List Comprehensions & Functions
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Find the first letter of each word:

    >>> words = 'this is a short sentence'.split()
    >>> [word[0] for word in words]
    ['t', 'i', 'a', 's', 's']

Convert each word in a list to lower case:

    >>> words = 'This sentence has some Capitalized words'.split()
    >>> [word.lower() for word in words]
    ['this', 'sentence', 'has', 'some', 'capitalized', 'words']

Define a function that counts the number of vowels in a word:

    >>> def vowels(word):
    ...     v = 0
    ...     for char in word:
    ...         if char in 'aeiouAEIOU':
    ...             v = v + 1
    ...     return v

Use the new 'vowels' function to find the number of vowels in each
word from a word list:

    >>> [vowels(word) for word in words]
    [1, 3, 1, 2, 5, 1]

Define a function that finds the average of a list of numbers.  

    >>> def avg(numbers):
    ...     return sum(numbers) / float(len(numbers)) # [1]

[1] Note that we used 'float()' to convert the denominator from an
    integer to a real number, since dividing by integers rounds to
    the nearest value:

        >>> print 10/3
        3

Find the average number of vowels in the world list.

    >>> print avg([vowels(word) for word in words])
    2.16666666667

Find the average length of words that begin with a vowel:

    >>> from nltk.corpus import brown
    >>> words = brown.tokenized('a', group_by_sent=False)
    >>> # Select only those words that start with a vowel:
    >>> words = [w for w in words if w[0].lower() in 'aeiouy']
    >>> # Find the length of each word:
    >>> lengths = [len(w) for w in words]
    >>> # Get the average:
    >>> print avg(lengths)
    4.00069249257