Introduction to Programming(CS1123)

By

Dr. Muhammad Aleem,

Department of Computer Science, Mohammad Ali Jinnah University, Islamabad

Fall 2013

What is a Computer?

• A computer is a electro-mechanical device that works semi-automatically to process input data according to the stored set of instructions and produces output or resultant data.

A Computer System

Instructions

DataResults

Components of a Computer System

Main Memory

CPU

Arithmetic and Logic Unit

Control Unit

Computer

Peripheral Devices / Connected devices

Keyboard Mouse Display CD Rom Hard Disk

Computer Instructions and Programs

• Instruction: A computer instruction is a command or directive given to a computer to perform specific task.

Examples: Add 2 and 5, Print “Hello World”

• Program: A program is sequence of instructions written in programming language that directs a computer to solve a problem

Examples: Draw a square, etc.

Computer Instructions and Programs

• Program “Draw a square”

1 – Draw a vertical line of length n inches

2 – Draw a horizontal line of n inches

3– Draw a vertical line of length n inches

4 – Draw a horizontal line of n inches

Computer Software System

Operating Systems (Windows, Linux, MAC, Solaris)

Compilers / Libraries (C++, C, Java)

Application Programs(.cpp, .c, .java,)

Computer Hardware

Programming Languages

• Classification of programming languages:

1. Machine language

2. Low-level languages

3. High-level languages

1. Machine level languages

• A computer understands only sequence of bits or 1’s and 0’s (the smallest piece of information)

• A computer program can be written using machine languages (01001101010010010….)

• Very fast execution

• Very difficult to write and debug

• Machine specific (different codes on different machines)

2. Low level languages

• English encrypted words instead of codes (1’s and 0’s)

• More understandable (for humans)

• Example: Assembly language

• Requires: “Translation” from Assembly code to Machine code

compare: cmpl #oxa,n cgt

end_of_loop acddl #0x1,nend_of_loop:

10010101010011011110010110010100010101011101001001101001101110111101100101010101

Assembler

Assembly Code Machine Code

3. High level languages

• Mostly machine independent

• Close to natural language (English like language keywords)

• Easy to write and understand programs

• Easy to debug and maintain code

• Requires compilers to translate to machine code

• Slower than low-level languages

3. High level languages

• Some Popular High-Level languages– COBOL (COmmon Business Oriented Language)– FORTRAN (FORmula TRANslation) – BASIC (Beginner All-purpose Symbolic Instructional Code) – Pascal (named for Blaise Pascal) – Ada (named for Ada Lovelace) – C (whose developer designed B first) – Visual Basic (Basic-like visual language by Microsoft) – C++ (an object-oriented language, based on C)– Java

Programming Paradigms

• Programming paradigm is the fundamental style of computer programming

1. Imperative2. Functional 3. Logical4. Object Oriented

Imperative Paradigm

• Machine based modes (Sequence of steps required to compute)

• Statements executed sequentially step-wise

• Emphasis on How is to compute

• How to obtain the required results or output

• Example languages: C, Pascal, Ada, etc.

Imperative Paradigm Languages

Functional/Logical Paradigm

• Specify what is to be computed using programming abstractions.

• Responsibility of a programmer is to specify What is to compute

• How to compute is implementation dependent and is managed by language compiler

• Example languages: Lisp, Scheme, Erlang, Haskell, Scala etc.

Object-Oriented Paradigm

• Programming with Abstract Data Types, to model real world

• Basic Program Unit: Class (a programmer defined type)– A entity that contains data and methods which work

on the data

• Basic Run-time Unit: Object– Instance of a class during execution

Problem Solving Steps

1. Understand the problem2. Plan the logic3. Code the program4. Test the program5. Deploy the program into production

1. Understanding the Problem

• Problems are often described in natural language like English.

• Users may not be able to specify needs well, and the needs may changing frequently

• Identify the requirements1. Inputs or given data-items

2. Required output(s) or desired results

3. Indirect inputs (may not be given directly, you have to calculate or assume)

1. Understanding the Problem

– Example: Calculate the area of a circle having the radius of 3 cm• Inputs: – Radius=3

• Output: – Area

• Indirect Inputs: – Pi=3.14

– Area = 3.14 * (3*3) = 28.27

2. Plan the Logic

• Identify/Outline small steps in sequence, to achieve the goal (or desired results)

• Tools such as flowcharts and pseudocode can be used:1. Flowchart: a pictorial representation of the logic

steps2. Pseudocode: English-like representation of the logic

• Walk through the logic before coding

3. Code the Program

• Code the program:

– Select the programming language

–Write the program instructions in the selected programming language

–Use the compiler software to translate the program into machine understandable code or executable file

– Syntax errors (Error in program instructions) are identified by the compiler during compilation and can be corrected.

4. Test the Program

• Testing the program–Execute it with sample data and check the

results

– Identify logic errors if any (undesired results or output) and correct them

–Choose test data carefully to exercise all branches of the logic (Important)

5. Deploy the Program

• Putting the program into production–Do this after testing is complete and all known

errors have been corrected

Introduction to Pseudocode

• One of the popular representation based on natural language

• Widely used– Easy to read and write–Allow the programmer to concentrate on the

logic of the problem

• Structured in English language (Syntax/grammar)

What is Pseudocode (continued...)

• English like statements

• Each instruction is written on a separate line

• Keywords and indentation are used to signify particular control structures.

