An Introduction toPython Programming

Dr. Mark Goadrich

Centenary College of Louisiana

http://mark.goadrich.com

NWLAPCUG - May 15th 2008

I’m Thinking of a Number…

• Can we guess the computer’s secret number between 0 and 100?

• Can the computer guess our secret number between 0 and 100?

• We need a common language to communicate.

Guess A Number Pseudocode

Pick a random number for the computer

Ask the user to guess the computer’s number

While they guess wrong,

Give feedback if number is too high or low

Ask for another guess from the user

Tell the user they are correct

Language of Programming

• English is a natural language– casual, slang, vague– “I went to the bank.” (money or river?)

• Computers need formal languages– strict, specific, clear, unambiguous

• Many language choices: C++, Java, Prolog, Scheme, Perl, Fortran, Cobol, Ruby, etc…

Python Programming

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

How to Talk to your Computer

Application CPU

User

Programmer

Python Interpreter

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Python and IDLE

Programming Components

• Five basic pieces to all programs – Data– User Interaction– Decisions– Repetition– Libraries

Data Storage and Memory

• Numbers903.141592 + 3 / 4 * 5.6

• Strings"The quick brown fox""$5.48"

• Variablesage = 31cat = "Felix"length = 5width = 10area = length * width

User Interaction

• We need to ask the user questions

age = input("How old are you? ")name = raw_input("What is your name? ")

• We want to tell the user the results

print "In dog years, you are " + str(age * 7)

Decisions

• Relate variables with logic (True, False)

temperature > 90legs == 2 and not tall

• Logic decides program path

if temperature > 90:print "Must be summer again . . ."

else:print "Looks like good weather."

?

Repetition

• Repeats commands whilea condition is true

count = 10while count > 0:

print countcount = count - 1

print "Blastoff!"

?

Including Libraries

• Import functions from other placesimport randomimport math

• Use these functions to help our programif (random.random() < 0.5):

print "Heads"else:

print "Tails"

Guess A Number Translation

Pick a random number for the computer

Ask the user to guess the computer’s number

While they guess wrong,

Give feedback if number is too high or low

Ask for another guess from the user

Tell the user they are correct

Guess A Number Translation

import randomnum = random.randrange(100)Ask the user to guess the computer’s number

While they guess wrong,

Give feedback if number is too high or low

Ask for another guess from the user

Tell the user they are correct

Guess A Number Translation

import randomnum = random.randrange(100)guess = input("Try to guess my number 0-99: ")

While they guess wrong,

Give feedback if number is too high or low

Ask for another guess from the user

Tell the user they are correct

Guess A Number Translation

import randomnum = random.randrange(100)guess = input("Try to guess my number 0-99: ")

while num != guess:

Give feedback if number is too high or low

Ask for another guess from the user

Tell the user they are correct

Guess A Number Translation

import randomnum = random.randrange(100)guess = input("Try to guess my number 0-99: ")

while num != guess:if guess < num:

print "Too Low!"else:

print "Too High!"Ask for another guess from the user

Tell the user they are correct

Guess A Number Translation

import randomnum = random.randrange(100)guess = input("Try to guess my number 0-99: ")

while num != guess:if guess < num:

print "Too Low!"else:

print "Too High!"guess = input("Guess again: ")

Tell the user they are correct

Guess A Number Translation

import randomnum = random.randrange(100)guess = input("Try to guess my number 0-99: ")

while num != guess:if guess < num:

print "Too Low!"else:

print "Too High!"guess = input("Guess again: ")

print "Correct!"

Running our Code

• Go to IDLE

• Open guess1.py

• Press F5 to run the program

Part II - Computer Guesses

• How can the computer guess our number?

• The same way we guessed– Start out with a range of numbers– Each guess, split the answers in half– Eventually, the range will be one number

• We’re now moving past programming to computer science . . .

Guess A Number II

Print instructions to the userInitialize high and low boundaries and status of guess

While guess is incorrect,Formulate a new guess halfway between boundariesAsk for user feedback on guessIf guess too high, reset upper boundary

If guess too low, reset lower boundary If correct, update status of guess

Otherwise ask for valid input from the user

When guess is correct, tell the user and exit

Guess A Number II

print "Pick a number between 0 and 99, I will guess it."print "If I am high, type H, low, type L, and correct type C."low, high, correct = 0, 100, Falsewhile not correct:

guess = (high + low) / 2answer = raw_input("I guess " + str(guess) + ", HLC? ")if answer == "H": high = guesselif answer == "L": low = guesselif answer == "C": correct = Trueelse: print "I don't understand, please enter H, L or C."

print "I found it!"

Other Examples

• Chaos and Fractals

• Can’t Stop the Monkeys

Further References

• Python Home Pagehttp://python.org

• How to Think Like a (Python) Programmerhttp://thinkpython.com

• Graphics Package for Pythonhttp://cs1graphics.org