What is a varialbe

• A variable is an abstraction for a memory location
• Allow programmer to use meaningful names and not memory addresses
• Variables have
  • Type: their category
  • Value: the contents
• Variables must be declared before they are used
• Variables value may change

Declaring and Initializing Variables

Naming variables

• Can contain letters, numbers, and underscores
• Must begin with a letter or underscore
  • cannot begin with a number
• Cannot use C++ reserved keywords i.e. int int
• Cannot redeclare a name in the same scope
  • C++ is case sensitive: upper case and lower case are different variables

Style and Best Practices

• Be consistent with naming conventions
  • myVariableName vs my_variable_name
  • avoid beginning names with underscores
• Use meaningful names
  • not too long and not too short
• Never use variables before initializing them
• Declare variables close to when you need them in your code

Global variables

• Declared outside functions
  • Declared inside a function is then local variables
• Global variable will not be used inside the fuction if variable is also declared locally
  • Below C++ code output 16 and not 18.

#include <iostream>

using namespace std;

int age {18};   // Global variable

int main () {
    int age {16}    // Local variable
    cout << age << endl;
    return 0;
}

C++ Primitive Data Types

• Fundamental data types implemented directly by the C++ language
• Character types
• Integer types
  • signed and unsigned
• Floating-point types
• Boolean type
• Size and precision is often compiler-dependent
  • #include <climits>

Type sizes

• Expressed in bits
• The most bits the more values that can be represented
• The more bits the more storage required

Size (in bits)	Representable Values	2 to the power of
8	256	2 8
16	65,536	2 16
32	4,294,967,296	2 32
64	18,446,744,073,709,551,615	2 64

Character Types

• Used to represent single characters, A, X, @
• Wider types are used to represent wide character sets

Type Name	Size/Precision
char	Exactly one byte. At least 8 bits
char16_t	At least 16 bits.
char32_t	At least 32 bits.
wchar_t	Can represent the largest available character set.

Integer Types

• Default signed integers

Type Name	Size/Precision
signed short int	At least 16 bits.
signed int	At least 16 bits.
signed long int	At leaset 32 bits.
signed long long int	At least 64 bits.

Type Name	Size/Precision
unsigned short int	At least 16 bits.
unsigned int	At least 16 bits.
unsigned long int	At least 32 bits.
unsigned long long int	At least 64 bits.

Floating-point Type

• Used to represent non-integer numbers
• Represented by mantissa and exponent (scientific notation)
• Precision is the number of digits in the mantissa
• Precision and size are compiler dependent

Type Name	Size/Typical Precision	Typical Range
float	/ 7 decimal digits	1.2 x 10 -38 to 3.4 x 10 38
double	No less than float / 15 decimal digits	2.2 x 10 -308 to 1.8 x 10 308
long double	No less than double / 19 decimal digits	3.3 x x 10 -4932 to 1.2 x 10 4932

Boolean Type

• Used to represent true and false
• Zero is false
• Non-zero is true

Type Name	Size/Precision
bool	Usually 8 bit: true or false (C++ keywords)

What is the Size of a Variable (sizeof)

Using the sizeof operator

• The sizeof operator determines the size in bytes of a type or variable
• Example

sizeof(int)
sizeof(double)

sizeof(some_variable)

sizeof some_variable    // () is optional for variables

• The sizeof operator gets information from climits, for integer types, and cfloat, for float types
• The climits and cfloat include files contain size and precision information about your implementation of C++
• The sizeof information varies between 32 bit compiler and 64 bit compiler

What is a constant

• Similar to C++ variables
  • have names
  • occupy storage
  • are usually typed
  • however, their value cannot change once declared!

Types of constants in C++

• Literal constants
  • i.e. x = 12;, y = 3.14;, name = "Anthony";, middle_initial = 'J';
  • Integer Literal Constants
    • an integer: 12
    • an unsigned integer: 12U or 12u
    • a long integer: 12L or 12l
    • a long long integer: 12LL or 12ll
  • Floating-point Literal Constants
    • a double: 12.1
    • a float: 12.1F
    • a long double: 12.1L
  • Character Literal Constatns (escape codes)
    • newline: \n
    • return: \r
    • tab: \t
    • backspace: \b
    • single quote: \'
    • double quote: \"
    • backslash: \\
    • i.e.

    cout << "Hello\tthere\nmy friend\n";
    // output: 
    Hello     there
    my friend

• Declared constants
  • const keyword
  • i.e.

  const double pi {3.1415926};
  const int months_in_year {12};
  
  pi = 2.5;   // Compiler error

• Constant expressions
  • constexpr keyword
• Enumerated constants
  • enum keyword
• Defined constants
  • #define
  • Don't use defined constants in Modern C++
  • i.e.

  #define pi 3.1415926