Programming and
Algorithms
CSE308
LECTURE 2: PROBLEM-SOLVING
BY: DR. MOSTAFA ABDELRAZIK
What is problem solving?
Problem-solving is a process of working
through the details of a problem to
reach a solution.
Problem Solving Steps
1) Define and Identify the
Problem
2) Analyze the Problem
3) Design the Solution
4) Implement the Code
5) Run and Debug
6) Documentation
The Problem-solving Process
Analysis
Problem
specification
Design
Algorithm
Implementation
Program
Compilation
Executable
(solution)
Analyze the Problem
We need to read it till we understand every detail
We need to dissect the problem into its component
parts (e.g. problems and sub-problems)
We need to remove any ambiguity, extra
information
We need to determine our knowns and our
unknowns
We need to be aware of any assumptions we are
making.
Analyze the Problem
Example. Making tea. Suppose we have a robot that
carries out household tasks. We wish to program the
Analyze the
robot to make a cup of tea. An initial attempt at an
algorithm might be:
Problem
1. Put tea leaves in a pot
2. Boil water
3. Add water to the pot
4. Wait 5 minutes
5. Pour the tea into cup
Design the solution
Design
Developing the algorithm that solves the
problem
the
• Identify alternative ways to solve the problem
solution
• Select the best way to solve the problem from the list
of alternative solutions
• List instructions that enable you to solve the problem
using selected solution
The algorithm is expressed a s flowchart or
pseudo-code
Design the solution
Implement the Code
Algorithm translated into a code using a high-
level language.
The choice depends on the type of the
problem
To get the exact result of the problem, proper
coding should be written
The codes written should be efficient and
concise.
It should be small enough.
What is debugging?
Programming errors are called bugs and the process of tracking them down
and correcting them is called debugging.
Three kinds of errors can occur in a program:
1. Syntax errors
▪ A program can only be executed if it is syntactically correct; otherwise, the process
fails and returns an error message.
▪ syntax refers to the structure of a program and the rules about that structure.
2. Runtime errors
▪ So called because the error does not appear until you run the program.
▪ These errors are also called exceptions because they usually indicate that something
exceptional (and bad) has happened.
3. Semantic errors
▪ If there is a semantic error in the program, it will run successfully, in the sense that the
computer will not generate any error messages, but it will not do the right thing. It will
do something else. Specifically, it will do what the programmer told it to do.
▪ But the written program does not solve the original problem. The meaning of the
program (its semantics) is wrong.
Testing the Algorithm
• Important distinction
➢ Mathematics
We test the answer
➢ Programs
We test the process
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Testing the Algorithm
It should be done to report that whether
the problem has met its desired
TESTING requirements or not.
The process of proving that the program
produces correct and meaningful
results for all possible data.
It should use cases that work with all
possible parts of the program.
DOCUMENTATION
• It is a Continuous process.
• Two kinds of documentation
* User-level documentation
One needs in order to use the program.
* Technical documentation
It is addressed to the programmer who may modify the program later.
Algorithms
Algorithm
A set of unambiguous instructions for solving a
problem or subproblem in a finite amount of
time using a finite amount of data
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Algorithms
A tool for solving a well-specified
computational problem
Input Algorithm Output
Algorithms must be:
Correct: For each input produce an appropriate
output
Efficient: run as quickly as possible, and use as little
memory as possible – more about this later
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What is a good algorithm?
❑ It must be correct
❑ It must be finite (in terms of time and size)
❑ It must terminate
❑ It must be unambiguous
▪ Which step is next?
❑ It must be space and time efficient
A program is an instance of an algorithm, written in some
specific programming language
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Correct and incorrect algorithms
Algorithm is correct if, for every input instance, it
ends with the correct output. We say that a correct
algorithm solves the given computational problem.
An incorrect algorithm might not end at all on some
input instances, or it might end with an answer
other than the desired one.
We shall be concerned only with correct
algorithms.
From Algorithms to Programs
Problem
Algorithm: A sequence
of instructions describing
how to do a task (or
process)
C++ Program
Pseudocode & Algorithm
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”
Pseudocode & Algorithm
Detailed Algorithm
Step 1: Input M1,M2,M3,M4
Step 2: GRADE (M1+M2+M3+M4)/4
Step 3: if (GRADE < 50) then
Print “FAIL”
else
Print “PASS”
endif
The Flowchart
(Dictionary) A schematic representation of a
sequence of operations, as in a manufacturing
process or computer program.
(Technical) A graphical representation of the
sequence of operations in an information system or
program.
Information system flowcharts show how data flows from
source documents through the computer to final
distribution to users.
Program flowcharts show the sequence of instructions in a
single program or subroutine. Different symbols are used to
draw each type of flowchart.
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 1
Statement 2
Statement 3
:
Flowchart – selection control
structure
No Yes
Condition
else- then-
statement(s) statement(s)
Flowchart – repetition control
structure
yes Loop
Condition Statement(s)
no
Example
START
Step 1: Input M1,M2,M3,M4
Input Step 2: GRADE (M1+M2+M3+M4)/4
M1,M2,M3,M4 Step 3: if (GRADE <50) then
Print “FAIL”
GRADE(M1+M2+M3+M4)/4
else
Print “PASS”
endif
N IS Y
GRADE<50
PRINT PRINT
“PASS” “FAIL”
STOP
Example 2
Write an algorithm and draw a flowchart to
convert the length in feet to centimeter.
Pseudocode:
Input the length in feet (Lft)
Calculate the length in cm (Lcm) by
multiplying LFT with 30
Print length in cm (LCM)
Example 2
Flowchart
Algorithm
START
Step 1: Input Lft
Step 2: Lcm Lft x 30 Input
Lft
Step 3: Print Lcm
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
Example 3
Algorithm
START
Step 1: Input W,L
Step 2: AL x W Input
W, L
Step 3: Print A
ALxW
Print
A
STOP
Flowchart – example 4
Begin
Read birth date
Calculate
Age = current year – birth date
Display
age
End
Flowchart – example 5
Begin
Read age
YES Age > 60? NO
print “Retired” print “Working”
End
Flowchart – example 6
Begin
sum = 0
current_number = 1
NO
current_number <= 10? print sum
YES
End
sum = sum + current_number
current_number = current_number + 1
