1. First Python Program
Professor Hugh C. Lauer CS-1004 — Introduction to Programming for Non-Majors
(Slides include materials from Python Programming: An Introduction to Computer Science, 2nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute)
CS-1004, A-Term 2014
First Python Program

2. Problem:– Find area of a disk
I.e., area of the part between inner circle and outer circle
How to do manually?
What value do you use for  ?
What if your have a lot of disks?
(i.e., with calculator)
CS-1004, A-Term 2014
First Python Program

3. Problem:– Find area of a disk (continued)
What about Excel?
This is, in fact, a form of programming!
Change value of cell  Excel reacts by changing values of all cells depending on that one!
A functional “language”
CS-1004, A-Term 2014
First Python Program

4. IDLE — Integrated Development Environment
Simple GUI for Python programmers
Python “shell”
I.e., command interpreter
Intelligent file editor
Understands Python syntax and formatting
Debugger
Single-step
View values of variables
CS-1004, A-Term 2014
First Python Program

5. More terms — Compiler A computer program that reads another program …
… line-by-line, character-by-character …
… analyzes it, …
… and converts it into internal machine instructions and data
CS-1004, A-Term 2014
First Python Program

6. More terms — Interpreter
A computer program that reads one or more lines of another program …
… analyzes just those lines …
… and executes them in place
Much in common with compiler
The Python shell is really an interpreter
CS-1004, A-Term 2014
First Python Program

7. Expressions
When typed into Python “shell”, evaluated immediately
Print Result
Much like (the right side of) any scientific or engineering equation
Operators
+, -, *, /, **
Special operator: //
Example: Area of disk
Inner radius = 3, outer radius = 5
What value to use for  ?
“import” Python’s own value called “pi”
CS-1004, A-Term 2014
First Python Program

8. Questions?
CS-1004, A-Term 2014
First Python Program

9. Variables
Definition:– a symbolic name used to refer to a value or to some computational object
Starts with a letter or underscore
Followed by any sequence of letters, digits, and underscore
Case sensitive
May not be reserved word — i.e., keyword See Table 2.1 (for all 33 keywords)
Subtly different from variables in C, Java, etc.
Python variable “refers to” or “points to” a value or object
C/Java variable is name of a location where value or object is stored
Using a variable
Anywhere in an expression or Python statement where the value could have been used instead
Examples
CS-1004, A-Term 2014
First Python Program

10. Assignment operator an action
variableName = any_valid_Python_expression
Makes variableName refer to the value of that expression
A specific action that happens at a specific time
I.e., when the Python interpreter gets to that particular statement
Variable retains what it refers to until changed (by another assignment operator)
Like C and Java
Unlike Excel
Example
Inner and outer radii of disk
CS-1004, A-Term 2014
First Python Program

11. Print statement Example print(3 + 2)
print(“The sum of 3 and 2 is “, 3 + 2)
May be any sequence of values
Converted into printable strings
“Printed” on output device
E.g., The IDLE window …
CS-1004, A-Term 2014
First Python Program

12. Questions?
CS-1004, A-Term 2014
First Python Program

13. Function
Definition: A sequence of Python statements that takes zero or more arguments and (optionally) returns a value that can be used in a Python expression
Useful when you need to repeat the same computation on different values
Example: area of disk
Also useful when you want to solve a (sub) problem and then put it out of your mind while working on another problem
Especially useful when sharing code
With teammates, colleagues, etc.
With people you don’t know!
CS-1004, A-Term 2014
First Python Program

14. Calling (i.e., invoking) a function
Function name used in Python expression or statement
Meaning:–
Suspend whatever you were doing
Execute the function with the argument values in place of parameters
When function “returns”, use its return value at the place where it was called.
Examples
Area of disk
CS-1004, A-Term 2014
First Python Program

15. Defining functions Indentation matters! return statement
Python uses indentation to recognize when still reading part of function vs whatever comes later!
return statement
Tells Python interpreter to stop interpreting the function and go back to what it was previously doing
(Optionally) delivers a value back to “caller” to be used at the point where function was called
CS-1004, A-Term 2014
First Python Program

16. Note “print” and “input” are functions!
Almost everything you do in Python will be part of a function
Almost every program your submit for homework will consist of one or more functions
Functions can (and usually do) call each other!
CS-1004, A-Term 2014
First Python Program

17. Questions?
CS-1004, A-Term 2014
First Python Program

18. This weekend Install Python 3.4 on your own computer or laptop
See course website for “cookbook”
DO IT TODAY!
GET HELP IF YOU NEED IT
Homework #1
Available on Course web-site
Need WPI password to access
Start ASAP
Due next Thursday, 6:00 PM
Read Chapter 1
Responsible for entire contents of chapter in quizzes
CS-1004, A-Term 2014
First Python Program