ALGORITHMS AND PROGRAMMING TECHNIQUES
 Introduction to Program Development and Problem Solution
 Computers do what we tell them to do NOT what we want them to do
 Computer Programming involves writing instructions and giving them to the
computer to complete a task.
 A computer program or software is:
 a set of instructions written in a computer language in order to
 be executed by a computer to perform a useful task
Ex: Application software packages, such as word processors, spreadsheets and databases are
all computer programs
 A Computer Programmer is
 is a person who translates the task you want a computer to do
 into a form that a computer can understand
 A well designed computer program must be:
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correct and accurate
easy to understand
easy to maintain and update
efficient
reliable
flexible
 The program development process involves the following steps:
 document the program
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program documentation is the text or graphics that provide description of
the purpose of a function or a particular step
the purpose of instruction in a program
determine the user needs
design the program specifications
review the program specifications
design the algorithm
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steps that will convert the available input into the desired output
step by step solution for a given problem is called the algorithm
a flowchart graphically details processing steps of a particular program
 code the program
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write the program in a programming language using a program editor
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a program editor is a program that allows to type, edit and save a program code on
a disk
 compile, test and debug the program
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in order to find out possible errors in the program
types of errors may be syntax errors, run-time errors and logic errors
 get program to the user
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install software on the users computer and offer training
 Types of programming errors
 Syntax of a programming language is the set of rules to be followed when
writing a program
 syntax error occurs when these rules are violated
 run-time errors occur when invalid data is entered during program execution
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e.g. program expects numeric data and alphabetic data is entered
program will crash
 logic error will not stop the program but results will be inaccurate
 The process of finding and correcting errors in a program is called debugging
 For successful programming
 give no ambiguity in program instructions
 give no possibility of alternative interpretations
 make sure there is only one course of action
 Programming task can be made easier
 by breaking large and complex programs
 into smaller and less complex subprograms (modules)
 Programming task can be separated into 2 phases (see Fig. 1)
 problem solving phase
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produce an ordered sequence of steps that describe solution of problem
this sequence of steps is called an algorithm
Example of an Algorithm: a recipe, to assemble a brand new computer ...
what else?
 implementation phase
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implement the program in some programming language (Pascal, Basic, C)
Figure 1: Problem Solving and Programming
 What is structured programming
 a programming technique that splits the program into smaller segments
(modules) to
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decrease program development time
decrease program maintenance cost
improve the quality of software
 structured programming achieves these goals by using
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top-down design and use of modules
use of limited control structures (sequence, selection and repetition)
management control
 Top-down design starts with major functions involved in a problem
 and divide them into sub-functions
 until the problem has been divided as much as possible
 Categories of programming languages are
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machine
low-level languages
assembly
high-level
fourth-generation
fifth-generation
}
 Machine language is
 made up of binary 1s and 0s
 this is the only programming language the computers can understand
 advantages of machine languages are:
fast execution speed and efficient use of main memory
 disadvantages of machine languages are
 writing machine language is tedious, difficult and time consuming
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 Types of language-translator programs
 assemblers
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translates the program in assembly-language into machine-language
 compilers
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translates high-level language program into machine-language all at once
 interpreters
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translates high-level language into machine-language a line at a time
 Major high-level languages are
 FORTRAN, COBOL, PL/I, BASIC, PASCAL, C, LISP, Prolog, Logo
 FORTRAN
 FORmula TRANslator introduced in 1957
 for use by scientists, engineers and mathematicians
 well suited for complex numeric calculations
 COBOL
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COmmon Business Oriented Language
a programming language used for business data processing
designed to handle large data files
 PL/I
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Programming Language One created in 1960
general purpose language for powerful computations and
sophisticated data structures
today largely used in the oil industry
 BASIC
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Beginners Allpurpose Symbolic Instruction Code created in 1960
easy to learn interactive language on a time-sharing computer
 PASCAL
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Named after Blaise Pascal and created in 1960
