4-Variables.pdf

STRUCTURED

PROGRAMMING

Content

2



Variables



C++ Built in data types



C++ variable declaration syntax



cin

statement



Local and global variables

Scope resolution operator (::)



Variables and RAM



Defining constants



Typecasting

Objectives

3

 By the end you should be able to:

 Understand what a variable is and why they are used

 Declare, naming, and initialize variables and constants

 Use C++ Built in data types to create program variables

 Differentiate between constants, and variables

 Recognize the ways that can be used to change the data stored in variables

 Apply C++ syntax rules in reading user input using cin stream and iostream library

 Explain what is meant by the scope of a variable

 Differentiate between local and global variables

 Recognize the ways how can we convert from one type to another

 Declare, initialize, and manipulate enumerated data types

Variable

 Location on computer’s memory to store data then use and change

its value in a program

 Each variable has

1. Name (identifier)

 Series of letters, digits, underscores

 Not a keyword

 Start with a letter

 Case sensitive

 Meaningful

2. Type

 Programmer-defined

 Built-in

C++ Built-in Data Types

 Called fundamental types or primitives types: numeric, character,

logical

bool

Data Type

 Has two values, true and false

 Manipulate logical (Boolean) expressions

 true and false are called logical values

 bool, true, and false are reserved words

char

Data Type

 Used for characters

 letters, digits, and special symbols

 Each character is enclosed in single quotes

 Examples: 'A', 'a', '0', '*', '+', '$', '&'

 A blank space is a character and is written ' ' with a space left

between the single quotes

Declaring Variables

 All variables must be declared anywhere in program with a name

and data type before they used

 Syntax rule: begin with a data type then variable name

 Variables of the same type can be declared in

 Multiple lines

 One line separated by commas

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int num1;

int num2;

int num3;

int num1, num2, num3;

string

Data Type

 Used to store data as a text ( a sequence of characters)

 Ex: “This is some text”

 String should be enclosed within double quotation “ ”

 To declare a variable of type string should refer to the header file

named string

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#include <string>

using namespace std;

int main() {

string studentName; return 0;

Initializing Variable

 Variables can be initialized once declared

 first and second are int variables with the values 13 and 10,

respectively

 ch is a char variable whose value is empty

 st is a string variable whose value is Amal

 x and y are double variables with 12.6 and 123.456, respectively

10

int first=13, second=10;

char ch=' ';

string st=“Amal”;

Using

cin

 Namespace

 std::

 Specifies using a name that belongs to “namespace” std  Can be removed through use of using statements

 Standard output stream object

 std::cin

 “Connected” to keyboard

Using

cin

(cont.)

 Stream extraction operator >>

 Value to left (left operand) inserted into right operand  Waits for user to input value then press Enter key  Example

 std::cin >> num1;

 Inserts the standard input from keyboard into variable num1

 Prints message before cin statement to direct the user to take a

specification called prompt

 cin and cout facilitate interaction between user and program

13 Enter first integer

45

Enter second integer 72

Sum is 117

1 // Fig. 2.5: fig02_05.cpp

2 // Addition program that display the sum of two numbers.

3 #include <iostream> // allow program to perform input and output

4

5 // function main begins program execution

6 int main()

7 {

8 // variable declaration

9 int number1; // first integer to add

10 int number2; // second integer to add

11 int sum; // sum of number1 and number2

12

13 std::cout << "Enter first integer: \n"; // prompt user for data

14 std::cin >> number1; // read first integer from user to number1

15

16 std::cout << "Enter second integer: \n"; // prompt user for data

17 std::cin >> number2; // read second integer from user to number2

18

19 sum = number1 + number2; // add the numbers; stor result in sum

20

21 std::cout << "Sum is " << sum << std::endl; // display sum; end line

22

23 return 0; // indicate that program ended successfully

24 } // end function main

Declare integer variables.

Use stream extraction

operator with standard input stream to obtain user input.

Stream manipulator

std::endl outputs a newline, then “flushes output buffer.”

Concatenating, chaining or cascading stream insertion operations.

