Lecture 02 Algorithmic Thinking Prof. Katherine Gibson Prof. - - PowerPoint PPT Presentation

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Lecture 02 Algorithmic Thinking Prof. Katherine Gibson Prof. - - PowerPoint PPT Presentation

CMSC201 Computer Science I for Majors Lecture 02 Algorithmic Thinking Prof. Katherine Gibson Prof. Jeremy Dixon Based on slides by Shawn Lupoli and Max Morawski at UMBC www.umbc.edu Last Class We Covered Syllabus Grading scheme,


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CMSC201 Computer Science I for Majors

Lecture 02 – Algorithmic Thinking

  • Prof. Katherine Gibson
  • Prof. Jeremy Dixon

Based on slides by Shawn Lupoli and Max Morawski at UMBC

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Last Class We Covered

  • Syllabus

– Grading scheme, expectations, etc. – Academic Integrity Policy

  • Computer System Components
  • Binary numbers

– Converting between binary and decimal

  • Algorithmic thinking

– Making sandwiches for aliens

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Any Questions from Last Time?

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Today’s Objectives

  • To practice thinking algorithmically
  • To understand and be able to implement

proper program development

  • To start learning about control structures
  • To be able to express an algorithm

using a flow chart

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What is an Algorithm?

  • Steps used to solve a problem
  • Problem must be

– Well defined – Fully understood by the programmer

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  • Steps must be

– Ordered – Unambiguous – Complete

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Developing an Algorithm

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Program Development

  • 1. Understand the problem
  • 2. Represent your solution (your algorithm)

– Pseudocode – Flowchart

  • 3. Implement the algorithm in a program
  • 4. Test and debug your program

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Step 1: Understanding the Problem

  • Input

– What information or data are you given?

  • Process

– What must you do with the information/data? – This is your algorithm!

  • Output

– What are your deliverables?

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“Weekly Pay” Example

  • Create a program to calculate the

weekly pay of an hourly employee

– What is the input, process, and output?

  • Input: pay rate and number of hours
  • Process: multiply pay rate by number of hours
  • Output: weekly pay

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Step 2: Represent the Algorithm

  • Can be done with flowchart or pseudocode
  • Flowchart

– Symbols convey different types of actions

  • Pseudocode

– A cross between code and plain English

  • One may be easier for you – use that one

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Step 2A: Pseudocode

  • Start with a plain English description, then…
  • 1. Display “Number of hours worked: ”
  • 2. Get the hours
  • 3. Display “Amount paid per hour: ”
  • 4. Get the rate
  • 5. Compute pay = hours * rate
  • 6. Display “The pay is $” , pay

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Flowchart Symbols

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Start End

Start Symbol End Symbol Data Processing Symbol Input/Output Decision Symbol Flow Control Arrows

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Step 2B: Flowchart

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pay = hours * rate

Start

Display “Number

  • f hours worked: ”

Get the hours

Display “Amount paid per hour: ”

Get the rate

Display “The pay is $” , pay

End

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Steps 3 and 4: Implementation and Testing/Debugging

  • We’ll cover implementation in detail next class
  • Testing and debugging your program involves

identifying errors and fixing them –We’ll talk about this later today

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Algorithms and Language

  • Notice that developing the algorithm

didn’t involve any Python at all

– Only pseudocode or a flowchart was needed – An algorithm can be coded in any language

  • All languages have 3 important control

structures we can use in our algorithms

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Control Structures

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Control Structures

  • Structures that control how the program

“flows” or operates, and in what order

  • Sequence
  • Decision Making
  • Looping

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Sequence

  • One step after another, with no branches
  • Already wrote one for “Weekly Pay” problem
  • What are some real life examples?

– Dialing a phone number – Purchasing and paying for groceries

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Decision Making

  • Selecting one choice from many based
  • n a specific reason or condition

– If something is true, do A … if it’s not, do B

  • What are some real life examples?

