sep 14
Questions ?
chapter 2 and 3
loops
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.)
data types
- internal representations
- type() function in python
float numbers are approximate - finite number of bits for storage
# (This is with older versions of python)
>>> 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
chapter 4 : strings
input
This fails:
# THIS WON'T WORK
name = input("What is your name? ")
print "You said ", name
because the input() command treats what you typed as if it were python code. (Oops).
Instead, you should do this:
name = raw_input("What is your name? ")
print "You said ", name
playing with strings
- a string s an array of characters.
- discuss subscript conventions: a[i], a[i:j], a[i:], a[:i], a[-1]
codes
- ord("a")
- chr(90)
- discuss text2numbers.py, pg 92
- ... and numbers2text.py, pg 93
string operations summary
"this " + "that" # concatenate
"x" * 10 # replicate
"This is a string"[0:3] # substrings
len("string") # how many characters?
for char in the_string: # loop over characters
# lots more
import string # grab a bunch of 'em
dir(string) # list all the string.yyy() functions
dir("xxx") # what the the "xxx".method() things?
# conversions
x = float("32.3") # convert string to float
y = int("101") # convert string to int
z = str(xxx) # convert anything to string
eval("1+2") # intepret string as python expression
formatting
Probably for next time ...
file processing
ditto