Calculations can be performed in output statements: alternative for lines 18 and 20:

Variable Scope

 Portion of the program where the variable can be used

 Scope can be

 Local

Local Variables

 Defined within a module

 Can be seen and used only by module itself

 Store temporally in memory

 Erased when the module terminates

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int main() {

int i; char a;

Global Variables

 Defined outside any module

 Used and seen by all modules

 Variable name can be duplicated within and outside a modules

 Differentiate between them by using unary scope resolution operator (::) 16

int i;

int main() {

char a;

Unary Scope Resolution Operator

 Denoted as ( :: )

 Used to declare local and global variables have a same name

 To avoid conflicts

 Syntax rule

 Not needed if names are different

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y = ::x + 3

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1 // Fig. 6.23: fig06_23.cpp

2 // using the unary scope resolution operator.

3 #include <iostream>

4 using std::cout;

5 using std::endl;

6

7 int number = 7; // global variable named number

8

9 int main()

10 {

11 double number = 10.5; // local variable named number

12

13 // display values of local and global variables

14 cout<< “local double value of number = “ << number

15 << “\nGlobal int value of number = “ << ::number << endl;

16 return 0; // indicate successful termination

17 } // end main

Variables and Memory

 Variables names correspond to location in the computer’s memory

(RAM)

 Every variable has name, type, size and value

 Placing new value into variable (memory location), overwrites old

value – called destructive

 Reading value of variable in memory – called nondestructive

Variables and Memory (cont.)

std::cin >> number1;

Assume user entered 45

std::cin >> number2;

Assume user entered 72

sum = number1 + number2;

20

number1 45

number1 45

number2 72

number1 45

number2 72

Constants

 Like variables

 data storage locations

 Unlike variables

 Values never changed during program execution

 Any attempt to change a const creates a compilation error

 Declared in two ways and follow identifier naming rules

 With const keyword

 With #define keyword

21

const char Gender = ‘F’;

Compatible C++ Data Types

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Data types

long double

double

float

unsigned long int (synonymous with unsigned long)

long int (synonymous with long)

unsigned int (synonymous with unsigned)

int

unsigned short int (synonymous with unsigned short)

short int (synonymous with short)

unsigned char

char bool

Highest

Converting Data Types

 Convert variables or expression of a given type into another type

 Two kinds of conversion

 Implicit conversion

 Explicit conversion

 May include

 Promotion: converting from low to high data type

Implicit Conversion

 Mixed Type Expressions

double avg = total / cnt ;

 Assignment statement

int x = 7.5 ;

24

double int

Promote to double

double int

Demote to int  fraction part is truncated

Explicit Conversion - Typecasting

 C-like notation

double b;

b = (double) a;

 Functional notation

 Using keyword static_cast

Enumerated Data Types

 Enable to create new types and then define variables of these types

 Syntax rule

 Write the keyword enum followed by new type name

 List legal values separated by a comma within braces

enum COLOR { RED, BLUE, GREEN, WHITE, BLACK };

Enumerated Data Types (cont.)

 Every enumerated constant has an integer value

enum COLOR { RED=100, BLUE, GREEN=500, WHITE, BLACK=700 };

 If you don’t specify integer values, the first constant has the value 0, and the

rest count up from there

 Variable declaration and manipulation

Color dress;

dress = RED;

Enumerated Data Types (cont.)

 Every enumerated constant has an integer value

enum Day { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY };

In memory...

MONDAY = 0 TUESDAY = 1 WEDNESDAY = 2 ..etc

 Using the Day declaration, the following code...

Day d = FRIDAY ;

cout << MONDAY << " " << WEDNESDAY << " " << d << endl;

Enumerated Data Types (cont.)

 You canNOT directly assign an integer value to an enum variable

Day workDay ;

workDay = 3; // Error!

 Instead, you must cast the integer:

workDay = static_cast<Day>(3);

 You CAN assign an enumerator to an int variable

int x; x = THURSDAY; x = workDay ;

.. What is the value of x in each statements above?

Exercise - 1

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1.

Write a program that declares two constant A and B

2.

Initialize A =1 and B=2.2

3.

Declare an int named C and float named D

4.

Initialize C =A and D=B

Included Sections

31