View source for Boston Python Workshop 7/Wordplay handout

===Wordplay goals===

* practice for loops
* practice using lists
* practice manipulating strings
* get experience with regular expressions
* have fun cheating at crosswords and Words with Friends

===Concept review===

====Indentation reminder====

In Python, indentation matters. Everything is indented by a multiple
of some number of spaces, often 4.

In <code>if</code> statements, you indent everything you want to be run if the if
conditional is <code>True</code>. For example:

<pre>
>>> James = 35
>>> Alice = 30
>>> if James > Alice:
...     print "James is older than Alice."
...
James is older than Alice.
>>></pre>

Because James really is older than Alice, the <code>if</code> conditional is <code>True</code>,
so Python does execute the code indented under the if line. In this
case we print "James is older than Alice."

<pre>
>>> James = 35
>>> Alice = 30
>>> if James < Alice:
...     print "James is younger than Alice."
...
>>></pre>

Because James is not older than Alice, the <code>if</code> conditional is <code>False</code>, so
Python does not execute the code indented under the if line.

In for loops, you indent everything you want to be run each loop For
example:

<pre>
>>> names = ["Jessica", "Adam", "Liz"]
>>> for name in names:
...     print "Hello " + name
...
Hello Jessica
Hello Adam
Hello Liz</pre>

The <code>print</code> line is indented 4 spaces under the <code>for</code>. That's how
Python knows to execute the print line for every name in names.

====range====

<pre>
>>> range(5)
[0, 1, 2, 3, 4]
>>> for i in range(5):
...     print "Hi" * i
...

Hi
HiHi
HiHiHi
HiHiHiHi</pre>

====<code>if</code> statements inside <code>for</code> loops====

<pre>
>>> for name in ["Alice", "Bob", "Cassie", "Deb", "Ellen"]:
...     if name[0] in "AEIOU":
...         print name + " starts with a vowel."
... 
Alice starts with a vowel.
Ellen starts with a vowel.</pre>

====<code>for</code> loops inside <code>for</code> loops====

<pre>
>>> letters = ["a", "b", "c"]
>>> numbers = [1, 2, 3]
>>> for letter in letters:
...     for number in numbers:
...         print letter * number
...
a
aa
aaa
b
bb
bbb
c
cc
ccc
>>> for number in numbers:
...     for letter in letters:
...         print number * letter
...
a
b
c
aa
bb
cc
aaa
bbb
ccc</pre>

====Imports====

Imports look like this:

<pre>
>>> import random
>>> import time</pre>

In the above example, <code>random</code> and <code>time</code> are both Python modules. Modules
are Python files outside of the current Python file that contain
Python code, like functions and variables. You can use code from
modules by first importing the module. Here's an example from the
<code>random</code> module:

<pre>
>>> import random
>>> random.randint(0, 10)
7
>>> random.randint(0, 10)
6
>>> random.randint(0, 10)
1
>>> random.randint(0, 10)
3
>>> random.randint(0, 10)
4
>>> random.randint(0, 10)
9</pre>

<code>randint</code> is a function in the <code>random</code> module. It takes a lower bound as
the first argument and an upper bound as the second argument and
returns a random integer between those bounds.

===New Wordplay material summary===

====<code>in</code> keyword====

<code>in is a keyword checking for containment. You can use it in a couple
of ways:

<pre>
>>> "a" in "apple"
True
>>> "z" in "apple"
False</pre>

<pre>
>>> dogs = ["pug", "boxer", "dalmation"]
>>> "tiger" in dogs
False
>>> "pug" in dogs
True</pre>

====Reversing lists====

Here's a quick way to reverse a list.

<pre>
>>> fruits = ["apples", "bananas", "cherries"]
>>> fruits[::-1]
['cherries', 'bananas', 'apples']</pre>

Let's break down why this works:

First, remember that we can get individual elements from lists:

<pre>
>>> fruits[0]
'apples'</pre>

We can also slice lists:

<pre>
>>> fruits[0:2]
['apples', 'bananas']
>>> fruits[:2]
['apples', 'bananas']
>>> fruits[1:]
['bananas', 'cherries']</pre>

We can also make a copy of the list by taking a slice from the
beginning to the end of the list:

<pre>
>>> my_fruits = fruits[:]
['apples', 'bananas', 'cherries']</pre>

There's an extended slicing syntax that let's you say what direction
you want to slice in. By default it is forward, but you can supply a
<code>-1</code> to say backwards:

<pre>
>>> fruits[::-1]
['cherries', 'bananas', 'apples']</pre>

====Regular expressions====

* ^ is an anchor that means "beginning"
* $ is an anchor that means "end"
* . means a single wildcard character
* .* means any number (including zero) of wildcard characters

Examples:

* <code>rstu</code> will match anything that contains "rstu", for example "understudy".
* <code>^aa</code> will match anything that starts with "aa", for example "aardvark".
* <code>a.b.c</code> will match anything containing "a", then any single character, then "b", then any single character, then "c", for example "iambic".
* <code>ss.*ss</code> will match anything containing "ss", then anything, then "ss", for example "messiness".
* <code>^c.d.e$</code> will match anything that starts with "c", then any single character, then d, then any single character, then ends with an "e", for example "cadre".


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