Sep 18
Reminder: the
resources page has links to the textbook website, which has downloadable versions of the code in the book and other goodies.
And remember the documentation:
Note that python currently has several different ways to format text.
Last week I described the "old" way, with % signs; the python 2.7 docs mostly describe the "new" way, with the "..."format.().
Here's an example of table formatting, using the old syntax:
from math import sqrt
print " %10s %10s " % ("n", "sqrt(n)")
print " %10s %10s " % ("-"*10, "-"*10)
for n in range(20):
print " %10i %10.4f " % (n, sqrt(n))
And here's the same with the new syntax:
from math import sqrt
print " {:10} {:10} ".format("n", "sqrt(n)")
print " {:10} {:10} ".format("-"*10, "-"*10)
for n in range(20):
print " {:10} {:10.4f} ".format(n, sqrt(n))
homework
Discuss the homework.
Here's the basic idea of the cipher assignments :
word = raw_input("What is the word to encode? (lowercase a-z) ")
cipher_key = input("What is the magic number? (1-25) ")
code_word = ""
for char in word:
char_number = ord(chard) - ord('a') # 0 to 26
code_number = (char_number + cipher_key) % 26 # wrap around at 'z'
code_letter = chr(code_number + ord('a')
code_word = code_word + code_letter
print "The coded word is '%s'." % code_word
And here are a few more detailed solutions.
We did this in class :
message = raw_input("Enter message: ")
cipher_key = input("What is the cipher key? ")
code_word = ""
for ch in message:
print " debug: char = ", ch
ascii = ord(ch)
print " ascii: ", ascii
number = ord(ch) - ord('a') # 0 to 25
print " number: ", number
new_number = (number + cipher_key) % 26
print " new number: ", new_number
new_char = chr(new_number + ord('a'))
print " new char = ", new_char
code_word = code_word + new_char
print
print
print "The coded word is '{}'.".format(code_word)
our story so far
where we've been :
- reading and writing to terminal and files
- numbers, strings
- variables, values, lists
- various built-in functions and operations
chap 5 : exploring "objects" with a graphics API
This is usually a fun week ...
objects
Object oriented programming is a popular paradigm these days. It's not the only one, but it's an important one.
An "object" is a abstration chunk of data (a "thing") with actions (called "methods") it can perform.
A "class" is a kind of object. Example: Point, Circle, GraphWin
An "instance" is a specific object: Example: that red circle right there.
In python, creating an instance is done this way :
p = Point(10, 20) # instance_variable = ClassName(argument1, arg2)
And once we have one, we can tell it do things like this:
p.move(2, 3) # change position (x,y)=(10,20) to (12,23)
An "API" (Application Programmer's Interface) is a description of which sorts of objects you can create, and what methods they have.
All this is pretty abstract, so let's see a concrete example with the attached files.
graphics.py
See the
resources or
Zelle's book page for both (1) the file you'll need to run this stuff (put in in the same directory), and (2) the API of what you can do with it.
It's possible that your python installation doesn't yet have the right system stuff - namely tkinter - to run Zelle's graphics library. So if this doesn't work for you, talk to Sam or Jim about getting things running.
You'll need the graphics.py module (i.e. file) from Zelle. And it needs to be somewhere where your python can find it, typically in the same folder as the python file you're running.
# You'll need to do this to use Zelle's graphics:
from graphics import *
# If that doesn't work, make sure you have his graphics.py file.
# Then make sure you're running python in the folder with graphics.py.
# You can see what the folder is with :
import os
print "The current directory is ", os.getcwd()
# And you can change the folder with
os.chdir(full_path_to_a_folder)
With that stuff running, we'll explore it interactively class, doing things like this :
- create a window
- with given size
- with given background
- create a circle
- give it a color
- draw it
- move it
- animate it
- "from time import sleep"
- "import random; r = Random(); r.randint(low,high)"
- get a mouse click
- draw an image from a .jpeg file
... and so on
Tic Tac Toe, anyone ?
an example
# wiggle.py
#
# a demo of Zelle's graphic library
#
# This uses two new python functions, randint and sleep,
# in addition to the graphic object API :
#
# from random import randint
# randint(low, high) # low <= random num <= high
#
# from time import sleep
# sleep(2) # do nothing for 2 sec
#
# Clicking in the window once it's stopped will quit cleanly.
# Or type "control-C" at the command prompt to interrupt it.
#
# Jim M | Sep 2012 | MIT License
from graphics import *
from time import sleep
from random import randint
def main():
window = GraphWin("wiggle", 300, 300)
dot = Circle(Point(150,150), 20)
dot.setFill("red")
dot.draw(window)
for i in range(100):
sleep(0.1) # in seconds
dx = randint(-5, 5) # in pixels
dy = randint(-5, 5)
dot.move(dx, dy)
wait = window.getClick()
main()
Exercise : Can you trace the path of the red circle? (One way is given in the attached wiggle2.py)
Are we having fun yet?
