Cs4hs2008 track a-programming

Introduction to
Programming with Python
Marty Stepp (stepp@cs.washington.edu)
University of Washington

Special thanks to Scott Shawcroft, Ryan Tucker, and Paul Beck for their work on these slides.
Except where otherwise noted, this work is licensed under:
http://creativecommons.org/licenses/by-nc-sa/3.0

Outline

• Programming languages and Python

• Basic programs and numeric data

• Control statements

• Text processing

• What's next? / CSE 142 at UW

2

Languages
• Some influential ones:
– FORTRAN
• science / engineering
– COBOL
• business data
– LISP
• logic and AI
– BASIC
• a simple language

3

Python
• Created in 1991 by Guido van Rossum (now at Google)
– Named for Monty Python

• Useful as a scripting language
– script: A program meant for use in small/medium projects

• Used by:
– Google, Yahoo!, Youtube
– Many Linux distributions
– Games and apps (e.g. Eve Online)

4

Installing Python
Windows: Mac OS X:
• Download Python from • Python is already installed.
http://www.python.org • Open a terminal and run python
• Install Python. or run Idle from Finder.
• Run Idle from the Start Menu.
Linux:
• Chances are you already have
Python installed. To check, run
python from the terminal.
• If not, install from your
distribution's package system.
Note: For step by step installation
instructions, see the course web site.

5

Interpreted Languages
• interpreted
– Not compiled like many other languages (Java, C, C++)
– Code is written and then directly executed by an interpreter
– Type commands into interpreter and see immediate results

Java: Runtime
Code Compiler Computer
Environment

Python: Code Interpreter Computer

6

The Python Interpreter
• Allows you to type commands one-at-a-time and see results
• A great way to explore Python's syntax

7

Basic Programs and
Numeric Data

The print Statement
• A Python program's code is just written directly into a file
print "text"
print (a blank line)

hello.py
1 print "Hello, world!"

swallows.py
2 print
3 print "Suppose two swallows carry it together."
4 print "African or European swallow?"

9

Comments
• Syntax:
# comment text (one line)

swallows2.py
1 # Suzy Student, CSE 142, Fall 2097
2 # This program prints important messages.
4 print # blank line
5 print "Suppose two swallows "carry" it together."
6 print 'African or "European" swallows?'

10

Expressions
• expression: A value or operation(s) to compute a value.
Example: 1 + 4 * 3

• Arithmetic operators:
– + - * / add, subtract/negate, multiply, divide
– ** exponentiate
– % modulus, a.k.a. remainder

• precedence: Order in which operations are computed.
– * / % ** have a higher precedence than + -
1 + 3 * 4 is 13
(1 + 3) * 4 is 16

11

Integer division
• When we divide integers with / , the quotient is an integer.
3 52
4 ) 14 27 ) 1425
12 135
2 75
54
21
• 35 / 5 is 7
• 84 / 10 is 8

• The % operator computes a remainder from integer division.
3 43
4 ) 14 5 ) 218
12 20
2 18
15
3
12

Variables
• variable: A named piece of memory that stores a value.

• assignment: Stores a value into a variable.
– Syntax:
name = expression
– Examples: x = 5
gpa = 3.14

x 5 gpa 3.14

– A variable can be used in expressions.
x + 4 is 9
13

Exercise
• This program's code is redundant. Improve it with variables:

print "Subtotal:"
print 38 + 40 + 30
print "Tax:"
print (38 + 40 + 30) * .09
print "Tip:"
print (38 + 40 + 30) * .15
print "Total:"
print 38 + 40 + 30 + (38 + 40 + 30) * .15 + (38 + 40 + 30) * .09

14

Data Types
• type: A category or set of data values.
– Constrains the operations that can be performed on the data
– Examples: integer, real number, text string

• Python is relaxed about types.
– A variable's type does not need to be declared.
– A variable can change types as a program is running.

Value Python type
42 int
3.14 float
"ni!" str

15

Parameters
• parameter: A value supplied to a command as you run it.
– Syntax:
command ( value )
command ( value, value, ..., value )

• Example:
print sqrt(25)
print sqrt(15 + 10 * 10 + 6)
x = 5
print sqrt(x + sqrt(16))

16

Math commands
Function name Description Constant Description
abs(value) absolute value e 2.7182818...
ceil(value) rounds up pi 3.1415926...
cos(value) cosine, in radians
floor(value) rounds down
log10(value) logarithm, base 10
max(value1, value2) larger of two values
min(value1, value2) smaller of two values
round(value) nearest whole number
sin(value) sine, in radians
sqrt(value) square root

• To use these commands, place this line atop your program:
from math import *

17

input
• input : Reads a number from the user's keyboard.
– You can store the result of input into a variable.

– Example:
age = input("How old are you? ")
print "Your age is", age
print "You have", 65 - age, "years until retirement"

Output:
How old are you? 53
Your age is 53
You have 12 years until retirement

• Exercise: Modify the restaurant program to ask the user
how much of a tip to leave.

18

if
• if statement: Executes a set of commands only if a certain
condition is True. Otherwise, the commands are skipped.
– Syntax:
if condition:
statements

– Example:
gpa = input("What is your GPA? ")
if gpa > 2.0:
print "Your application is accepted."

20

if/else
• if/else statement: Executes one set of statements if a
certain condition is True, and a second set if it is False.
– Syntax:
if condition:
statements
else:
statements

– Example:
gpa = input("What is your GPA? ")
if gpa > 2.0:
print "Welcome to Mars University!"
else:
print "Your application is denied."

