Finding the most frequent character in a string

Question

I found this programming problem while looking at a job posting on SO. I thought it was pretty interesting and as a beginner Python programmer I attempted to tackle it. Howe

Answer 1

If you want to have all the characters with the maximum number of counts, then you can do a variation on one of the two ideas proposed so far:

import heapq  # Helps finding the n largest counts
import collections

def find_max_counts(sequence):
    """
    Returns an iterator that produces the (element, count)s with the
    highest number of occurrences in the given sequence.

    In addition, the elements are sorted.
    """

    if len(sequence) == 0:
        raise StopIteration

    counter = collections.defaultdict(int)
    for elmt in sequence:
        counter[elmt] += 1

    counts_heap = [
        (-count, elmt)  # The largest elmt counts are the smallest elmts
        for (elmt, count) in counter.iteritems()]

    heapq.heapify(counts_heap)

    highest_count = counts_heap[0][0]

    while True:

        try:
            (opp_count, elmt) = heapq.heappop(counts_heap)
        except IndexError:
            raise StopIteration

        if opp_count != highest_count:
            raise StopIteration

        yield (elmt, -opp_count)

for (letter, count) in find_max_counts('balloon'):
    print (letter, count)

for (word, count) in find_max_counts(['he', 'lkj', 'he', 'll', 'll']):
    print (word, count)

This yields, for instance:

lebigot@weinberg /tmp % python count.py
('l', 2)
('o', 2)
('he', 2)
('ll', 2)

This works with any sequence: words, but also ['hello', 'hello', 'bonjour'], for instance.

The heapq structure is very efficient at finding the smallest elements of a sequence without sorting it completely. On the other hand, since there are not so many letter in the alphabet, you can probably also run through the sorted list of counts until the maximum count is not found anymore, without this incurring any serious speed loss.