Sep 6
Questions?
Next assignment, from chap 2 & 3, is posted (due Tuesday).
chap 1 homework
Talk about and when to turn in work - several of you had not turned things in by last night.
chap 2
software development cycle
- understand problem (can be harder than you think)
- create specs : inputs? outputs? (be very specific)
- choose algorithm: what does it do?
- implement (write the code)
- test and debug (again, can be harder than you expect)
- maintain (if used over time, needs will usually change)
- aside: python code in wiki pages
- does this have any bugs? Hmmm.
first_name # underbar convention
FirstName # camelcase convention
3people # WRONG - can't start with number
site43_bldg2 # OK - can have embedded numbers
n="Jim Mahoney" # BAD - later will be hard to tell 'n' means.
- variables (those names) have values (numbers, strings, lists, ...).
- expressions let you manipulate the values
- example1: (2.0 + 0.1/20)**3
- example2: "Mr " + first_name + " " + last_name
- operators: + - / * ** ... and others
- types: int float (i.e. decimal) strings ... and others
$ python
>>> type(1)
<type 'int'>
>>> type(1.0)
<type 'float'>
- tuples
- fairly specific to the python language
$ python
>>> a = 1,2,3
(1, 2, 3)
- Try these to see what they do :
>>> a * 2
>>> 2 * a
>>> a + 2
>>> 2 + a
>>> type(a)
- output and output - printing to the terminal
a = input("What is a? ")
print "OK, a is ", a
(We'll look at various sorts of inputs and outputs in more detail later.)
Aside: the "print" statement in python, parens, and what it can be confusing.
interest_rate = 3.0 # percent
start_amount = amount = 100.00
periods = 10
for i in range(periods):
amount = amount * (1 + interest_rate/100.0)
print amount, " at ", \
interest_rate, "% ", \
periods, " times is ", amount
# (The "\" character at the end of a line
# continues it on the next line.)
We will look at the pieces of this more
carefully soon; for now, the point is to
have a first exposure and to get the general
idea. We'll continue to fill in the details
and go over specific pieces, like range().
chapter 3
- data types
- internal representations
- type() function in python
- float numbers are approximate - finite number of bits for storage
>>> 0.3
0.29999999999999999
- but in python, integers grow as needed ... and become another type
>>> 2**10, 2**20, 2**30, 2**40
(1024, 1048576, 1073741824, 1099511627776L)
(Notice the "L" at the end of the last number: that's a new "long" type.)
Discussion: why is the switch near 2**30 ? why not always use "long" types for integers? why have different storage for floats and ints?
math library
First a bit about names, namespaces, and dir()
$ python
>>> dir() # what
['__builtins__', '__doc__', '__name__']
>>> dir(__builtins__)
... long list of built-in things ...
>>> from math import *
>>> dir()
['__builtins__', '__doc__', '__name__', 'acos', 'asin', 'atan', 'atan2', 'ceil', 'cos', 'cosh', 'degrees', 'e', 'exp', 'fabs', 'floor', 'fmod', 'frexp', 'hypot', 'ldexp', 'log', 'log10', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh']
The math functions like sin(), cos(), sqrt(), aren't available in python by default. Instead, you must "import" them from a "module" called "math".
The book does this instead :
>>> import math
>>> math.sqrt(3.0)
which leaves things from the math module with "math." before their names. If you do things that way, sin(pi/2) is math.sin(math.pi/2).
accumulating results in a loop
sum = 0
numbers = [1, 10, 20, 18, 17, 34, 22]
for number in numbers:
sum = sum + number
print "The sum is ", sum
Look at this carefully to understand what's going on.
In class: run this. Then put in more print statements
to see exactly what is going on during the loop.
(The loop is the "for" statement and the indented part
after it.)
![[paper clip]](/courses/source/wiki_images/paper_clip_tilt.png)
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