Introductory Programming
Fall 2004

For Thursday you should

1) Read Section 9.1 -- 9.4 and Chapter 10


Homework 5 Solution
-------------------

move and nudge are fruitless functions.

They do stuff (like move the Amoeba) but they have no
return statement.

The return value from a fruitless function is a special value called None.

def move(a, limit=10):
    # move the amoeba by a random amount
    x = a.x + random_coordinate(1)
    y = a.y + random_coordinate(1)

    x = bound(x, limit)
    y = bound(y, limit)
    teleport(a, x, y)

def nudge(a, b, fraction=0.02):
    # move a just a little bit closer to b
    dx = a.x - b.x
    a.x -= dx * fraction
    dy = a.y - b.y
    a.y -= dy * fraction


You use fruitless functions as if they were statements:

        move(amy)
        move(bob)

        nudge(amy, bob)
        nudge(bob, amy)


distance and bound are fruitful functions.

They have no visible effect, but they return values.

def distance(a, b):
    # compute the distance between amoebas
    dx = a.x - b.x
    dy = a.y - b.y
    return sqrt(dx*dx + dy*dy)

def bound(x, limit):
    # make sure that x is between -limit and +limit
    if x > limit: x = limit
    if x < -limit: x = -limit
    return x


You use fruitful functions as part of an expression:

    x = bound(x, limit)
    y = bound(y, limit)

and

        d = distance(amy, bob)
        if d<1.0:
            mate(amy, bob, world, n-i)


This homework involved some code you may not have understood,
which I think made it hard to get started.

I hope that Homework 6 is easier to get started on.


Evaluation 5
------------

Let's see how this went and behave accordingly!


Homework 6
----------

Let's work on this in class and let me help people out a little.