# question: how do you sort on the second letter of a list of strings?
>>> ls = [  "ahi", "gbc", "def" ]
>>> ls
Out[1]: ['ahi', 'gbc', 'def']
>>> import operator
>>> sorted(ls, key=operator.itemgetter(1))
Out[1]: ['gbc', 'def', 'ahi']


# because YOU asked :)
# different ways to square elements of a list
>>> values = [ 1, 2, 3]
>>> values
Out[1]: [1, 2, 3]
# traditional for loop, fine for all situations in this class
>>> valuesSquared = []
>>> for x in values:
…     valuesSquared.append(x * x)
>>> valuesSquared
Out[1]: [1, 4, 9]
# standard list comprehension, "squaring" is part of the definition
>>> [ x * x for x in values ]
Out[1]: [1, 4, 9]
# define a function to compute squared value
>>> def square (x):
…     return x * x
# more general list comprehension, "squaring" can be anything function does
>>> [ square(x) for x in values ]
Out[1]: [1, 4, 9]
# map function, apply given function to each element in sequence
>>> map(square, values)
Out[1]: [1, 4, 9]
# map function, apply "inline, anonymous" function to each element in sequence
# primarily for those that do not want to define function separately
# NEVER required in this course
>>> map(lambda x: x * x, values)
Out[1]: [1, 4, 9]


# dictionary basics
# empty
>>> d = { }
# some initial values, using colon as separator
>>> d = { 'a':13, 'b':24, 'c':39 }
>>> d
Out[1]: {'a': 13, 'b': 24, 'c': 39}
# accessing values using keys
>>> d['a']
Out[1]: 13
>>> d['b']
Out[1]: 24
>>> d['c']
Out[1]: 39
# error to use non-existent key
>>> d['z']
Traceback (most recent call last):
  File "/Users/guestuser/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/IPython/core/interactiveshell.py", line 2721, in run_code
    exec code_obj in self.user_global_ns, self.user_ns
  File "<ipython-input-1-da9eddaf4274>", line 1, in <module>
    d['z']
KeyError: 'z'

# accessing all contents
>>> for (k,v) in d.items():
...     print(k + '\t' + str(v))    
a	13
c	39
b	24

# changing a value
>>> d['c'] = 45
>>> d['c']
Out[1]: 45
>>> d['c'] += 1
>>> d['c']
Out[1]: 46
# error to update non-existent key
>>> d['z'] += 1
Traceback (most recent call last):
  File "/Users/guestuser/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/IPython/core/interactiveshell.py", line 2721, in run_code
    exec code_obj in self.user_global_ns, self.user_ns
  File "<ipython-input-1-b3482c982944>", line 1, in <module>
    d['z'] += 1
KeyError: 'z'

# keys must be immutable
>>> d = { (1,2):24, [1,2]:13 }
Traceback (most recent call last):
  File "/Users/guestuser/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/IPython/core/interactiveshell.py", line 2721, in run_code
    exec code_obj in self.user_global_ns, self.user_ns
  File "<ipython-input-1-f58f7debf8a7>", line 1, in <module>
    d = { (1,2):24, [1,2]:13 }
TypeError: unhashable type: 'list'
# tuples, numbers, and strings are immutable
>>> d = { (1,2):24, 13:[1,2], 'abc':'def' }