2. Introduction
• Computers are used for solving various day-to-day problems and thus
problem solving is an essential skill that a computer science student should
know.
• It is pertinent to mention that computers themselves cannot solve a
problem.
• Precise step-by-step instructions should be given by us to solve the problem.
• Thus, the success of a computer in solving a problem depends on how
correctly and precisely we define the problem, design a solution (algorithm)
and implement the solution (program) using a programming language.
• Thus, problem solving is the process of identifying a problem, developing
an algorithm for the identified problem and finally implementing the
algorithm to develop a computer program.
6. Analyzing the problem
• It is important to clearly understand a problem before we begin to find
the solution for it.
• We need to read and analyze the problem statement carefully in order to
list the principal components of the problem and decide the core
functionalities that our solution should have.
• By analyzing a problem, we would be able to figure out what are the
inputs that our program should accept and the outputs that it should
produce.
7. Developing an Algorithm
• It is essential to device a solution before writing a program code for a
given problem.
• The solution is represented in natural language and is called an algorithm.
• We start with a tentative solution plan and keep on refining the algorithm
until the algorithm is able to capture all the aspects of the desired
solution.
• For a given problem, more than one algorithm is possible and we have to
select the most suitable solution.
8. Coding
• After finalizing the algorithm, we need to convert the algorithm into the
format which can be understood by the computer to generate the desired
solution.
• Different high level programming languages can be used for writing a
program.
9. Testing and Debugging
The program created should be tested on various parameters.
• The program should meet the requirements of the user.
• It must respond within the expected time.
• It should generate correct output for all possible inputs.
In the presence of syntactical errors, no output will be obtained.
In case the output generated is incorrect, then the program should be checked for logical
errors, if any. This is to ensure that the software meets all the business and technical
requirements and works as expected.
10. Algorithm
• In our day-to-day life we perform activities by following certain sequence
of steps.
• To complete each activity, we follow a sequence of steps.
• A finite sequence of steps required to get the desired output is
called an algorithm.
11. Before developing the algorithm, let us first identify the input, process and
output:
• Input: Number whose square is required
• Process: Multiply the number by itself to get its square
• Output: Square of the number
Algorithm to find square of a number.
Step 1: Input a number and store it to num
Step 2: Compute num * num and store it in square
Step 3: Print square
12. Why do we need an Algorithm?
• The programmer first prepares a roadmap of the program to be written, before actually
writing the code. Such a roadmap is nothing but the algorithm which is the building
block of a computer program.
• For example, searching using a search engine, sending a message, finding a word in a
document, booking a taxi through an app, performing online banking, playing computer
games, all are based on algorithms.
• Writing an algorithm is mostly considered as a first step to programming.
• If the algorithm is correct, computer will run the program correctly, every time. So, the
purpose of using an algorithm is to increase the reliability, accuracy and efficiency of
obtaining solutions.
13. Characteristics of a good algorithm
• Precision -the steps are precisely stated or defined.
• Uniqueness -results of each step are uniquely defined and only
depend on the input and the result of
the preceding
steps.
• Finiteness -the algorithm always stops after a finite number of
steps.
• Input -the algorithm receives some input.
• Output -the algorithm produces some output.
14. While writing an algorithm, it is required to clearly identify
the following:
• The input to be taken from the user
• Processing or computation to be performed to get the
desired result
• The output desired by the user
15. Representation of Algorithms
There are two common methods of representing an algorithm —
flowchart and pseudo code.
Either of the methods can be used to represent an algorithm while
keeping in mind the following:
• It showcases the logic of the problem solution, excluding any
implementation details
• It clearly reveals the flow of control during execution of the program
16. Flowchart
• A flowchart is a visual representation of an algorithm.
• A flowchart is a diagram made up of boxes, diamonds and other shapes,
connected by arrows.
• Each shape represents a step of the solution process and the arrow
represents the order or link among the steps.