suited to both scientific and file processing applications
designed to teach structured programming and top-down design
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Developed at Bell Labs in 1970s
used advantages of both high-level and low-level languages
C gives programmers extensive control over computer hardware
incorporates sophisticated control and data structures
C++ is the object oriented version of C
C
LISP
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is a language that processes symbol sequences (lists) rather than numbers
designed to handle data strings more efficiently than others
 Prolog
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is a language for symbol processing
 Logo
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is an interactive education oriented language
designed to teach inexperienced users logic and programming techniques
includes list processing capabilities
employs a triangular object called turtle
to draw, animate and color images very simply
 Object-Oriented Programming Languages (OOPL)
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there are two main parts of a program, instructions and data
traditional prog. languages treat instructions and data as separate entities
an OOPL treats a program as a series of objects and messages
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to qualify as an OOPL a language must incorporate concepts of
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is the ability of a programming language to define a new object
that has all the attributes of an existing object
programmer inherits the existing object
writes the code that describes how the new object differs from the existing one
polymorphism is
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combining data and instructions into a reusable structures
encapsulation generates prefabricated components of a program
inheritance
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1. encapsulation
2. inheritance
3. polymorphism
encapsulation is
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an object is a combination of data and instructions that work on data
and is stored together as a reusable unit
messages are instructions sent between objects
the ability of an object to respond to a message in many ways
for example, say you have a circle object and a square object
each object has the same characteristics of drawing
when a drawing message is sent to a circle object it draws a circle
when a drawing message is sent to a square object it draws a square
thus, each object has the same characteristics of drawing
but the characteristics is implemented differently
Advantages of OOPL
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code re-use rather than reinvention and adaptable code
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these speed development & maintenance of applications and reduce cost
Smaltalk, Objective-C and C++ are examples of OOPL
ALGORITHMS AND FLOWCHARTS
 A typical programming task can be divided into 2 phases:
 Problem solving phase
produce an ordered sequence of steps that describe solution of problem
 this sequence of steps is called an algorithm
Examples of Algorithm: a recipe, to assemble a brand new computer ...
what else?
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 Implementation phase
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implement the program in some programming language
 Steps in Problem solving
 First produce a general algorithm (You can use pseudocode)
 Refine the algorithm successively to get step by step detailed algorithm that is
very close to a computer language.
Pseudocode is an artificial and informal language that helps programmers develop
algorithms. Pseudocode is very similar to everyday English.
Example 1: Write an algorithm to determine the students final grade and indicate
weather it is passing or failing. The final grade is calculated as the average of four
marks.
Solution:
Pseudocode:
Input a set of 4 marks
Calculate their average by summing and dividing by 4
if average is below 50
Print “FAIL”
else
Print “PASS”
Detailed Algorithm:
Step 1:
Step 2:
Step 3:
Input M1, M2,M3,M4
GRADE = (M1+M2+M3+M4)/4
if (GRADE <50) then
Print “FAIL”
else
Print “PASS”
The Flowchart
 Easier to convey ideas by picture than by words
 Examples:
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It is easier to show how to go somewhere on a map than explaining
It is easier to construct a toy if diagrams are shown
It is easier to construct a new PC if diagrams are provided
 Hence use pictorial format for describing an algorithm
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this is called a FLOWCHART
 A Flowchart
 shows logic of an algorithm
 emphasises individual steps and their interconnections
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e.g. control flow from one action to the next
 standard flowchart symbols are shown below
Example 1: Write an algorithm to determine the students final grade and indicate
weather it is passing or failing. The final grade is calculated as the average of four
marks.
Pseudocode:
Input a set of 4 marks
Calculate their average by summing and dividing by 4
if average is below 50
Print “FAIL”
else
Print “PASS”
Detailed Algorithm:
Flowchart:
Step 1:
Step 2:
Step 3:
Input M1, M2,M3,M4
GRADE = (M1+M2+M3+M4)/4
if (GRADE <50) then
Print “FAIL”
else
Print “PASS”
START
TRACE TABLE:
Input
M1,M2,M3,M4
GRADE=(M1+M2+M3+M4)/4
MARK
GRADE
M1=80
80
M2=30
+ 30
M3=40
+ 40
M4=30
+ 30 /4
= 45
N
IS
GRADE<50
PRINT
“PASS”
Y
PRINT
“FAIL”
STOP
STATUS
FAIL
Example 2: Write an algorithm and draw a flowchart to convert the length in feet to
centimeter.
Pseudocode:
Input the lenght in feet (Lft)
Calculate the length in cm (Lcm) by multiplying LFT with 30
Print LCM
Algorithm:
Step 1:
Step 2:
Step 3:
Flowchart:
Input Lft
Lcm = Lft x 30
Print Lcm
START
Input
Lft
Lcm = Lft x 30
Print
Lcm
STOP
Example 3: Write an algorithm and draw a flowchart that will read the two sides of
a rectangle and calculate its area.
