基础的 if else

习题6.1

循环输入字符，统计输入了多少个空格字符，如果输入.字符则停止统计

#include <iostream>
int main()
{
	using namespace std;
	char ch;
	int spaces = 0;
	int total = 0;
	cin.get(ch);
	while (ch != '.')
	{
		if (ch == ' ')
			++spaces;
		++total;
		cin.get(ch);
	}
	cout << spaces << " spaces, " << total;
	cout << " character total in sentence";
	return 0;
}

习题6.2

加密字符，循环输入字符，对除了换行符外的所有字符加密，加密方式为对应ASCII码+1，如果输入.字符则停止。

#include <iostream>
int main()
{
	using namespace std;
	char ch;
	cout << "Type, and I shall repeat.\n";
	cin.get(ch);
	while (ch != '.')
	{
		if (ch == '\n')
		{
			std::cout << ch;
		}
		else
			std::cout << ++ch;
		std::cin.get(ch);
	}
	std::cout << "\npls excuse me for the slight confusion";
	return 0;
}

注意一点，++ch不能替换为ch+1，因为1为整型，若ch+1返回的值为整型数字而不再是字符。

同样的，++ch也不能替换为ch++，因为ch++在代码中的效果会是，代码先使用ch输出，再使ch++

习题6.3

猜数字游戏，给定一个数字，输入数字猜该数字，猜错了会提示高了还是低了，直到猜对为止。

#include <iostream>
const int Fave = 27;
int main()
{
	using namespace std;
	int n;
	cout << "Enter a number in the range 1-100 to find my favorite number: ";
	do
	{
		cin >> n;
		if (n < Fave)
			cout << "Too low!";
		else if (n > Fave)
			cout << "Too high!";
		else
			cout << "You are right!\n";
	} while (n != Fave);

	return 0;
}

课后习题

100分

改进与加强

习题6.3中使用的循环为do while，请你使用for循环和while循环分别重写习题6.3，体会一下阿稳说do while在此处的便利之处。

桂桂答题：

1.采用for循环：
#include<iostream>
const int Fave = 89;
int main()
{
	using namespace std;
	int n;
	cout << "Enter a number in the range 1-100 to find my favorite number: ";
	for (int n = 0; n != Fave;)
	{
		cin >> n;
		if (n < Fave)
			cout << "Too low!\n";
		else if (n > Fave)
			cout << "Too high!\n";
		else
			cout << Fave << " is right!\n";
	}

	return 0;
}
2.采用while循环：

	using namespace std;
	int n;
	cout << "Enter a number in the range 1-100 to find my favorite number: ";
	while (n != Fave)
	{
		cin >> n;
		if (n < Fave)
			cout << "Too low!\n";
		else if (n > Fave)
			cout << "Too high!\n";
		else
			cout << Fave << " is right!\n";
	}

	return 0;
}

总结

三个循环本质没有太多区别，do while 先做了do后面语句块内容后，再由while内部条件进行判断；for循环内部需要对变量进行初始化、条件以及判断次数；while循环只需要条件。

巩固与提高

设计一个单个密码锁，密码为提前设定好的单个字符，用户可以输入字符，如果输错密码，提示密码错误。

如果你想还可以增加循环使其反复输入密码直至正确才退出

桂桂答题：

#include<iostream>
const char ch1 = 'c';
int main()
{
	using namespace std;
	char ch2 = 0;
	cin >> ch2;
	for (; ch2 != ch1;)
	{
		cout << "Your code is wrong.\n";
		cin >> ch2;
	}
		cout << "The code is right!";
	
	return 0;
	
}

答案

	//for循环版本
	cin >> n;
	if (n < Fave)
		cout << "Too low!";
	else if (n > Fave)
		cout << "Too high!";
	else
		cout << "You are right!\n";
	for (; n != Fave;)
	{
		cin >> n;
		if (n < Fave)
			cout << "Too low!";
		else if (n > Fave)
			cout << "Too high!";
		else
			cout << "You are right!\n";
	}
	//while循环版本
	cin >> n;
	if (n < Fave)
		cout << "Too low!";
	else if (n > Fave)
		cout << "Too high!";
	else
		cout << "You are right!\n";
	while (n != Fave)
	{
		cin >> n;
		if (n < Fave)
			cout << "Too low!";
		else if (n > Fave)
			cout << "Too high!";
		else
			cout << "You are right!\n";
	}

	char ch;
	cout << "pls guess the code:";
	cin >> ch;
	if (ch != Code)
	{
		cout << "code wrong";
	}
	else
	{
		cout << "welcome!";
	}