2.1 关系运算符

==, !=
>, >=
<, <=

2.2 条件语句

02.score_rater.cpp

#include <iostream>
using namespace std;

int main()
{
	if (true)
	{
		cout << "This is always displayed.\n\n";
	}

	if (false)
		cout << "This is never displayed.\n\n";

	int score = 1000;

	if (score)
	{
		cout << "At least you didn't score zero.\n\n";
	}

	if (score >= 250)
		cout << "You scored 250 or more. Decent.\n\n";

	if (score >= 500)
	{
		cout << "You scored 500 or more. Nice.\n\n";

		if (score >= 1000)
			cout << "You scored 1000 or more. Impressive!\n";
	}

	return 0;
}

02.score_rater2.cpp

#include <iostream>
using namespace std;

int main()
{
	int score;
	cout << "Enter your score: ";
	cin >> score;

	if (score >= 1000)
		cout << "You scored 1000 or more. Impressive!\n";
	else
		cout << "You scored less than 1000.\n";

	return 0;
}

02.score_rater3.cpp

#include <iostream>
using namespace std;

int main()
{
	int score;
	cout << "Enter your score: ";
	cin >> score;

	if (score >= 1000)
	{
		cout << "You scored 1000 or more. Impressive!\n";
	}
	else if (score >= 500)
	{
		cout << "You scored 500 or more. Nice.\n";
	}
	else if (score >= 250)
	{
		cout << "You scored 250 or more. Decent.\n";
	}
	else
	{
		cout << "You scored less than 250. Nothing to brag about.\n";
	}

	return 0;
}

2.5 switch语句

只用于比较int型（或可当作int型处理的值，char型或枚举型）。

02.menu_chooser.cpp

#include <iostream>
using namespace std;

int main()
{
	cout << "Difficulty Levels\n\n";
	cout << "1 - Easy\n";
	cout << "2 - Normal\n";
	cout << "3 - Hard\n\n";

	int choice;
	cout << "Choice: ";
	cin >> choice;

	switch (choice)
	{
	default:
		cout << "You made an illegal choice.\n";
		break;
	case 1:
		cout << "You picked Easy.\n";
		break;
	case 2:
		cout << "You picked Normal.\n";
		break;
	case 3:
		cout << "You picked Hard.\n";
		break;
	}

	return 0;
}

2.6 while循环

02.play_again.cpp

#include <iostream>
using namespace std;

int main()
{
	char again = 'y';
	while (again == 'y')
	{
		cout << "\n**Played an exciting game**";
		cout << "\nDo you want to play again? (y/n): ";
		cin >> again;
	}

	cout << "\nOkay, bye.";

	return 0;
}

2.7 do循环

02.play_again2.cpp

#include <iostream>
using namespace std;

int main()
{
	char again;
	do
	{
		cout << "\n**Played an exciting game**";
		cout << "\nDo you want to play again? (y/n): ";
		cin >> again;
	} while (again == 'y');

	cout << "\nOkay, bye.";

	return 0;
}

2.8 break和continue语句

02.finicky_counter.cpp

#include <iostream>
using namespace std;

int main()
{
	int count = 0;
	while (true)
	{
		count += 1;

		// end loop if count is greater than 10
		if (count > 10)
		{
			break;
		}

		// skip the number 5
		if (count == 5)
		{
			continue;
		}

		cout << count << endl;
	}

	return 0;
}

2.9 逻辑运算符

!
&&
||
注意优先级和短路现象

02.designers_network.cpp

#include <iostream>
#include <string>
using namespace std;

int main()
{
	string username;
	string password;
	bool success;

	cout << "\tGame Designer's Network\n";

	do
	{
		cout << "\nUsername: ";
		cin >> username;

		cout << "Password: ";
		cin >> password;

		if (username == "S.Meier" && password == "civilization")
		{
			cout << "\nHey, Sid.";
			success = true;
		}

		else if (username == "S.Miyamoto" && password == "mariobros")
		{
			cout << "\nWhat's up, Shigeru?";
			success = true;
		}

		else if (username == "W.Wright" && password == "thesims")
		{
			cout << "\nHow goes it, Will?";
			success = true;
		}

		else if (username == "guest" || password == "guest")
		{
			cout << "\nWelcome, guest.";
			success = true;
		}

		else
		{
			cout << "\nYour login failed.";
			success = false;
		}
	} while (!success);

	return 0;
}

2.10 随机数

02.die_roller.cpp

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;

int main()
{
	srand(static_cast<unsigned int>(time(0))); // seed random number generator based on current time

	int randomNumber = rand(); // generate random number

	int die = (randomNumber % 6) + 1; // get a number between 1 and 6
	cout << "You rolled a " << die << endl;

	return 0;
}

2.12 Guess My Number游戏

02.guess_my_number.cpp

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;

int main()
{
	srand(static_cast<unsigned int>(time(0))); // seed random number generator

	int secretNumber = rand() % 100 + 1; // 生成随机数
	int tries = 0;
	int guess;

	cout << "\tWelcome to Guess My Number\n\n";

	do
	{
		cout << "Enter a guess: ";
		cin >> guess; // 获取用户猜测
		++tries;	  // 增加猜测次数

		// 告知猜测情况
		if (guess > secretNumber)
		{
			cout << "Too high!\n\n";
		}
		else if (guess < secretNumber)
		{
			cout << "Too low!\n\n";
		}
		else
		{
			cout << "\nThat's it! You got it in " << tries << " guesses!\n";
		}

	} while (guess != secretNumber); // 是否猜对

	return 0;
}