Software Design
Fall 2004

For next time:

1) finish Chapter 3 of "How to think..."

2) continue work on hw01


Linux review
------------

User and superuser

Accounts and environments

The prompt  (typeahead)

Files and directories

The "working" directory

pwd, ls and cd

Absolute and relative paths

Commands, arguments and options



Syllabus
--------

Administrivia for the whole family.

Past projects (never too early...)

Feedback on the surveys.



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


modulus
-------

x % y   is the remainder when x is divided by y (y is an integer)

What is it good for?

1)

2)

3)

4)

5)



for loops                       (an early bird special)
---------

What do the following statements do?

for c in 'allen':
    print c,                


for i in range(4):
    print i         


for statement 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.

bob = Turtle(world)
for color in ['red', 'yellow', 'blue']:
    print color                            #inside the loop
    bob.set_color(color)                   #inside the loop
bob.fd(100)                                #outside the loop


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

In Linux, create a console 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?