Pythonic way to count sequences

[Chris Angelico]

On Thu, Apr 25, 2013 at 3:05 PM, CM <cmpython at gmail.com> wrote:
> I have to count the number of various two-digit sequences in a list
> such as this:
>
> mylist = [(2,4), (2,4), (3,4), (4,5), (2,1)]  # (Here the (2,4)
> sequence appears 2 times.)
>
> and tally up the results, assigning each to a variable.

You can use a tuple as a dictionary key, just like you would a string.
So you can count them up directly with a dictionary:

count = {}
for sequence_tuple in list_of_tuples:
    count[sequence_tuple] = count.get(sequence_tuple,0) + 1

Also, since this is such a common thing to do, there's a standard
library way of doing it:

import collections
count = collections.Counter(list_of_tuples)

This doesn't depend on knowing ahead of time what your elements will
be. At the end of it, you can simply iterate over 'count' and get all
your counts:

for sequence,number in count.items():
	print("%d of %r" % (number,sequence))

ChrisA