[C++] Lecture 3. Variables and Data Types

Number Systems

 

n bits이 있으면 2^n 개의 data를 나타낼 수 있음

e.g.) 8 bits: data range는 0 부터 2^8-1 = 255, 총 256개

 

Digits	Bytes	Data Range
8	1	0~255
16	2	0~65,535
32	4	0~2^32-1 = 4,294,967,295
64	8	0~2^64-1

 

Binary number system (base = 2, 1개의 bit으로 하나의 value를 나타냄, 1, 2)

Octal number system (base = 8, 3개의 bit으로 하나의 value를 나타냄, 0-7)

Decimal number system (base = 10)

Hexadecimal number system (base = 16, 4개의 bit으로 하나의 value를 나타냄, 0-9, A-F)

 

int n1 {15};			// Decimal
int n2 {017};			// Octal, 1*8^1 + 7*8^0 = 15
int n3 {0x0f};			// Hexadecimal, 0*16^1 + 15*16^0 = 15
int n4 {0b00001111};		// Binary

 

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int

 

stores decimals, typically occupies 4 or more bytes in memory

variable: a named piece of memory that you use to store specific types of data

 

#include <iostream>

int main(){
    int age1 {2.9};			// braced initialization --> compile error
    int age2 (2.9);			// functional initialization --> information lost
    int age3 = 2.9;			// assignment initialization --> information lost
    std::cout << age2 << std::endl;	// output 2 (chop off decimal point)
    std::cout << age3 << std::endl;	// output 2 (chop off decimal point)
    
    return 0;
}

 

#include <iostream>

int main(){
    int age {9};
    std::cout << sizeof(int) << std::endl;	// 4 bytes
    std::cout << sizeof(age) << std::endl; 	// 4 bytes
    return 0;
}

 

Type with modifier	Bytes in memory	Data range
unsigned short int	2	0~2^16-1
signed short int	2	-2^15~2^15-1
unsigned int	4	0~2^32-1
signed int	4	-2^31~2^31-1
signed long int	4 or 8	2^64-1 (if 8)
unsigned long int	4 or 8	-2^63~2^63-1 (if 8)
unsigned long long int	8	2^64-1
signed long long int	8	-2^63~2^63-1

 

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Fractional Numbers

 

Type	Size (Bytes in memory)	Precision	Comment
float	4	7 decimal digits	-
double	8	15-16 decimal digits	Recommended default
long double	8 or 16	> double	-

 

n (floating point) / 0 --> inf (+/-)

0.0 / 0.0 --> nan

 

#include <iostream>
#include <iomanip>

int main(){

    float n1 {1.12345678901234567890f};
    double n2 {1.12345678901234567890};
    long double n3 {1.12345678901234567890L};

    std::cout << std::setprecision(20);
    std::cout << n1 << std::endl;			// 1.1234568357467651367
    std::cout << n2 << std::endl; 			// 1.1234567890123456912
    std::cout << n3 << std::endl;			// 1.1234567890123456912

    std::cout << sizeof(float) << std::endl;		// 4
    std::cout << sizeof(double) << std::endl; 		// 8
    std::cout << sizeof(long double) << std::endl;	// 8
    
    return 0;
}

 

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Boolean

 

bool occupies 8 bits (1 byte) in memory.

A byte can store 256 different values. Using it just to cover two states (true/false) is wasteful, especially devices with hard memory constraints (embedded devices). There are techniques to pack even more data into a byte.

Print 하면 true --> 1, false --> 0 으로 나옴

 

#include <iostream>

int main(){

    bool red_light {true};
    bool green_light {false};

    std::cout << red_light << std::endl;		// 1
    std::cout << green_light << std::endl; 		// 0

    std::cout << std::boolalpha;
    std::cout << red_light << std::endl;		// true
    std::cout << green_light << std::endl; 		// false

	return 0;
}

 

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Characters and Text

 

char occupies 8 bits (1 byte) in memory. Each value (from 256 possible values) match to some character.

 

#include <iostream>

int main(){

    char c1 {'a'};
    char c2 {65};						// ASCII character code 'A'

    std::cout << c1 << std::endl;				// A
    std::cout << static_cast<int>(c2) << std::endl;		// 65

}

 

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Auto Keyword

 

auto lets the compiler to deduce the type

 

#include <iostream>

int main(){

    auto v1 {15};		// int: 4 bytes
    auto v2 {13.0};		// double: 8 bytes
    auto v3 {14.0f};		// float: 4 bytes
    auto v4 {15.0l};		// long double: 8 or 16 bytes
    auto v5 {'c'};		// char: 1 byte

    auto v6 {123u};		// unsigned int: 4 bytes
    auto v7 {100ul};		// unsigned long int: 8 bytes
    auto v8 {123ll};		// long long int: 8 bytes
	
    return 0;
}

 

*Need to careful about auto assignments (auto keyword를 이용해서 variable 정의한 다음에 deduced type에 맞지 않는 value로 reassign 하면 compile error 없이 garbage value를 저장함)

 

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Reference

• https://www.youtube.com/watch?v=8jLOx1hD3_o