Data Types and Variables

Data Types, Primitive Types in C++, Variables – Declaration, Initialization, Scope

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Table of Contents How computers work on data

Variables, example of using a variable in C++

What is a Data Type?

Identifiers in C++

Primitive Data Types in C++ and Using Them

Declaring Variables in C++ Variable Scope

Initializing Variables in C++ Default values

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How Computers Work on Data

Variables, Using variables in C++, Identifiers

How Computing Works?

Computers are machines that process data Data is stored in the computer

memory in variables

Variables have name, data type and value

Example of variable definition and assignment in C++

4

int count = 5;Data type

Variable name

Variable value

What Is a Data Type? A data type:

Is a domain of values of similar characteristics

Defines the type of information stored in the computer memory (in a variable)

Examples:

Positive integers: 1, 2, 3, …

Alphabetical characters: a, b, c, …

Days of week: Monday, Tuesday, …5

Data Type Characteristics

A data type has: Name (C++ keyword) Size (how much memory is used) Default value

Example: Integer numbers in C++ Name: int Size: 32 bits (4 bytes) – on Windows* Default value: 0

* int is equal to the system “word”. E.g. x86 has 32-bit words. Windows always sets int to 32 bits 6

Primitive Data TypesRepresenting Integer, Floating-point

and Symbolic values

Integer Types Integer types:

Represent whole numbers

May be signed or unsigned

Have range of values, depending on the size of memory used

8

Name Description Size* Range*

charCharacter or small integer.

1byte signed: -128 to 127unsigned: 0 to 255

short int (short)

Short Integer. 2bytes signed: -32768 to 32767

unsigned: 0 to 65535

int Integer. 4bytessigned: -2147483648 to 2147483647unsigned: 0 to 4294967295

long int (long) Long integer. 4bytessigned: -2147483648 to 2147483647unsigned: 0 to 4294967295

Integer Types – Explained

int – the most common integer type

char – symbolic & integer type short int (or just short)

Smaller type than int

long int (or just long) Same as int on most systems

long long int (or just long long) Double the size of int on most

systems 9

Integer Types – Sizes C++ has no strict data type size on

any type Sizes determined by the system’s

data types

i.e. int may be 32, 64 or even 16 bits

C++ standards before C++11 only guarantee:

Generally, using int is the best option

sizeof(char) <=sizeof(short)sizeof(short) <=sizeof(int) sizeof(int) <=sizeof(long)sizeof(long) <=sizeof(long long)sizeof(char) == 1 //in bytes

10

Integer Types – Char C++ works with two char types

One for storing 8-bit numbers

One for storing characters

Writing simply char invokes the character type

If we need to store numbers Should write down "signed" or

"unsigned"

Tell the compiler we need char for numbers

Note: don’t store numbers in char, unless you have a very good reason

11

Using Integer and Symbol Types

Live Demo

Boolean Type – bool C++ has a Boolean type

bool – a value which is either true or false

Always takes up 1 byte 1 bit would be enough, but memory

is addressed per bytes, not per bit

Takes true, false, or numeric values Any non-zero numeric value is

interpreted as true

Zero is interpreted as false13

Using bool

Live Demo

Floating-Point Types Floating-Point numbers:

Represent real numbers (approximations) 2.3, 0.7, -Infinity, -1452342.2313,

etc.

Range of values, depending on memory used

Accuracy, depending on memory used

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Name Description Size* Range*

float Floating point number. 4bytes ±1.5 × 10−45 to ±3.4 × 1038 (~7

digits)

doubleDouble precision floating point number.

8bytes ±5.0 × 10−324 to ±1.7 × 10308 (~15 digits)

long doubleLong double precision floating point number.

8bytes ±5.0 × 10−324 to ±1.7 × 10308 (~15 digits)

Floating-Point Types – Explained

float – fast, lower-precision double – slower, higher precision long double

High precision on some systems, not widely used

No guarantee on exact size (as with integers)

sizeof(float)<=sizeof(double)<=sizeof(long double)

16

Using Floating-Point Types

Live Demo

Declaring And Using Variables

Declaring Variables When declaring a variable we:

Specify its type

Specify its name (called identifier)

May give it an initial value

The syntax is the following:

Example:

19

<data_type> <identifier> [= <initialization>];

int height = 200;

Identifiers Variable names are called identifiers

All "words" in a language are identifiers Data types

Operators

Functions

Identifiers in C++ are case-sensitive THis != THIS != this

(the last being a reserved keyword)

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Identifiers in C++ (1) C++ identifiers

One or more letters, digits or underscores

Start with a letter or underscore Avoid starting underscores & double

underscores

May be compiler-reserved

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Identifiers in C++ (2) C++ identifiers

Can’t be a standard reserved keyword

Can’t be a name of operator representations

Can’t be a reserved compiler keyword Some compilers reserve their own

keywords

Bad C++ identifiers are detected compile-time Don’t need to remember all rules

Compiler will warn you about errors

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Identifiers in C++ (3) Standard reserved identifiers in C++

Reserved identifiers for alternate operator representations (not always reserved)

asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete, do, double, dynamic_cast, else, enum, explicit, export, extern, false, float, for, friend, goto, if, inline, int, long, mutable, namespace, new, operator, private, protected, public, register, reinterpret_cast, return, short, signed, sizeof, static, static_cast, struct, switch, template, this, throw, true, try, typedef, typeid, typename, union, unsigned, using, virtual, void, volatile, wchar_t, while

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and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq

Identifiers – Examples Examples of correct identifiers:

Examples of incorrect identifiers:

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int new; // new is a keywordint 2Pac; // Cannot begin with a digit

int New = 2; // Here N is capitalint _2Pac; // This identifiers begins with _

string greeting = "Hello";

int n = 100; // Undescriptiveint numberOfClients = 100; // Descriptive

// Overdescriptive identifier:int numberOfPrivateClientOfTheFirm = 100;

Valid C++ IdentifiersLive Demo

Variable Scope Scope of a variable

Lines in code, where the identifier is valid

i.e. "where the variable is still alive"

Two kinds of scope in C++ Global (almost) – the variable is

visible and usable by all functions in the program

Local – the variable is visible and usable only in the current block i.e. in the inner-most { … }

26

Variable Scope Making a local variable – declare it in

a block

Making a global variable – declare outside any function or class

int main() {int a = 5; //local variablereturn 0;

}

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#include <iostream>int a = 5; //global variableint main(){

a++;return 0;

}

Initializing VariablesWays to initialize, Default values

Initializing Variables C++ supports two ways of initializing

Through the assignment operator

Through calling the type constructor

Both ways are equivalent in the case of primitives

int a = 5;

29

int a (5);

Initializing Variables – Defaults

What happens to uninitialized variables?

C++ allows operating on uninitialized variables Actually variables get initialized

sometimes

Even if not initialized, variables get values

Whatever was in that part of memory

Where the variable is placed

int a; cout << a;

30

Initializing Variables – Defaults

Initialization depends on the variable scope

If the variable is global Initialized to default type value

E.g. for integers 0 If the variable is local

Undefined in standard

Usually garbage values from memory

Whatever values in memory marked free

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Initializing VariablesLive Demo

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Data Types and Variables

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