Introduction

1. Introduction George Mason University

2. Today’s topics • Review of Introduction chapter • Go over examples and questions • Introduction to Python

3. Introduction • What is a program? Let’s draw a diagram • What does a computer know how to do?

4. Examples • What examples did you have trouble with?

5. 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

6. 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

7. 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

8. Example run

9. 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:

10. 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

11. 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)

12. 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()

13. Exercise 12 • Let’s draw what memory looks like, and why it’s important to understand the concept of storage for programming

14. 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

15. 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

16. 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

17. 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()

18. 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

19. 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

20. 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