《Beginning C++17》-学习笔记-Chapter 04-Making Decisions

There are only two possible bool values, true and false. True and false are keywords and are literals of type bool. They are
sometimes called Boolean literals.

#include <iostream>

int main()
{

	bool isValid{ true }; // Define and initialize a logical variable to true.

	bool isFalse{}; // Define and initialize a logical variable to false.

	std::cout << isValid << std::endl;//输出1
	std::cout << std::boolalpha;//This statement makes bool values output as true and false
	std::cout << isFalse << std::endl;//输出false
	std::cout << std::noboolalpha;// This statement returns output of bool values to the default setting, which is o or 1
	std::cout << (4<5) << std::endl;//输出1

}

If you cast true to an integer type, the result will be 1; casting false to an integer results in 0. Conversely, you can also convert numerical values to type bool. Zero converts to false, and any nonzero value converts to true. When you have a numerical value where a bool value is expected, the compiler will insert an implicit conversion to convert the numerical value to type bool.

---

#include <iostream>
#include <cctype>  /*functions in this header return values of type int. The value will be nonzero (true) if the character is of the type being tested for, and 0 (false) if it isn’t. The reason they don’t return a bool value is that they originate from the C Standard Library and predate type bool in C++.*/

int main()
{
	char letter{}; // Store input here
	std::cout << "Enter a letter: "; // Prompt for the input
	std::cin >> letter;

	if (std::isupper(letter))
	{
		std::cout << "You entered an uppercase letter." << std::endl;
		std::cout << "Convert it to uppercase as:" << static_cast<char>(std::tolower(letter));/* The result of function "tolower()" will be returned as type int, so you need to explicitly cast it if you want to store it as type char*/
		return 0;
	}

	else
	{
		std::cout << "You entered a lowercase letter." << std::endl;
		std::cout <<"Convert it to uppercase as:" << static_cast<char>(std::toupper(letter));
		return 0;
	}
}

An else always belongs to the nearest preceding if that’s not already spoken for by another else.

When an else block is another if, writing else if on one line is an accepted convention.

---

The binary logical operators are so-called short-circuit operators.

If the first operand to a binary logical expression already determines the outcome, the compiler will make sure no time is wasted evaluating the second operand. This property of the logical operators && and || is called short-circuit evaluation.

The second operand of && is evaluated only after the first operand evaluates to true, and the second operand of || only after the first evaluates to false.

---

// Using the conditional operator to select output.
#include  <iostream>

int main()
{
	int apple{};               // 苹果数量

	std::cout << "How many apple(s) do you have? ";
	std::cin >> apple;

	std::cout << "I have " << apple
		<< (((apple == 1 ) || (apple ==0))? " apple" : " apples")
		<< " in total." << std::endl;
}

---

You can only switch on values of integral (int, long, unsigned short, etc.), character (char, etc.), and enumeration types.

Each case value must be a constant expression, which is an expression that the compiler can evaluate at compile time.
 

// Using the switch statement
#include <iostream>
#include <cctype>		// for std::isalpha() and tolower()

int main()
{
	char letter{};
	std::cout << "Enter a letter: ";
	std::cin >> letter;

	if (std::isalpha(letter))//检查输入的字符是否是字母
	{
		switch (std::tolower(letter))
		{
		case 'a': 
			std::cout << "You entered vowel 'a'." << std::endl;
			[[fallthrough]];/*can add a [[fallthrough]] statement in the same place where you would otherwise add a break statement to prevent compiler to issue a warning when compiling your program. This feature is available from C++ 17.*/
		case 'e': case 'i': case 'o': case 'u':
			std::cout << "You entered a vowel." << std::endl;
			break;/*This statement will break out of the if-else and exit the program. A break statement is not the only way to move control out of a switch statement. If the code following a case label contains a return statement, control instantly exits not only the switch statement but the surrounding function as well. */
		default:
			std::cout << "You entered a consonant." << std::endl;
			break;
		}
	}
	else
	{
		std::cout << "You did not enter a letter." << std::endl;
	}
}

Selecting Language Standard
To enable the latest features of the C++ language outlined in this book it is necessary to manually change the language standard setting for your project. You can do this by first going to Project ➤ Properties to bring up the Property pages. From there, navigate to Configuration Properties ➤ C/C++ ➤ Language ➤ C++ Language Standard. Select the ISO C++17 standard from the drop-down list. Click OK and the project will now be configured to compile according to the C++17 language standard.

 

---

Any variable declared within a block ceases to exist at the end of the block, so you cannot reference it outside the block

In general, it is considered good coding style to limit the scope of variables to the region in which they are used.

#include <iostream>

int main()
{
	int input;
	std::cout << "Enter a number" << std::endl;
	std::cin >> input;
	if (int local_value{input}; local_value > 0) /*This is a new way, beginnig with C++ 17, of initializing a variable within the scope of an if-else.*/
	{
		std::cout << "value entered is larger than 0." << std::endl;
	}
	else
	{
		std::cout << "value entered is not larger than 0." << std::endl;

	}

}

 

/*Even though starting with C++14, C++ supports binary integral literals (of form 0b11001010), C++ standard output functions and streams do not support outputting integral values in binary format. They mostly do support hexadecimal and octal formatting—for std::cout, for instance, you can use the std::hex and std::oct output manipulators defined in <ios>. But to output a character in binary format, you’ll thus have to write some code yourself. A char normally has only eight bits, remember? You can just stream these bits one by one.*/

#include <iostream>

int main() {
	char ch{};
	std::cout << "Enter a letter: ";
	std::cin >> ch;

	// Output the character code as binary
	std::cout << "The binary code for '" << ch << "' is "
		<< ((ch & 0b10000000) ? 1 : 0)
		<< ((ch & 0b01000000) ? 1 : 0)
		<< ((ch & 0b00100000) ? 1 : 0)
		<< ((ch & 0b00010000) ? 1 : 0)
		<< ((ch & 0b00001000) ? 1 : 0)
		<< ((ch & 0b00000100) ? 1 : 0)
		<< ((ch & 0b00000010) ? 1 : 0)
		<< ((ch & 0b00000001) ? 1 : 0)
		<< std::endl;

	return 0;
}