• Multiple conditions can be chained with elif

21

Logic
Operator Meaning Example Result
== equals 1 + 1 == 2 True
!= does not equal 3.2 != 2.5 True
< less than 10 < 5 False
> greater than 10 > 5 True
<= less than or equal to 126 <= 100 False
>= greater than or equal to 5.0 >= 5.0 True

– Logical expressions can be combined using logical operators:
Operator Example Result
and (9 != 6) and (2 < 3) True
or (2 == 3) or (-1 < 5) True
not not (7 > 0) False

22

for loops
for name in range(start, end):
statements
for name in range(start, end, step):
statements
– Repeats for values start (inclusive) to end (exclusive)
>>> for i in range(2, 6):
print i
2
3
4
5
>>> for i in range(15, 0, -5):
print i, "squared is", (i * i)
15 squared is 225
10 squared is 100
5 squared is 25

23

Cumulative loops
• Some loops incrementally compute a value.
– sometimes called a cumulative loop

sum = 0
for i in range(1, 11):
sum = sum + (i * i)
print "sum of first 10 squares is", sum

Output:
sum of first 10 squares is 385

24

Exercise
• Write a program that reads a student's homework scores as
input and computes the student's homework percentage.
This program computes your average homework grade.
How many assignments were there? 3

Assignment 1
Points earned? 12
Points possible? 15

Assignment 2
Points earned? 10
Points possible? 20

Assignment 3
Points earned? 4
Points possible? 5

Your total score: 26 / 40 : 65 %

25

while
• while loop: Executes as long as a condition is True.
– good for indefinite loops (repeat an unknown number of times)

• Syntax:
while condition:
statements

• Example:
number = 1
while number < 200:
print number,
number = number * 2

– Output:
1 2 4 8 16 32 64 128

26

Random numbers
– from random import *

– randint(min, max)
Produces a random value between min and max (inclusive)

– Example:
coinflip = randint(1, 2)
if coinflip == 1:
print "Heads"
else:
print "Tails"

27

Exercise
• Write a program that plays a guessing game with the user:
Let's play a game. Try to guess my number from 1-100!
Your guess? 40
Too low.
Your guess? 85
Too high.
Your guess? 68
Too high.
Your guess? 51
Too low.
Your guess? 57
Too low.
Your guess? 63
Too high.
Your guess? 61
You got it right in 7 guesses!

28

Strings
• string: A sequence of text characters in a program.
– Strings start and end with quote " or apostrophe ' characters.
"hello"
"This is a string"
"This, too, is a string. It can be very long!"

• A string can represent special characters with a backslash.
– " quotation mark character
– t tab character
– backslash character
"Bob said, "Hello!" to Susan."

30

Indexes
• Characters in a string are numbered with indexes :
name = "P. Diddy"
index 0 1 2 3 4 5 6 7
character P . D i d d y

• Accessing an individual character from a string:
variable [ index ]

print name, "starts with", name[0]

Output:
P. Diddy starts with P

31

String properties
• len(string) - number of characters in a string
(including spaces)
• str.lower(string) - lowercase version of a string
• str.upper(string) - uppercase version of a string

• Example:
name = "Martin Douglas Stepp"
big_name = str.upper(name)
print big_name, "has", len(big_name), "characters"

Output:
MARTIN DOUGLAS STEPP has 20 characters

32

raw_input
• raw_input : Reads a string of text from the user's keyboard.

– Example:
name = raw_input("Howdy. What's yer name? ")
print name, "... what a silly name!"

Output:
Howdy. What's yer name? Paris Hilton
Paris Hilton ... what a silly name!

33

Text processing
• text processing: Examining, editing, formatting text.
– Often uses loops that examine characters one by one.

• A for loop can examine each character in a string in order.
– Example:
for c in "booyah":
print c
Output:
b
o
o
y
a
h

34

Strings and numbers
• ord(text) - Converts a string into a number.
– Example: ord("a") is 97, ord("b") is 98, ...

– Characters use standard mappings such as ASCII and Unicode.

• chr(number) - Converts a number into a string.
– Example: chr(99) is "c"

35

Exercise
• Write a program that "encrypts" a secret message by shifting
the letters of the message by 1:
– e.g. "Attack" when rotated by 1 becomes "buubdl"

36

Further programming
• What do students learn next?
– Arrays, data structures
– Objects and object-oriented programming
– Algorithms: searching, sorting, recursion, etc.
– Graphical user interfaces, drawing, event-driven programming

38

CSE 142 Homework
• Our homeworks process interesting data and text

39

Student pictures
• Around the 3rd week, we assign a graphical program
• Two parts:
– draw a particular figure using loops and parameters
– draw any figure you want
• figures posted on course web site and voted on by students

40

Critter simulation
• Students write several
small classes to
represent animals in a
simulation world

• Each student writes a
custom "Husky" animal
with their choice of
behavior

• The best huskies
compete in a
tournament for prizes

41

Facebook integration

42

Links
• Python for math teachers:
– http://showmedo.com/videos/series?id=101

• Physics and 3D in Python:
– http://showmedo.com/videos/series?name=pythonThompsonVPythonSeries
• Handbook of the Physics Computing Course (Python):
– http://www-teaching.physics.ox.ac.uk/computing/handbook_Python/handbook_Python.html

• Biopython - Python tools for biology:
– http://biopython.org/wiki/Main_Page
• Python course in Bioinformatics:
– http://www.pasteur.fr/recherche/unites/sis/formation/python/index.html

43

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