*******
Find the average length of all strings in a list of strings

["apple", "bear", "cat", "friends"]

5 + 4 + 3 + 6 = 18/4 = 4.5

def avg(words):
   tot = 0
   for x in words:
       tot = tot + len(x)
   return 1.0*tot/len(words)

def avg(words):
   return 1.0*sum([len(x) for x in words])/len(words)

*******

Is a number prime? Prime numbers are only divisible
by one and themselves, 49 isn't prime (divisible by 7), 53 is prime

def is_prime(n):
    for div in range(2,n):
        if n % div == 0:
            return False
    return True

********

Read a file and return a list of all words with
more than three vowels

def v3(fname):
    f = open(fname)
    str = f.read()
    ret = []
    for word in str.split():
      if lotsvowels(word):
          ret.append(word)
    return ret
---
write lotsvowels:

def lotsvowels(word):
    tot = 0
    for let in word:
        if "aeiou".find(let) >= 0:
            tot = tot + 1
    return tot > 3

********

find the longest runof H's in a string of H,T

longh("HHHTTTHHHHHTTHHHHHHTTTTHH") is 6

def longh(st):
  best = 0
  run = 0
  for ch in st:
      if ch == 'H':
          run = run + 1
          if run > best:
              best = run
      else:
          run = 0
   return best

********

create acronym from last letter in a list of string
["this", "pony", "cub", "luna", "car", "radii", "cat", "bee"]

produces: "sybarite"

def lastac(words):
  w = ""
  for word in words:
      w = w + word[-1]
  return w

*********
Read a file, count how many times each word in the
file occurs using a dictionary, return a list of
words sorted by frequency

def words(fname):
    f = open(fname)
    d = {}
    for line in f:
        parts = line.strip().split()  # list of words
        for word in parts:
            if word not in d:
                d[word] = 0
            d[word] += 1

    #d is a dictionary of word/frequency pairs, sort by frequency:

    ret = sorted(d.items(), key=operator.itemgetter(1))
    return ret

************
compute the words in common to two lists a and b

a = ['apple', 'bear', 'cherry, 'lion']
b = ['bear', 'lion', 'tiger', 'giraffe']

common = list(set(a) & set(b))

# common = ['bear', 'lion']

combo = list(set(a) | set(b))

# combo = ['apple', 'bear', 'cherry, 'lion', 'tiger', 'giraffe']



************
store words in a file and a list of line numbers on which
words occur, list is sorted with each line number occuring once

def words(fname):
    f = open(fname)
    d = {}
    line_num = 0
    for line in f:
        parts = line.strip().split()  # list of words
        line_num += 1
        for word in parts:
            if word not in d:
                d[word] = set()
            d[word].add(line_num)

    for key in d:
        d[key] = sorted(d[key])

    return d