Introduction to Algorithms
Yonglei Tao
What is an Algorithm?

A step-by-step description about a solution to a
problem
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

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Unambiguous
In a logical order
Executable
A program is the expression of an algorithm in a
programming language
Examples of Algorithm

Making a phone call
1)
2)
3)
4)
5)
6)

Pick up the receiver
Hear the dial-tone
Dial a phone number
Wait for the reply
Talk
Replace the receiver
Other examples
 Programming a VCR
 A recipe for a cake
Building Instructions for LEGO
Developing an Algorithm
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How to develop an algorithm?
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Problem solving
How to describe an algorithm?
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In English?
In a programming language?
Pseudocode
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English statements + basic control structures in
programming languages
Good compromise

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Simple, readable, no strict rules, don’t worry about
punctuation
Let you think at an abstract level about the problem
Allow you to focus on what needs to be done without
having to deal with the syntax and grammar of a
programming language
Pseudocode vs. Code

Pseudocode

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Get the rate from user
VB code

dblRate = Convert.ToDouble(txtRate.Text)
Building Blocks for Pseudocode
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Basic statements
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Get the input (from user)
Print the output (on the screen)
Perform basic calculations
Conditional and iterative statements
Variables
Variables

How to do (2 + 3) * (5 – 1) using a calculator?
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A variable is a memory location
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Can store a value
Addressable by its name
Examples
count
sum
Basic Statements

Assign values to variables using basic arithmetic
operations
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Get input from user
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Set the value of x to 3 or x = 3
Set the value of y to x/10 or y = x/10
Set the value of z to x +25 or z = x + 25
Get the value of x
Print results to the screen
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Print the value of y, z
Print “CIS160 Visual BASIC”
Example

Write pseudocode that asks the user for three
numbers, computes their sum and average and
outputs the results.
1.
2.
3.
4.
Get the values of a, b, c from user
avg = ( a + b + c ) / 3
sum = a + b + c
Print sum, avg
Example

Write pseudocode that reads the value of a circle
radius from the user, and prints the area of a circle
with that radius
1.
2.
3.
Get the value of radius from user
area = pi * radius * radius
Print “The area of your circle is “ & area
Exercise

Write pseudocode that inputs the length and width
of a carpet in feet and the price in dollar per square
yard. The algorithm should print out the total cost
of the carpet, including a 6% sales tax.

1 square yard = 9 square feet