20. Draw a flowchart to solve the problem of a non-functioning light bulb
21. Pseudocode
• A pseudocode is another way of representing an algorithm.
• It is considered as a non-formal language that helps programmers to write
algorithm.
• It is a detailed description of instructions that a computer must follow in a
particular order.
• It is intended for human reading and cannot be executed directly by the
computer.
• No specific standard for writing a pseudocode exists.
• The word “pseudo” means “not real,” so “pseudocode” means “not real
code”.
22. Following are some of the frequently used keywords while writing pseudocode:
• INPUT
• COMPUTE
• PRINT
• INCREMENT
• DECREMENT
• IF/ELSE
• WHILE
• TRUE/FALSE
23. Write an algorithm to display the sum of two numbers entered by
user, using both pseudo code and flowchart.
Pseudo code for the sum of two numbers will be:
INPUT num1
INPUT num2
COMPUTE Result = num1 + num2
PRINT Result
24. Write an algorithm to calculate area and perimeter of a rectangle, using both
pseudo code and flowchart.
Pseudo code for calculating area and perimeter of a rectangle.
INPUT length
INPUT breadth
COMPUTE Area = length * breadth
PRINT Area
COMPUTE Perim = 2 * (length + breadth)
PRINT Perim
25. Benefits of Pseudo code
• Before writing codes in a high level language, a pseudo code of a program
helps in representing the basic functionality of the intended program.
• By writing the code first in a human readable language, the programmer
safeguards against leaving out any important step.
• Besides, for non-programmers, actual programs are difficult to read and
understand, but pseudo code helps them to review the steps to confirm
that the proposed implementation is going to achieve the desire output.
26. Flow of Control
• The flow of control depicts the flow of events as represented in the flow
chart.
• The events can flow in a sequence, or on branch based on a decision or
even repeat some part for a finite number of times.
27. Sequence
• The sequence construct means the statements are being executed
sequentially .
• Algorithms where all the steps are executed one after the other are said
to execute in sequence.
• It is simply performing one step after another.
28. SELECTION
• Selection construct means the execution of statements depending upon
a condition.
• If a condition evaluates to true, set of statements is followed otherwise a
different set of statements are followed.
• This construct is also called decision construct because it helps in
making decision about which set of statements is to be executed.
• It is used to make yes/no or true/false decisions logically.
29. Write an algorithm to check whether a number is odd or even.
• Input: Any number
• Process: Check whether the number is even or not
• Output: Message “Even” or “Odd”
Pseudocode of the algorithm can be written as follows:
PRINT "Enter the Number"
INPUT number
IF number MOD 2 == 0 THEN
PRINT "Number is Even"
ELSE
PRINT "Number is Odd"
30. Write a pseudocode and draw a flowchart where multiple conditions are checked to categorise a
person as either
child (<13), teenager (>=13 but <20) or adult (>=20),based on age specified.
• Input: Age
• Process: Check Age as per the given criteria
• Output: Print either “Child”, “Teenager”, “Adult”
Pseudocode is as follows:
INPUT Age
if Age < 13 then
PRINT "Child"
else if Age < 20 then
PRINT "Teenager"
else
PRINT "Adult"
31. The syntax of if statement is:
if condition:
statement(s)
Example of if statement is,
age = int(input("Enter your age "))
if age >= 18:
print("Eligible to vote")
32. Syntax of if else :
if condition:
statement(s)
else:
statement(s)
Example of if else :
age = int(input("Enter your age: "))
if age >= 18:
print("Eligible to vote")
else:
print("Not eligible to vote")
33. Syntax of if else if :
if condition:
statement(s)
elif condition:
statement(s)
elif condition:
statement(s)
else:
statement(s)
Example of if else if:
number = int(input("Enter a number: ")
if number > 0:
print("Number is positive")
elif number < 0:
print("Number is negative")
else:
print("Number is zero")
34. Algorithm for a card game called “Dragons and Wizards”. Make two teams DRAGONS
and WIZARDS The rules for the game are as follows:
• If the card drawn is a diamond or a club, Team DRAGONS gets a point
• If the card drawn is a heart which is a number, Team WIZARDS gets a point
• If the card drawn is a heart that is not a number, Team DRAGONS gets a point
• For any other card, Team WIZARDS gets a point
• The team with highest point is the winner
Let us identify the following for a card:
Input: shape, value
Process: Increment in respective team scores by one based on the outcome of the card
drawn, as defined in the rules.
