Question

Lists

Lists are exactly as the name implies. They are lists of objects. The objects can be any data type (including lists), and it is allowed to mix data types. In this way they are much more flexible than arrays. It is possible to append, delete, insert and count elements and to sort, reverse, etc. the list.

a_list = [1,2,3,"this is a string",5.3]
b_list = ["A","B","F","G","d","x","c",a_list,3]
print(b_list)

print(b_list[7:9])

a = [1,2,3,4,5,6,7]
a.insert(0,0)
print(a)
a.append(8)
print(a)
a.reverse()
print(a)
a.sort()
print(a)
a.pop()
print(a)
a.remove(3)
print(a)
a.remove(a[4])
print(a)

Just like with strings, elements are indexed beginning with 0.

Lists can be constructed using ‘for’ and some conditional statements. These are called, ‘list comprehensions’. For example:

even_numbers = [x for x in range(100) if x % 2 == 0]
print(even_numbers)

List comprehensions can work on strings as well:

first_sentence = "It was a dark and stormy night."
characters = [x for x in first_sentence]
print(characters)

For more on comprehensions see: https://docs.python.org/2/tutorial/datastructures.html?highlight=comprehensions

Another similar feature is called ‘map’. Map applies a function to a list. The syntax is

map(aFunction, aSequence). Consider the following examples:

def sqr(x): return x ** 2
a = [2,3,4]
b = [10,5,3]
c = map(sqr,a)
print(c)
d = map(pow,a,b)
print(d)

Note that map is usually more efficient than the equivalent list comprehension or looping contruct.

Tuples

Tuples are like lists with one very important difference. Tuples are not changeable.

a = (1,2,3,4)
print(a)
a[1] = 2

a = (1,"string in a tuple",5.3)
b = (a,1,2,3)
print(a)
print(b)

As you can see, all of the other flexibility remains - so use tuples when you have a list that you do not want to modify.

One other handy feature of tuples is known as ‘tuple unpacking’. Essentially, this means we can assign the values of a tuple to a list of variable names, like so:

my_pets = ("Chestnut", "Tibbs", "Dash", "Bast")
(aussie,b_collie,indoor_cat,outdoor_cat) = my_pets
print(aussie)
cats=(indoor_cat,outdoor_cat)
print(cats)

Dictionaries

Dictionaries are unordered, keyed lists. Lists are ordered, and the index may be viewed as a key.

a = ["A","B","C","D"] #list example
print(a[1])

a = {'anItem': "A", 'anotherItem': "B",'athirdItem':"C",'afourthItem':"D"} # dictionary example
print(a[1])

a = {'anItem': "A", 'anotherItem': "B",'athirdItem':"C",'afourthItem':"D"} # dictionary example
print(a['anItem'])

print(a)

The dictionary does not order the items, and you cannot access them assuming an order (as an index does). You access elements using the keys.

Sets

Sets are unordered collections of unique elements. Intersections, unions and set differences are supported operations. They can be used to remove duplicates from a collection or to test for membership. For example:

from sets import Set
fruits = Set(["apples","oranges","grapes","bananas"])
citrus = Set(["lemons","oranges","limes","grapefruits","clementines"])
citrus_in_fruits = fruits & citrus   #intersection
print(citrus_in_fruits)
diff_fruits = fruits - citrus        # set difference
print(diff_fruits)
diff_fruits_reverse = citrus - fruits  # set difference
print(diff_fruits_reverse)
citrus_or_fruits = citrus | fruits     # set union
print(citrus_or_fruits)

a_list = ["a", "a","a", "b",1,2,3,"d",1]
print(a_list)
a_set = Set(a_list)  # Convert list to set
print(a_set)         # Creates a set with unique elements
new_list = list(a_set) # Convert set to list
print(new_list)        # Obtain a list with unique elements

More examples and details regarding sets can be found at: https://docs.python.org/2/library/sets.html