List Exercises

About List Exercises

• note that the following exercises create and return new lists!
• there are built-in functions and constructs in Python that make these exercises easier
  • for now, we'll do it the difficult way
  • we'll go over some of these constructs in-depth in later classes
• these exercises will map, filter or reduce a list…

Pluralize All (Map)

Implement a function called pluralize_all: →

• it will take a list of strings called words
• it should return a list of strings
• the resulting list will have plural versions of every string passed in
• assume that each element in the list is a string that's a singular word
• you can use a naive method of pluralization (just add an 's'!)
• it will return an empty list if an empty list is inputted
• remember to use assertions

Pluralize All … Output

print(pluralize_all(["zebra", "cow", "tiger"]))
['zebras', 'cows', 'tigers']

Pluralize All Potential Solution

def pluralize_all(words):
	new_list = []
	for word in words:
		new_list.append(word + 's')
	return new_list

More Characters Than (Filter)

Implement a function called more_characters_than: →

• it will take a list of strings, called words, and an integer, called min, as an input
• it should return a list of strings
• the resulting list will only have strings with more than min characters
• it will return an empty list if an empty list is inputted
• ignore the case where a list has non-string types or if min is not an integer
• remember to use assertions

More Characters Than Example

print(more_characters_than(["zebra", "cow", "tiger"], 4))
['zebra', 'tiger']

More Characters Than Potential Solution

def more_characters_than(words, min):
	new_list = []
	for word in words:
		if len(word) > min:
			new_list.append(word)
	return new_list

assert ['zebra', 'tiger'] == more_characters_than(["zebra", "cow", "tiger"], 4), "only strings with more than 4 characters"
assert [] == more_characters_than([], 4), "an empty list returns an empty list"

Average (Reduce)

• define a function called average
• it should take a list of numbers as an argument
• it will return the average of the numbers in the list
• empty list will have an average of 0
• remember to use assertions

Implementing Average

def average(numbers):
	sum = 0
	for n in numbers:
		sum += n
	return sum / len(numbers)

assert 9 == average([2, 3, 4]), "takes a list of integers and returns the average value"

Using the built-in sum:

def average(numbers):
	return sum(numbers) / len(numbers)

Notes

Interested in other ways to do this? Check out… →

• List Comprehensions (we'll check this out in a later class)
• map()
• filter()

Lists and Mutability