Software Design
Fall 2004

For next time:

1) read Chapter 8 of "How to think..."

2) prepare for a quiz covering Chapters 2-7, probably
   involving a stack diagram

The first exam will be Thursday 30 September, during class time.


Homework 3 Tips
---------------

As we learn more about strings and lists, you might want to
go back and look for easier ways.

The in operator:

>>> 'e' in 'Allen'
True


String operations:

from string import *
s = lower('ALLEN')

http://www.python.org/doc/current/lib/module-string.html


String methods:

s = 'ALLEN'
s2 = s.lower()

http://www.python.org/doc/current/lib/string-methods.html


List methods:

http://www.python.org/doc/current/lib/typesseq-mutable.html


Homework 2 Solutions
--------------------

wget http://wb/sd/code/polygon.py
wget http://wb/sd/code/Wanderer_soln.py


polygon.py highlights:

def polyline(t, n, length, angle):
    for i in range(n):
        fd(t, length)
        lt(t, angle)

def polygon(t, n, length):
    angle = 360.0/n
    polyline(t, n, length, angle)


What's wrong with the following, INCORRECT, version of arc?

def arc(t, r, theta=360.0, length=1):
# draw part of a circle with radius r and angle theta.
# using segments with the given length
    circum = 2 * pi * r * theta / 360
    n = circum / length
    length = circum / n
    angle = theta / n
    polyline(t, n, length, angle)


Wanderer.py highlights:

def out_of_bounds(t):
    # which way is toward the center?
    away = atan2(t.y, t.x) * 180 / pi
    toward = (away + 180) % 360

    # choose two possible headings
    d1 = t.heading + rand_turn(t.clumsiness)
    d2 = t.heading + rand_turn(t.clumsiness)

    # choose the one that is more toward the center
    diff1 = angle_distance(toward, d1)
    diff2 = angle_distance(toward, d2)

    if diff1 < diff2:
        t.heading = d1
    else:
        t.heading = d2



Looping
-------

Chapter 6 talks mostly about while loops, because 
control flow is explicit:

i = 0
while i<3:
    print i
    i = i + 1

However, the for loop is far more common in Python.
We have already seen some examples:

for i in range(4):
    print i

The range command returns a list:

>>> range(4)
[0, 1, 2, 3]
>>> range(2,5)
[2, 3, 4]
>>> range(2,10,2)
[2, 4, 6, 8]

So the previous example is equivalent to

for i in [0, 1, 2, 3]:
    print i


And we saw the following example in class:

def koch(t, n):
    if n<4:
        fd(t, n)
        return
    for angle in [60, -120, 60, 0]:
        koch(t, n/3.0)
        lt(t, angle)

We also saw this example in notes02 (a list of strings!):

for color in ['red', 'yellow', 'blue']:
    bob.set_color(color)

for loops also traverse the letters of a string:

for letter in 'allen':
    print letter


The while statement is more general than the for loop,
because you can generate the sequence "on the fly":

while n != 1:
    print n,
    if n%2 == 0:
        n = n/2
    else:
        n = n*3+1

Can you express this as a for loop?