for循环
简单for循环
#include <iostream>
using namespace std;
int main()
{ // 5.1
int i;
for (i = 0; i < 5; i++)
cout << "C++ knows loops. \n";
cout << "C++ knows when to stop.\n";
return 0;
}
for循环组成部分
#include <iostream>
using namespace std;
int main()
{
//5.2
cout << "Enter the starting countdown value: ";
int limit;
cin >> limit;
int i;
for (i = limit; i; i--)
cout << "i = " << i << "\n";
cout << "Done now that i = " << i << "\n";
return 0;
}i = 0 自动类型转换成bool类型的false
表达式和语句
x = y = z = 0; 赋值操作从右往左
age = 100 是表达式
age = 100; 是语句
for (expression, expression, expression)
statement;
#include <iostream>
using namespace std;
int main()
{
// 5.3
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);
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;
}
for循环计算阶乘
#include <iostream>
using namespace std;
int main()
{
// 5.4
const int ArSize = 16;
double factorials[ArSize];
factorials[1] = factorials[0] = 1.0;
int i;
for (i = 2; i < ArSize; i++)
factorials[i] = i * factorials[i - 1];
for (i = 0; i < ArSize; i++)
cout << i << "! = " << factorials[i] << endl;
return 0;
}
ArSize 修改就可以快速获得所有的常数的修改
修改步长
#include <iostream>
using namespace std;
int main()
{
// 5.5修改步长
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;
}
使用for循环访问字符串
#include <iostream>
using namespace std;
int main()
{
// 使用for循环 访问字符串
cout << "Enter a word: ";
string word;
cin >> word;
for (int i = word.size() - 1; i >= 0; i--)
cout << word[i];
cout << "\nBye.\n";
return 0;
}
递增操作符和递减操作符
#include <iostream>
using namespace std;
int main()
{
int a = 20;
int b = 20;
cout << "a = " << a << "; b = " << b << "\n";
cout << "a++ = " << a++ << "; ++b = " << ++b << endl;
cout << "a = " << a << "; b = " << b << "\n";
return 0;
}
a++ 先给出表达式结果然后再加1； ++b 先加1然后用新的值来给出表达式结果
副作用(side effect)和顺序点(sequence point)
int guests = 9;
while (guests++ < 10)
printf("%d \n", guests);
guests 和10 比较之后加1；对于类而言，前缀版本的效率更高
递增递减操作符和指针
#include <iostream>
using namespace std;
int main()
{
double arr[5] = {21.1, 32.8, 23.4, 45.2, 37.4};
double *pt = arr;
++pt;
cout << pt << endl;
cout << *pt << endl;
// ++*pt; //int 类型的++
*pt++;
cout << pt << endl;
cout << *pt << endl;
return 0;
}
组合赋值操作符
i = i + by 等价于 i += by
int k = 5;
k += 3;
int *pa = new int[10];
pa [4] = 12;
pa [4] +=6;
*(pa + 4) += 7;
pa += 2;
a += b 等价于 a = a + b
a -= b 等价于 a = a - b
a *= b 等价于 a = a * b
a /= b 等价于 a = a / b
a %= b 等价于 a = a % b
复合语句
#include <iostream>
using namespace std;
int main()
{
// program5.8
cout << "The Amazing Account 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++)
{
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";
cout << "The Amazing Account bids you adieu!\n";
return 0;
}
外部语句定义的变量在内部是被定义，反之是没有被定义的
int x = 20;
{
cout << x << endl;
int x = 100;
cout << x << endl;
}
cout << x << endl;
return 0;逗号操作符
将两个表达式放在原来只能放一个表达式的地方
#include <iostream>
using namespace std;
int main()
{
cout << "Enter a word: ";
string word;
cin >> word;
char temp; // 在外部声明变量可以提高速度
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;
}
i = 20, j = 2 * i 逗号表达式的值是第二部分， 并且逗号表达式的优先级是最低的
关系表达式
可能犯的错误
#include <iostream>
using namespace std;
int main()
{
//== 等于， ！= 不等于 5.10
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++)
cout << "quiz " << i << " is a 20 \n";
return 0;
}
C-风格字符串的比较
word == "mate" 数组名是数组的地址，所以左边的判断表达式表示的是是否存储在相同的地址上。
strcmp() 可以判断两个字符串是否相等
不能关系操作符比较字符串，但是字符是整形所以 是可以比较的
for (char ch = 'a'; ch < 'z'; ch++)
cout << ch;
return 0;
#include <iostream>
#include <cstring>
using namespace std;
int main()
{
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;
} 
strcmp(str1, str2) 如果这两个是不同的则输出是true, 同时还可以比较顺序
比较 string类字符串
#include <iostream>
#include <string>
using namespace std;
int main()
{
// 5.12 比较 string类字符串
string word = "?ate";
for (char ch = 'a'; word != "mate"; ch++)
{
cout << word << endl;
word[0] = ch;
}
cout << "After loop ends, word is " << word << endl;
// C++ 使用String类型不用提前规定大小
}
while 循环
while 循环
while(test-condition)
body
#include <iostream>
const int ArSize = 20;
using namespace std;
int main()
{
// 5.12
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;
}
for 循环可以知道所有的信息， while 循环无法预先知道所执行的次数
while (for) 后面加循环，则变成了空循环
等待一段时， 延时循环
#include <iostream>
#include <ctime>
using namespace std;
int main()
{
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;
} 
do while 循环
do
body
while (test-expression)
#include <iostream>
using namespace std;
int main()
{
int n;
cout << "Enter numbers in the range 1-10 to find ";
cout << "My favorite number\n";
do
{
cin >> n;
} while (n != 7);
cout << "Yes, 7 is my favorite number. \n";
return 0;
}
循环和文本输入
cin输入
#include <iostream>
using namespace std;
int main()
{
// 哨兵字符，作为停止标记
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;
}
输出没有空格
cin.get()输入
修改代码
#include <iostream>
using namespace std;
int main()
{
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;
}
cin.get()的两个使用方法
文件尾条件
#include <iostream>
using namespace std;
int main()
{
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;
}
嵌套和二维数组
二维数组每一个元素本身就是一个数组。
#include <iostream>
using namespace std;
const int Cities = 5;
const int Years = 4;
int main()
{
// 二维数组初始化，逗号隔开
const char *cities[Cities] = // 字符串类型指针
// char cities[Cities][25] =
// const string cities[Cities] =
{
"Gribble City",
"Gribble town",
"New Gribble",
"San Gribble",
"Gribble Vista"};
int maxtemps[Years][Cities] =
{
{54, 53, 86, 100, 104},
{95, 97, 90, 106, 102},
{96, 100, 940, 107, 105},
{97, 102, 89, 108, 104}};
cout << "Maximum temperatures for 2002-2005 \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;
}