Sep 25

Discuss the homework. Questions? Comments?

We did the rectangle draw and file input problem in class; see the attached files.

Here's another animation example : ball_arc.py (Note various types of docs within the file.)

chap 6 : functions

Here's an example :

# Define a function.
def print_info(x):
  print " " + str(type(x)) + " has value '"+ str(x) + "'"

# Call it several times.
print_info(3.2)
print_info('a')
print_info("alpha")
print_info([1,2,3])

Running that gives


 <type 'float'> has value '3.2'
 <type 'str'> has value 'a'
 <type 'str'> has value 'alpha'
 <type 'list'> has value '[1, 2, 3]'

This is one of the key ideas of the semester
It's a "mini program"

Functions can take "arguments".

Go over process of invoking a function, and what happens to the values "passed in" to a function.

Functions let you avoid repitition.

Happy birthday example.
Look over previous graphics programs, and do same.

Functions break a program into bite-sized chunks.

Each with own documentation.
Each with own tests. (More on this in a bit.)

Functions may "return" a value.

def double(x):
  return 2*x

def print_double(x):
  print "Twice " + str(x) + " is " + str(double(x))

for i in range(4):
  print_double(i)

Functions may have side effects ... but you should usually avoid that.

Discuss what a "side effect" is.
And why they're bad: hard to tell what function does, and so hard to debug.

Discuss "Global" vs "Local" variables, and "scope".

And that different programming languages behave differently in how they treat them.
Including what happens to passed arguments that are modified.

What does this do?

# Define a global variable.
a = 6

# Define a function that alters its argument.
def increment_argument(x):
  x = x + 1

# Does it work?
print "a is " + str(a)
increment_argument(a)
print "now a is " + str(a)

How about this?

# Define a global variable.
a = ["zero", "one"]

# Define a function that alters its argument.
def alter_array(x):
  x[1] = x[1] + " ... changed"

# Does it work?
print "a is " + str(a)
alter_array(a)
print "now a is " + str(a)