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Introduction. George Mason University. Today’s topics. Review of Introduction chapter Go over examples and questions Introduction to Python. Introduction. What is a program? Let’s draw a diagram What does a computer know how to do?. Examples. What examples did you have trouble with?.

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Introduction
Introduction

George Mason University


Today s topics
Today’s topics

  • Review of Introduction chapter

  • Go over examples and questions

  • Introduction to Python


Introduction1
Introduction

  • What is a program? Let’s draw a diagram

  • What does a computer know how to do?


Examples
Examples

  • What examples did you have trouble with?


Converting your code to python
Converting your code to python

  • Solution to exercise 1:

    1. num1 = get the first number from the user

    2. num2 = get the second number from the user

    3. difference = num1 – num2

    4. return difference

  • In python:

    num1 = input(“Please enter the first number: ”)

    num2 = input(“Please enter the second number: ”)

    difference = num1 – num2

    return difference


Input
input( )

  • input(“prompt”) displays the prompt to the user

    • waits for them to type in something with the keyboard

    • returns whatever was typed in when the user hits enter

    • this value is often stored


Running your code
Running Your Code

  • Place your code inside the template file available on the course syllabus

    • Indentation is important!

  • Save the file (keep the name template.py)

  • Open a terminal and change directory to the folder containing your template.py

  • Type the following to run your code:

    python template.py

  • This will print the returned result to the screen



Errors
Errors

  • If we make a mistake in our code, it might crash or print out the wrong value

  • Let’s modify our code to do something wrong:


Errors1
Errors

  • There are many many types of errors

    • If you encounter one, you are allowed to Google the error (cut and paste just the error message) or ask a friend what that error message means

      • You could Google “SyntaxError: invalid syntax”

    • You are NOT allowed to Google for a solution, or ever show your actual code to a friend

    • Understanding what an error message means is different than knowing how to fix it

    • Basically, only share/show/ask about what is on the terminal


Exercise 3
Exercise 3

1. radius = get the radius from the user

2. area = radius x radius x 3.14

3. return area

  • In python:

    radius = input(“Enter the radius”)

    area = radius * radius * 3.14

    return area

    In python, multiplication is represented with *

    In python, integer division represented with / or //

    In python, normal division represented with x/(y*1.0)


Exercise 9
Exercise 9

1. number = get the number from the user

2. product1 = number x 3

3. product2 = product1 x 3

4. product3 = product2 x 3

5. return product1 + “ “ + product2 + “ “ + product3

  • In python:

    number = input(“Please enter a number”)

    product1 = number * 3

    product2 = product1 * 3

    product3 = product2 * 3

    return str(product1) + “ “ + str(product2) + “ “ + str(product3)

    You can add together numbers and strings, but you must convert the number to a string before adding it to the string, with str()


Exercise 12
Exercise 12

  • Let’s draw what memory looks like, and why it’s important to understand the concept of storage for programming


Writing code in python
Writing code in python

  • A value is stored in memory by using the assignment operator, =

    • The left side is the label for a memory location, called a variable

    • The right side is always a value (or something that simplifies to a value), that is stored in memory

    • num1 is a variable that stores the value entered by the user


Variables
Variables

  • You pick their names from letters, numbers, or the underscore (no spaces!)

    • cannot start with a number

    • Python is case-sensitive, so num1 and Num1 are different variables

  • Descriptive variable names are preferred to single letters

    • so difference is better than diff, d, or a


Expressions
Expressions

  • Python simplifies expressions, like num1 – num2, to be used in an assignment or return statement

  • Like mathematical expressions, precedence matters; use parentheses as need

    • 3 + 2 * 5 is different than (3 + 2) * 5


Expressions and types
Expressions and types

  • Python cannot mix string and numeric types in expressions

    • convert a number to a string using str()

    • convert a string to an integer with int()or rational number with float()


Comments
Comments

  • Commentsstart with # and are not run

    • They can occur on the same line as code; anything to the right of # does not get run

  • You can put English notes inside your code like this


What else is in this example
What else is in this example?

  • Python cares about indentation, but we will learn why later

  • Your text editor will highlight strings and comments in different colors (pink and gray here)

  • keywordshave special meaning in python, and cannot be used as variable names; your text editor will highlight them in blue

  • We will learn about lines 1 and 11 later this semester


Questions
Questions?

  • If we have time, let’s convert the exercises from today into python code

  • One of these will be the coding part of the quiz in the next lab


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