Wed Sep 13
chapter 2
- development cycle
- understand problem
- create specification : inputs? outputs?
- design algorithm : how?
- implement
- test/debug
- maintain
- "convert" program
- does it have any bugs? Hmmmm
- program elements
- names : this_is_a_name abc123 CamelCaseVariableName
- expressions : (2.0 + .1/20)**3
- operators: + - * / ** etc
- assignment : x = x + 1
- Note that this is not a test for equality!
- It means
- calculate the thing on the right
- put it in the storage location described on the left
- Python tuples
- fairly specific to Python language
>>> a = 1,2,3
(1, 2, 3) # This is a "tuple" with 3 parts
>>> a * 2 # Do these do anything?
>>> a + 2 # If so, what?
>>> a**2
- Lets you do things like this
>>> a = 10
>>> b = 20
>>> a,b = b,a # swap the values in 'a' and 'b'
for <var> in <sequence>:
<body>
Here's an example
for odd in [1,3,5,7,9]:
print odd*odd
Here's another way to make a sequence :
>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Jim's aside: print
Python's "print" is a bit funky, IMHO
Language X :
print("hi there")
Usually I expect either always parens,
or optional parens
a = sqrt 2
But at first glimpse, Python seems to allow either.
>>> print "hi there"
hi there
>>> print("hi there")
hi there
Until you look closer.
>>> a=10
>>> print "a is", 10
a is 10
print("a is",10)
('a is', 10)
Oops - what was that?
It turns out that "print" in Python isn't a function,
like sqrt(); all Python's functions require parens.
Instead, "print" in python is a built-in language
primitive, like "if". And it never takes parens.
What? But we saw it work with parens! Remember,
>>> print("hi there")
hi there
Ah. Well, what you thought you saw was this:
>>> x = ("hi there")
>>> print x
hi there
Where the parens are used to group things together.
In this case, there just happens to be one thing.
The place you can really see this happen is with
Python "tuples", which are groups of things clumped
together. Like this :
>>> point = 10,20
>>> point
(10, 20)
Ah hah. Theere are those parens again. This is
the way Python always outputs a tuple.
So now we see why we have
>>> print(1,2)
(1, 2)
because it's the same as
>>> point = (1,2)
>>> print point
(1, 2)
