Tue Sep 16
chap 2 & 3 homework
... is due today.
The submissions I've seen didn't include
what running the code does ... please do
put that in for future assignments.
Generally speaking, you should plan on trying to get
the homework in before Tuesday's class, since we'll
typically go over it then. I will accept work after
that, but since I may have posted my solutions and/or
we'll have discussed the problems in class, it
will likely be worth less.
Please do use that as an example of what I want:
- readable code (attachments or pasted in)
- evidence that it works, i.e. what happens when it runs
- cut'n'paste from a terminal session
- or tests (more on this idea later)
- or screenshots (if a graphics program)
I remind you that Sam Judson has tutorial hours
over the weekend - see his emails - to answer questions and help.
Also be aware of the python docs on resources page including :
As I showed last week, you can also ask python for the names it knows.
$ python
>>> dir() # list of defined names
>>> dir(__builtins__) # list of built-in functions
There is also help within the python prompt.
$ python
>>> help
>>> help(dir) # describe the dir() built-in function
>>> help()
help> keywords
Questions about the homework? Parts you'd like to discuss?
Any other questions about anything so far?
Homework for next Tuesday has been posted: chapter 4 stuff on strings.
That brings us to ...
chapter 4 : strings
(We'll see how far we get with all this today, and continue next class.)
At this point you should understand :
- creating / editing programs (i.e. XXX.py files)
- running python programs
- integers and floats (decimals), and some operations on 'em
- basic input and output from the command line
- basic loops and "accumulating" something during the loop
Next: working with strings.
So, part one of this week's assignment: read chapter 4, and bring questions to class.
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]
ascii
ASCII : numeric values for 1 byte characters;
- wikipedia: ascii
- printable are 32 (space) through 126 (~).
- In python: ord() and chr() convert between characters and ASCII.
>>ord('a') # convert character to integer (ascii)
97
>>chr(90) # convert integer (ascii) to character
'Z'
, pg 92
, pg 93
(Aside: you can download the python programs in the textbook from Zelle's website; see the links on the
resources page.
string operations summary
There are many ways to manipulate strings in python.
'this ' + 'that' # concatenate
'x' * 10 # replicate
'This is a string'[0:3] # substrings; 'Thi'
len('string') # how many characters?
for char in the_string: # loop over characters
print char, ' is ', ord(char) # ... and do something with each
# 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(anything) # convert anything to a string
eval("1+2") # evaluate string as python expression; return result
Discuss the name.method(arg1, arg2) convention we just started seeing.
We'll be getting lots more of that.
And remember the dir() trick to see what
.method() things there are for different things in the language.
More practice with string manipulations.
import string
text = "This is some text."
text_list = list(text) # ['T', 'h', ...]
test_list2 = string.split(text, '') # same thing
back_to_text = string.join(text_list, '')
hello_there = "hello" + " " + "there"
number = ord('h')
letter = chr(number)
word_dashes = 'one-two-three'
word_list = string.split(word_dashes, '-') # ['one', 'two', 'three']
word_commas = string.join(word_list, ',') # 'one,two,three'
Aside :
There are actually three 'places' that the string functions live, which can be confusing :
>>> number = ord('h') # built-in function
>>> import string; string.upper('hello') # within the string module
>>> 'hello'.upper() # (!)
The last form we haven't talked about yet, but will do lots more with later.
(This is actually "object-oriented" programming, in which we think
of the string 'hello' as an "object" and upper as one of its "methods".)
Many of the string manipulations are available in both the string module
and as methods of strings. To see the difference
>>> import string # load the string module
>>> dir(string) # functions and variables in the string module
>>> dir('hello') # names that can be used as 'hello'.name
text formatting
Making the output look nice.
There are several ways to do this within python:
the old way (in your text), and the new way (recommended).
In both cases, markers are put within a string to
indicate where values should go, and then variables
are "interpolated" into the string in some given
format. The details get messy.
the old way : ("x=%f" % 3)
I'll mention this and then (mostly) ignore it.
print "Compare %f and %0.20f \n" % (3.14, 3.14)
Here the "%" symbol is placed between (a) the string with its markers
and (b) a sequence of the values to be interpolated.
The funky symbols within the string (%i, %f, %d, %s) give the format (integer, float, decimal, string).
the new way : "x={}".format(3)
The python language has been transitioning to a new syntax
for string formatting, the .format() string method and {}
for interpoloation.
The new version looks like this :
print "Compare {} and {:0.20} ".format(3.14, 3.14)
This is what you should do. For the details see the docs.
Files
A file is essentially a list of lines. Once you've "opened" one, you can read from it and/or print to it.
inputfile = open('filename', 'r')
# In that last line :
# 'r' for 'read' access', i.e. input,
# 'w' for 'write access', i.e. output
# Input: use one of these three :
whole_file = inputfile.read()
next_line = inputfile.readline()
array_of_lines = inputfile.readlines()
outputfile = open('other_filename_here', 'w')
outputfile.write("this will be the first line in that file. \n") # \n is newline
summary of strings, text, files
Putting all this together you get a very typical program :
- open a file
- read stuff in (typically text or numbers)
- do something to it (put it into a secret code, for example)
- write it back out
Example: write together in class a program to do file conversion for rot13
- see wikipedia:rot13
- Ask for input file name
- create file with same name, but 'rot13_' on front.
- Read input to a string.
- In a loop, convert (and store) the thing to its rot13 version
- Output new string to new file
Are we having fun yet?
