int j = 0; j < Months; j++) { cout << "Enter the sales of " << months[j] << ": "; cin >> sale[i][j]; sum += sale[i][j]; } cout << "Sales for this year: " << sum << endl << endl; } return 0; }*/ //question 7 /*struct car { string name; int year; }; int main() { cout << "How many cars do you wish to catalog? "; int num; (cin >> num).get(); car* ps = new car[num]; for (int i = 0; i < num; ++i) { cout << "Car #" << i + 1 << ":\n"; cout << "Please enter the make: "; getline(cin, ps[i].name); cout << "Please enter the year made: "; (cin >> ps[i].year).get(); } cout << "Here is your collection:\n"; for (int i = 0; i < num; ++i) cout << ps[i].year << " " << ps[i].name << endl; delete[] ps; return 0; }*/ //question 8 /*int main() { char word[20]; int sum = 0; cout << "Enter words (to stop,type the word done):\n"; cin >> word; while (strcmp(word, "done")) { sum++; cin >> word; } cout << "You entered a total of " << sum << " words.\n"; return 0; }*/ //question 9 /*int main() { string word; int sum = 0; cout << "Enter words (to stop, type the word done):\n"; cin >> word; while (word != "done") { sum++; cin >> word; } cout << "You entered a total of " << sum << " words.\n"; return 0; }*/ //question 10 /*int main() { cout << "Enter number of rows:"; int num; cin >> num; for (int i = 0; i < num; i++) { for (int j = num - i; j > 1; j--) cout << "."; for (int k = 0; k <= i; ++k) cout << "*"; cout << endl; } return 0; }*/

 

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