Software Design
Spring 2006

For today, you should have:

1) read Chapter 2 of How to Think, and preferably Chapter 3

2) read Chapter 1 of the Olinux manual

3) started on Homework 1

4) chosen and ordered a Python book


Today:

1) the ultra-secret point of Homework 1

2) variables, values and types

3) defining and calling functions


For next time:

1) read Chapter 3 of How to Think

2) read Chapter 2 of the Olinux manual


Next Tuesday, there will be two sessions:

10:00 - 10:25  project planning for people who want to start early
10:25 - 10:50  help with Homework 1

You can come for either session or both (or even neither :()


Homework 1
----------














Values and types
----------------

Everything is an object.

Names refer to objects, so we draw them with arrows.

x = 1
y = 1

What does the UML state diagram look like?

All values have types.

The expression 1 creates a reference to an integer object.

               1.0                      floating-point

               1L                       arbitrary-precision integer!

               'allen'                  string (double quotes work, too)


Variables do not have types.  Any name can refer to any type.

x = 1
x = 1.0
x = 'allen'

You don't have to declare variables, but you do have to
avoid using keywords as variable names.

Choosing good names is surprisingly important.


Expressions
-----------

Expressions are made up of operands and operators.

The effect of the operator sometimes depends on
the types of the operands.

1 + 1         integer addition
2.0 + 2.0     floating point addition
'a' + 'llen'  string concatenation
'a' * 3       string multiplication?

Gotchas

0) string operators: hoax or scam?

1) what if the operands don't match?

2) beware integer division

3) precedence: learn it, live it, forget it


Try it out!
-----------

In Linux, create a terminal and type python.

Python 2.2.2 (#1, Feb 24 2003, 19:13:11) 
[GCC 3.2.2 20030222 (Red Hat Linux 3.2.2-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

The chevron >>> is a prompt.  You can type a python expression
and the python interpreter will print the result.

>>> 'allen' * 10
'allenallenallenallenallenallenallenallenallenallen'

If you are not sure whether something is legal, try it!

>>> 'allen' / 10
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: unsupported operand type(s) for /: 'str' and 'int'

What does this error message mean to you?


function definitions
--------------------

function def is an example of a compound statement

1) header ending in :
2) indented body

By convention, indentation is 4 spaces.

End of body marked by an outdented line.

def twice(s):
    print s, s

def fourtimes(s):
    twice(s)
    twice(s)

A function definition doesn't DO anything except:

1) create a function object

2) create a name that refers to the object


function calls
--------------

To execute the body of the function, you have to call/invoke it

1) built-in functions

len('allen')

2) functions from a module

import math
print math.sin(math.pi/4) - math.sqrt(2)/2

3) user defined functions

fourtimes('allen')


arguments are expressions:
 
s = 'downey'
fourtimes('allen' + ' ' + s)


make some errors
----------------

Try out the following and see what goes wrong:

1) leave off the colon :

def fun()
    x = 1

2) mess up the indentation

def fun():
    x = 1
  y = 1

def fun():
    x = 1
  y = 1

3) call a function with the wrong number of arguments

len('allen', 'downey')

4) call something that is not a function

x = 5
x(17)


function names are variables
----------------------------

Try this out, and see if you can make sense of it.
You might want to draw a state diagram.

def is_divisible(x, y):
    if x%y == 0:
        print 'yes!'

print is_divisible

f = is_divisible

f(100,10)

is_divisible = 'banana'

print f

print is_divisible
    

functions as parameters       (early bird special)
-----------------------

Try this out, and see if you can make sense of it.

def do_twice(f, s):
    f(s)
    f(s)

def print_twice(s):
    print s, s

do_twice(print_twice, 'allen')


Hint: see http://www.allenallenvolleyball.com/