3.4.1 (v3.4.1:c0e311e010fc, May 18 2014, 00:54:21) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)]
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# Topic: list comprehensions.
# They take this general form:
# [ <expression> for <variable> in <iterable> ]

# Set up for some examples.
L = [5, 13, -2, 77, 9]

# Examples:
[item + 100 for item in L]
[105, 113, 98, 177, 109]

[item**2 for item in L]
[25, 169, 4, 5929, 81]

# Imagine doing this without a list comprehension.
answer = []
for item in L:
    answer.append(item**2)
answer
[25, 169, 4, 5929, 81]

# Much nicer to do it in one quite easy-to-understand line.
[item**2 for item in L]
[25, 169, 4, 5929, 81]

# Here we'll work with a list of lists.
matrix = [[1, 2, 3,], [4, 5, 6], [7, 8, 9]]
[row[0] for row in matrix]
[1, 4, 7]
# Generalize it to any column.
col = 2
[row[col] for row in matrix]
[3, 6, 9]
matrix
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

# Remember the sum function?
sum([4, 7, 99, 199])
309

# We can use it in our list comprehension to sum up the rows of our matrix.
[sum(row) for row in matrix]
[6, 15, 24]

# We can sum that to get the grand total of the values in the matrix -- cool!
sum([sum(row) for row in matrix])
45

# What if it was a less tidy list, with different levels
# of nesting?
L = [8, [4, 5, 6], [1, 2, 3, [8, 8], 4], [[[[5]]]]]
# We'll figure that one out next week.

# ---- And a couple of examples I didn't get to show you in lecture:

# We can iterate over anything.  But a list comprehension will 
# always compose a list.
# Example iterating over a str:
s = 'cool'
[char + '!' for char in s]
['c!', 'o!', 'o!', 'l!']

# Example iterating over a dict:
d = {1: 'a', 2: 'b', 3: 'c', 4: 'd'}
[k/2.0 for k in d.keys()]
[0.5, 1.0, 1.5, 2.0]