Pseudocode:
Input the width (W) and Length (L) of a rectangle
Calculate the area (A) by multiplying L with W
Print A
Algorithm:
Step 1:
Step 2:
Step 3:
Flowchart:
START
Input
W, L
A=LxW
Print
A
STOP
Input W,L
A=L x W
Print A
Example 4: Write an algorithm and draw a flowchart that will calculate the roots of
a quadratic equation ax2 + bx + c = 0
Hint: d = sqrt (b2 – 4ac), and the roots are: x1 = (–b + d)/2a and x2 = (–b – d)/2a
Pseudocode:
Input the coefficients (a, b, c) of the quadratic equation
Calculate the discriminant d
Calculate x1
Calculate x2
Print x1 and x2
Algorithm:
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
Flowchart:
Input a, b, c
d = sqrt (b2 – 4 x a x c)
x1 = (–b + d) / (2 x a)
x2 = (–b – d) / (2 x a)
Print x1 and x2
START
Input
a, b, c
d = sqrt(b2 – 4 x a x c)
x1 = (–b + d) / (2 x a)
X2 = (–b – d) / (2 x a)
Print
x1 and x2
STOP
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 of
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print the value of A (see Fig. 2)
 if A>B is false (if A is not greater than B) we take the action on right
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print the value of B (see Fig. 2)
Y
Figure 2:
Print A
is
A>B
N
Print B
IF–THEN–ELSE STRUCTURE
 The structure is as follows
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If condition
then true alternative
else false alternative
endif
 The algorithm for the flowchart in figure 2 is as follows:
If A>B
then print A
else print B
endif
Here “>” is called the relational operator. Table 1 gives the possible relational operators:
Relational Operators
Operator
Meaning
>
<
=



Greater than
Less than
Equal to
Greater than or equal to
Less than or equal to
Not equal to
Example 5: Write an algorithm that reads two values, determines the largest value
and prints the largest value with an identifying message.
Algorithm:
Step 1:
Step 2:
Step 3:
Input VALUE1, VALUE2
if VALUE1 > VALUE2
then MAX = VALUE1
else MAX = VALUE2
endif
Print “The largest value is”, MAX
Flowchart:
START
Input
VALUE1,VALUE2
Y
is
N
VALUE1>VALUE2
MAX = VALUE1
MAX = VALUE2
Print
“The largest value is”, MAX
STOP
NESTED IFS
 One of the alternatives within an IF–THEN–ELSE statement
 may involve further IF–THEN–ELSE statement
Example 6: Write an algorithm that reads three numbers and prints the value of the
largest number.
Algorithm:
Step 1:
Step 2:
Step 3:
Input N1, N2, N3
if N1>N2
then if N1>N3
then MAX = N1 [N1>N2, N1>N3]
else MAX = N3 [N3>N1>N2]
endif
else if N2>N3
then MAX = N2 [N2>N1, N2>N3]
else MAX = N3 [N3>N2>N1]
endif
endif
Print “The largest number is”, MAX
Flowchart: Draw the flowchart of the above Algorithm.
Example 7: Write and algorithm and draw a flowchart to
a) read an employee name (NAME), overtime hours worked (OVERTIME), hours
absent (ABSENT) and
b) determine the bonus payment (PAYMENT).
Bonus Schedule
OVERTIME – (2/3)*ABSENT
Bonus Paid
>40 hours
$50
$40
>30 but  40 hours
$30
>20 but  30 hours
$20
>10 but  20 hours
$10
 10 hours
Algorithm: Step 1:
Step 2:
Step 3:
Input NAME,OVERTIME,ABSENT
if OVERTIME–(2/3)*ABSENT > 40
then PAYMENT = 50
else if OVERTIME–(2/3)*ABSENT > 30
then PAYMENT = 40
else if OVERTIME–(2/3)*ABSENT > 20
then PAYMENT = 30
else if OVERTIME–(2/3)*ABSENT > 10
then PAYMENT = 20
else PAYMENT = 10
endif
endif
endif
endif
Print “Bonus for”, NAME “is $”, PAYMENT
Flowchart: Draw the flowchart of the above algorithm?