STRUCTURED

PROGRAMMING

Variables

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

Enumerated data types

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

Create, trace, and debug programs containing input and output

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

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C++ Built-in Data Types

Called fundamental types or primitives types: numeric, character,

logical

5

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

6

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

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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;

dataType varName;

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

9

#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”;

double x=12.6, y=123.456;

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

Defined in input/output stream header file <iostream>

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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

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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

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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:

std::cout << "Sum is " << number1 + number2 << std::endl;

Variable Scope

Portion of the program where the variable can be used

Scope can be

Local

Global

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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;

return 0;

}

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;

return 0;

}

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

::variable

18

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

Local double value of number = 10.5

Global int value of number = 7

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

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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

sum 117

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

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const char Gender = ‘F’;

#define studentsPerClass 15

Compatible C++ Data Types 22

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

Lowest

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

Demotion: converting from high to low data type

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Implicit Conversion

Mixed Type Expressions

double avg = total / cnt ;

Assignment statement

int x = 7.5 ;

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double int

Promote to double

double int

Demote to int fraction part is truncated

warning : conversion from 'double' to 'int',

possible loss of data

Explicit Conversion - Typecasting

C-like notation

int a = 2000;

double b;

b = (double) a;

Functional notation

b = double (a);

Using keyword static_cast

b = static_cast<double>(a);

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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 };

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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;

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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;

..will produce this output: 0 2 4

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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?

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Exercise - 1 30

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

5. Write statements to print C and D to screen

Included Sections 31

Chapter 2: from section 4 and 5