Cse115 lecture03problemsolving

Lecture 03
Problem Solving
CSE115: Computing Concepts

Introduction to Problem Solving
•Problem solving is the process of transforming the
description of a problem into a solution by using our
knowledge of the problem domain and by relying on
our ability to select and use appropriate problem-
solving strategies, techniques and tools.
•Computers can be used to help us solving problems

Software Development Method (SDM)
1. Specification of needs
2. Problem analysis
3. Design and algorithmic representation
4. Implementation
5. Testing and verification
6. Documentation

Specification of Needs
•To understand exactly:
• what the problem is
• what is needed to solve it
• what the solution should provide
• if there are constraints and special conditions.
I want a
train…

Problem Analysis
• In the analysis phase, we should identify the following:
• Inputs to the problem, their form and the input media to be
used
• Outputs expected from the problem, their form and the output
media to be used
• Special constraints or conditions (if any)
• Formulas or equations to be used

Design and Algorithmic Representation
• An algorithm is a sequence of a finite number of steps
arranged in a specific logical order which, when executed,
produces the solution for a problem.
• An algorithm must satisfy these requirements:
• It may have an input(s)
• It must have an output
• It should not be ambiguous (there should not be different
interpretations to it)
• Every step in algorithm must be clear as what it is supposed to
do

Design and Algorithmic Representation
• It must be general (it can be used for different inputs)
• It must be correct and it must solve the problem for which it is
designed
• It must execute and terminate in a finite amount of time
• It must be efficient enough so that it can solve the intended
problem using the resource currently available on the computer
• An algorithm can be represented using pseudocodes or
flowcharts.

Cse115 lecture03problemsolving

Pseudocodes
• A pseudocode is a semiformal, English-like language with limited
vocabulary that can be used to design and describe algorithms.
• Criteria of a good pseudocode:
• Easy to understand, precise and clear
• Gives the correct solution in all cases
• Eventually ends

Pseudocodes: The Sequence control structure
• A series of steps or statements that are executed in the order they
are written in an algorithm.
• The beginning and end of a block of statements can be optionally
marked with the keywords begin and end.
• Example 1:
Begin
Read the birth date from the user.
Calculate the difference between the
birth date and today’s date.
Print the user age.
End

Pseudocodes: The Selection control structure
• Defines two courses of action depending on the outcome of a
condition. A condition is an expression that is, when computed,
evaluated to either true or false.
• The keyword used are if and else.
• Format:
if condition
then-part
else
else-part
end_if
Example:
if age is greater than 55
print “Retire”
else
print “Work Work Work”
end_if

•Sometimes in certain situation, we may omit the
else-part.
if number is odd number
print “This is an odd number”
end_if

• Nested selection structure: basic selection structure that
contains other if/else structure in its then-part or else-
part.
if number is equal to 1
print “One”
else if number is equal to 2
print “Two”
else if number is equal to 3
print “Three”
else
print “Other”
end_if

Pseudocodes: The Repetition control structure
• Specifies a block of one or more statements that are repeatedly
executed until a condition is satisfied.
• The keyword used is while.
• Format:
while condition
loop-body
end_while

• Example: Summing up 1 to 10
set cumulative sum to 0
set current number to 1
while current number is less or equal to 10
add the cumulative sum to current number
add 1 to current number
end_while
print the value of cumulative sum

• Subsequently, we can write the previous pseudocode with
something like this.
• Example: Summing up 10 numbers
cumulative sum = 0
current number = 1
while current number is less or equal to 10
cumulative sum = cumulative sum + current number
current number = current number + 1
end_while
print the value of cumulative sum
• Note that in this algorithm, we are using both the
sequence and repetition control structure

• Example:
Begin
number of users giving his birth date = 0
while number of users giving his birth date < 10
Read the birth date from the user.
Calculate the difference between the birth
date and today’s date.
Print the user age.
if the age is greater than 55
print “retire”
else
print “keep working”
end_if
number of user giving his birth date + 1
end_while
End

Flowcharts
• Flowcharts is a graph used to depict or show a step by
step solution using symbols which represent a task.
• The symbols used consist of geometrical shapes that are
connected by flow lines.
• It is an alternative to pseudocoding; whereas a
pseudocode description is verbal, a flowchart is graphical
in nature.

Flowchart Symbols
Terminal symbol - indicates the beginning and
end points of an algorithm.
Process symbol - shows an instruction other than
input, output or selection.
Input-output symbol - shows an input or an output
operation.
Disk storage I/O symbol - indicates input from
or output to disk storage.
Printer output symbol - shows hardcopy printer
output.

Flowchart Symbols cont…
Selection symbol - shows a selection process
for two-way selection.
Off-page connector - provides continuation
of a logical path on another page.
On-page connector - provides continuation
of logical path at another point in the same
page.
Flow lines - indicate the logical sequence of
execution steps in the algorithm.

Flowchart – sequence control structure
Statement 2
Statement 1
Statement 3
:

Flowchart – selection control structure
Condition
else-
statement(s)
then-
statement(s)
YesNo

Flowchart – repetition control structure
Condition
Loop
Statement(s)
yes
no

Flowchart – example 1
Begin
Read birth date
Calculate
Age = current year – birth date
Display
age
End

Begin
Read age
End
Age > 55? NOYES
print “retired” print “keep working”

Begin
End
current_number <= 10?
NO
YES
sum = 0
current_number = 1
sum = sum + current_number
current_number = current_number + 1
print sum

Implementation
• The process of implementing an algorithm by writing a
computer program using a programming language (for
example, using C language)
• The output of the program must be the solution of the
intended problem
• The program must not do anything that it is not supposed to
do
• (Think of those many viruses, buffer overflows, trojan horses, etc.
that we experience almost daily. All these result from programs doing
more than they were intended to do)

Testing and Verification
• Program testing is the process of executing a program to
demonstrate its correctness
• Program verification is the process of ensuring that a program
meets user-requirement
• After the program is compiled, we must run the program and
test/verify it with different inputs before the program can be
released to the public or other users (or to the instructor of this
class)
Awesome!!!

Documentation
• Contains details produced at all stages of the program
development cycle.
• Can be done in 2 ways:
• Writing comments between your line of codes
• Creating a separate text file to explain the program
• Important not only for other people to use or modify your
program, but also for you to understand your own program
after a long time (believe me, you will forget the details of your
own program after some time ...)

Documentation
• Documentation is so important because:
• You may return to this program in future to use the whole of or a part
of it again
• Other programmer or end user will need some information about
your program for reference or maintenance
• You may someday have to modify the program, or may discover some
errors or weaknesses in your program
• Although documentation is listed as the last stage of software
development method, it is actually an ongoing process which
should be done from the very beginning of the software
development process.

Homework
• Read the value of the height, width and length of a box from
the user and print its volume.
• Convert temperature (provided as input) from degree Celsius
to degree Fahrenheit.
• Find the minimum of three given numbers.
• Find the maximum of three given numbers.
• Take a, b and c as input and determine the roots for the
equation ax2+bx+c.
• Read a positive integer n from the user and then compute and
print the sum of all integers between 1 and n.
• Read 100 numbers from the user and print their sum.
• Determine whether a given integer is a prime number or not.
• Find the smallest integer n such that, 1+2+3+...+n is equal to
or greater than 1000.

Cse115 lecture03problemsolving

Recommended

More Related Content

What's hot (20)

Similar to Cse115 lecture03problemsolving (20)

Recently uploaded (20)

Cse115 lecture03problemsolving