Output: Winning team
35. INPUT shape
INPUT value
SET Dpoint = 0, Wpoint = 0
IF (shape is diamond) OR (shape is club) THEN
INCREMENT Dpoint
ELSE IF (shape is heart) AND (value is number)THEN
INCREMENT Wpoint
ELSE IF (shape is heart) AND (value is not a
number)THEN
INCREMENT Dpoint
ELSE
INCREMENT Wpoint
END IF
If Dpoint > Wpoint THEN
PRINT "Dragon team is the winner"
ELSE
PRINT "Wizard team is the winner"
36. ITERATION
• The iteration construct mean repetition of a set-of-statements depending
upon a condition-test.
• Iteration is a looping construct
• Iteration is a combination of decision and sequence and can repeat steps
• Iteration can be thought of as “while something is true, do this, otherwise
stop”
• Iteration logic is used when one or more instructions may be executed
several times depending on some condition .
37. Syntax of the For Loop:
for <variable> in <sequence>:
statements to_repeat
Example of the For Loop:
for letter in 'PYTHON':
print(letter)
Syntax of the while Loop:
while test_condition:
body of while
38. Write pseudocode and draw a flowchart to accept 5 numbers and find their
average.
Pseudocode will be as follows:
Step 1: Set count = 0, sum = 0
Step 2: While count <5 , repeat steps 3 to 5
Step 3: Input a number to num
Step 4: sum = sum + num
Step 5: count = count + 1
Step 6: Compute average = sum/5
Step 7: Print average
39. Write pseudocode and draw flowchart to accept numbers till the user enters 0 and then
find their average.
Pseudocode is as follows:
Step 1: Set count = 0, sum = 0
Step 2: Input num
Step 3: While num is not equal to 0, repeat Steps 4 to 6
Step 4: sum = sum + num
Step 5: count = count + 1
Step 6: Input num
Step 7: Compute average = sum/count
Step 8: Print average
40. Verifying Algorithms
• To verify, we have to take different input values and go through all the steps
of the algorithm to yield the desired output for each input value and may
modify or improve as per the need.
• The method of taking an input and running through the steps of the
algorithm is sometimes called dry run.
• Dry run will help us to:
1. Identify any incorrect steps in the algorithm
2. Figure out missing details or specifics in the algorithm
• It is important to decide the type of input value to be used for the
simulation.
• In case all possible input values are not tested, then the program will fail.
41. Comparison of Algorithm
• There can be more than one approach to solve a problem using
computer and hence we can have more than one algorithm.
• Algorithms can be compared and analysed on the basis of the
amount of processing time they need to run and the amount of
memory that is needed to execute the algorithm. These are
termed as time complexity and space complexity, respectively.
• The choice of an algorithm over another is done depending on
how efficient they are in terms of processing time required (time
complexity) and the memory they utilise (space complexity)
42. Coding
• Coding is the process of converting algorithm to the syntax of the
given programming language.
• The ordered set of instructions is written in that programming
language by following its syntax.
• Syntax is the set of rules or grammar that governs the formulation
of the statements in the language, such as spellings, order of
words, punctuation, etc.
• A program written in a high-level language is called source code.
43. Decomposition
• The basic idea of solving a complex problem by decomposition is to
'decompose' or break down a complex problem into smaller sub problems.
• Finally, the sub-problems are combined in a logical way to obtain the
solution for the bigger, main problem.
• Each sub problem can be solved independently and by different persons
