3.3 while loop

将一段代码重复执行，就会用到循环语句，常用的循环语句有while 、for。先回顾一下while循环

while loop

// Syntax :
while( expression )
{
    //...
}

while关键词后面跟表达式，表达式就是一个条件，如果条件成立，就会执行花括号内部的程序。

If the condition is true, the statement (loop body) will be executed.
跟if不同的是，花括号内部程序执行结束，会继续判断条件是否成立，如果还是成立，还会继续执行代码，直到条件不成立。如果该条件一直为true，那么该循环就会陷入死循环中，要极力避免此种情况。

#include <iostream>
using namespace std;
int main()
{
    int num = 10;
    while (num > 0)
    {
        cout << "num = " << num << endl;
        num--;
    }
    return 0;
}

do-while loop

The test takes place after each iteration in a do-while loop.
The test takes place before each iteration in a while loop.

int num = 10;
do
{
    cout << "num = " << num << endl;
    num--;
}while (num > 0);

do-while语法中，花括号内部的程序会先执行，然后判断条件是否成立。无论条件是否成立，都会先执行括号内的程序。

break and continue

如果我们想停止循环，可以用break语句

int num = 10;
while (num > 0)
{
    if (num == 5)
        break;
    cout << "num = " << num << endl;
    num--;
}

Skip the remaining part of the loop body and continue the next iteration.

int num = 10;
while (num > 0)
{
    if (num == 5)
        continue;
    cout << "num = " << num << endl;
    num--;
}

continue之后，当前循环停止，继续下次循环，上面代码中num=5 不会打印，会陷入死循环。
So，The Condition, Be Careful!

Can you find any problem from the down code?

size_t num = 10;
while(num >= 0)
{
    cout << "num = " << num << endl;
    num--;
}

size_t是无符号整数，当num自减到0时，在—的话，理论上是负数，但是其是无符号整数，—的话真实值是能表示的最大的整数。

bool flag = true;
int count = 0;
while(flag = true)
{
    cout << "count = " << count++ << endl;
    // and do sth
    if (count == 10) //meet a condition
    flag = false; //set flag to false to break the loop
}

flag=true 是赋值操作，而不是判断条件；

这是不会发生编译错误的

• Expression 3+4 has a value;
• Expression a+b has a value;
• Expression (a==b) has value (true or false);
• a=b is an assignment赋值操作, also an expression and has a value b

The follow code can be compiled successfully!

int b = 0;
int m = (b = 8);
cout << "m="<< m << endl;