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 <iostream>
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 <iostream>
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 <iostream>
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 <iostream>
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 <iostream>
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<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";
return 0;
}5.7 递增运算符
//plus_one.cpp -- the increment operator
#include "stdafx.h"
#include<iostream>
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<iostream>
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<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)
{
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<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";
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<iostream>
#include <cstring>
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<iostream>
#include <string>
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<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;
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<iostream>
#include <ctime>
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<iostream>
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<iostream>
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<iostream>
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<iostream>
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<iostream>
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 <iostream>
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;
}*/