• Written from top to bottom, with only one entry and one exit

• Groups of statements may be formed into modules

Some Basic Constructs

• Print Output is to be sent to the Printer

• Write Output is to be written to a file

• Display Output is to be written to the screen

• Prompt required before an input instruction Get, causes the message to be sent to the screen

• Compute /Calculate Calculation performed by computer

• Calculation operations: +, -, *, /, ()

Some Basic Constructs

• Set Used to set inital value, E.g., Set Marks to 0

• = Place values from right hand-side item to left hand side

E.g., Marks = 67

• Save Save the variable in file or disk E.g., Save Marks

Comparison/Selection

IF student_Marks are above 50 THENResult = “Pass“

ELSEResult = “Fail“ENDIF

• Making decision by comparing values• • Keywords used: • IF, THEN, ELSE

IF student_Marks are above 50 THENResult = “Pass“

ELSEResult = “Fail“

ENDIF

Comparison/Selection

CASE of <condition-variable> Case <valueA>

Do Step1 Case <valueB>

Do Step2 Case <valueC>

Do Step3ENDCASE

• Case Strtucture Multiple banching based on value of condition

• Three keywords used: • CASE of, Case, ENDCASE

Comparison/Selection

• Case Strtucture Example...

Repeat a group of Statements

DOWHILE student_total < 50Read student recordPrint student nameAdd 1 to student_total

ENDDO

• Repeating set of instruction base on some condition

• Keyword used: • DOWHILE, ENDDO

Example Pseudocode Program

• A program is required to read three numbers, add them together and print their total.

Inputc Processing Output

Number1Number2Number3

total

Solution Algorithm

Add_three_numbersSet total to 0

Read number1 Read number2 Read number3Total = number1 + number2 + number3Print total

END

Repeat Until (Loop)• Repeat-Until statement (loop)

REPEAT Do StepA Do StepB

UNTIL conditionN is True

- What is the Difference between (While & Repeat) ?

Example:…

Flowcharts• “A graphic representation of a sequence of

operations to represent a computer program”

• Flowcharts show the sequence of instructions in a single program or subroutine.

A Flowchart• A Flowchart– Shows logic of an algorithm or problem– Shows individual steps and their interconnections– E.g., control flow from one action to the next

Name Symbol Description

Oval Beginning or End of the Program

Parallelogram Input / Output Operations

Rectangle Processing for example, Addition, Multiplication, Division, etc.

Diamond Denotes a Decision (or branching) for example IF-Then-Else

Arrow Denotes the Direction of logic flow

Flowchart Symbols

Example 1

Step 1: Input M1,M2,M3,M4Step 2: GRADE = (M1+M2+M3+M4)/4 Step 3: if (GRADE < 50) then

Print “FAIL” else

Print “PASS” endif

START

InputM1,M2,M3,M4

GRADE(M1+M2+M3+M4)/4

ISGRADE<50

STOP

YN

Print“FAIL”

Print“PASS”

Example 2• Write an algorithm and draw a flowchart to

convert the length in feet to centimeter.

Example 2

Algorithm • Step 1: Read Lft• Step 2: Lcm = Lft x 30 • Step 3: Print Lcm

Flowchart

START

Read Lft

Lcm = Lft x 30

STOP

Print LCM

Example 3• Write an algorithm and draw a flowchart that will read

the two sides of a rectangle and calculate its area.

Example 3

Algorithm • Step 1: Read W,L• Step 2: A = L x W • Step 3: Print A

START

ReadW, L

A L x W

STOP

PrintA

DECISION STRUCTURES • The expression A>B is a logical expression

• It describes a condition we want to test

• if A>B is true (if A is greater than B) we take the action on left

• Print the value of A

• if A>B is false (if A is not greater than B) we take the action on right

• Print the value of B

IF–THEN–ELSE STRUCTURE

• The algorithm for the flowchart is as follows:If A>B then

print AElse

print Bendif

isA>B

Y N

Print A Print B

• Multiple branching based on a single condition

CASE STRUCTURE

CASE Condition

Do Step A Do Step B Do Step C Do Step D

Relational / Logical Operators

Relational Operators

Operator Description> Greater than

< Less than

= Equal to

Greater than or equal to Less than or equal to

Not equal to

Example 4• Write an algorithm that reads two values, finds largest

value and then prints the largest value.

ALGORITHMStep 1: Read VALUE1, VALUE2Step 2: if (VALUE1 > VALUE2) then

MAX VALUE1else

MAX VALUE2endif

Step 3: Print “The largest value is”, MAX

-- DRAW the Flow Chart for the Program

Selection Structure• A Selection structure can be based on

1. Dual-Alternative (two code paths)2. Single Alternative (one code path)

isA>B

Y N

Print A Print B

Dual Alternative Example

Selection Structure• Single Alternative Example

Pseudocode: IF GPA is greater than 2.0 ThenPrint “Promoted ”

End IF

GPA > 2.0

N Y

Print “Promoted”

Loop Structure• Repetition (WHILE structure)– Repeats a set of actions based on the answer to a

question/condition pseudocode: DoWHILE <Some-True-Condition>

Do SomethingENDDO

Loop Structure• REPEAT-UNTIL structure– Repeats a set of actions until a condition remains True

– pseudocode: REPEAT Do-Something

UNTIL <Some True Condition>