– Walking around campus (construction!) – Choosing where to eat lunch

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Decision Making: Pseudocode

  • Answer the question “Is a number positive?”

– Start with a plain English description

  • 1. Display “Enter the number: ”
  • 2. Get the number (call it num)
  • 3. If num > 0
  • 4. Display “It is positive”
  • 5. Else
  • 6. Display “It is negative”

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Decision Making: Flowchart

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Start

Display “Enter the number: ” Get the number

num > 0

End

Display “It is positive” Display “It is negative” TRUE FALSE

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Looping

  • Doing something over and over again
  • Used in combination with decision making

– Otherwise we loop forever

  • This is called an “infinite loop”
  • What are some real life examples?

– Doing homework problem sets – Walking up steps

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Looping: Pseudocode

  • Write an algorithm that counts from 1-20

– Start with a plain English description

  • 1. Set num = 1
  • 2. While num <= 20
  • 3. Display num
  • 4. num = num + 1
  • 5. (End loop)

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num >= 20

Looping: Flowchart

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Start End

Display num FALSE

num = 1

num >= 20

TRUE

num = num + 1

There’s an error in this flowchart… do you see it?

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num <= 20

Looping: Flowchart

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Start End

Display num FALSE

num = 1

TRUE

num = num + 1

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Debugging

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A Bit of History on “Bugs”

  • US Navy lab – September 9, 1947
  • Grace Hopper and her colleagues

were working on the Harvard Mark II

– Or trying to… it wasn’t working right

  • They found a literal bug inside the machine

– Taped the bug (a moth) into their log book

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Errors (“Bugs”)

  • Two main classifications of errors
  • Syntax errors

– Prevent Python from understanding what to do

  • Logical errors

– Cause the program to run incorrectly, or to not do what you want

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Syntax Errors

  • “Syntax” is the set of rules followed by a

computer programming language

– Similar to grammar and spelling in English

  • Examples of Python’s syntax rules:

– Keywords must be spelled correctly True and False, not Ture or Flase or Truu – Quotes and parentheses must be closed: ("Open and close")

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Syntax Error Examples

  • Find the syntax errors in each line of code below:

1 prnit("Hello") 2 print("What"s up? ") 3 print("Aloha!) 4 print("Good Monring")

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Syntax Error Examples

  • Find the syntax errors in each line of code below:

1 prnit("Hello") 2 print("What"s up? ") 3 print("Aloha!) 4 print("Good Monring")

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not actually a syntax error

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Logical Errors

  • Logical errors don’t bother Python at all…

they only bother you!

  • Examples of logical errors:

– Using the wrong value for something currentYear = 2013 – Doing steps in the wrong order

  • “Close jelly jar. Put jelly on bread. Open jelly jar.”

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Exercise

  • Write an algorithm that asks a user for their

name, then responds with “Hello NAME”

  • You can use a flowchart or pseudocode

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Start End

Data Processing Input/Output Decision Flow Control

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Exercise #2

  • Write an algorithm that asks a user for their

grade, and tells them their letter grade.

A: 100 - 90 C: <80 - 70 F: <60 - 0 B: <90 - 80 D: <70 - 60

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Start End

Data Processing Input/Output Decision Flow Control

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Announcements

  • Your Lab 0 is an in-person lab this week!

–You need to go to your labs during your assigned lab time

  • Homework 1 is out

– Due by next Monday (Feb 8th) at 8:59:59 PM

  • Both of these assignments are on Blackboard

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Practice Problems

  • Complete the 2 exercises on the previous slides
  • Modify our “count to 20” algorithm so that it counts

from 0 to 100, in increments of 5

  • Design an algorithm that finds the average of

three exam scores (pseudocode or flowchart)

  • Advanced: Design an algorithm that asks the user for two

numbers, and then asks them if they want to multiply, add, or subtract the numbers from each other; perform the operation the user wanted, and show them the result

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