An Introduction To Software Development Using Python

Spring Semester, 2015

Class #21:Sets

What Is A Set?

• A set is a container that stores a collection of unique values.

• Unlike a list, the elements or members of the set are not stored in any particular order and cannot be accessed by position.

• The operations available for use with a set are the same as the operations performed on sets in mathematics.

• Because sets need not maintain a particular order, set operations are much faster than the equivalent list operations

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An Example Of SetsCheese Pizza Pepperoni Pizza Meat Lover’s Pizza

• Marinara sauce

• Pepperoni

• Italian sausage

• Slow-roasted ham

• Hardwood smoked bacon

• Seasoned pork and beef

• Marinara sauce

• Cheese

• Pepperoni

• Creamy garlic Parmesan sauce

• Cheese

• Toasted Parmesan

Image Credit: https://order.pizzahut.com/site/menu/pizza

Creating & Using Sets

• To create a set with initial elements, you can specify the elements enclosed in braces,just like in mathematics:

cheesePizza = {“Creamy Garlic Parmesan Sauce”, “Cheese”, “Toasted Parmesan”}

Image Credit: https://order.pizzahut.com/site/menu/pizza

Creating & Using Sets

• Alternatively, you can use the set function to convert any list into a set:

cheesePie = [“Creamy garlic Parmesan sauce”, “Cheese”, “Toasted Parmesan”]

cheesePizza = set(cheesePie)

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Creating & Using Sets

• For historical reasons, you cannot use {} to make an empty set in Python.

• Instead, use the set function with no arguments: cheesePizza = set()

• As with any container, you can use the len function to obtain the number of elements in a set: numberOfIngredients = len(cheesePizza)

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Is Something In A Set?• To determine whether an element is contained in the set, use

the in operator or its inverse, the not in operator:

if "Toasted Parmesan" in cheesePizza : print(“A cheese pizza contains Toasted Parmesan")else : print(“A cheese pizza does not contain Toasted Parmesan")

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Sets & Order• Because sets are unordered, you cannot access the elements of a set by position

as you can with a list.

• Instead, use a for loop to iterate over the individual elements: print("The ingredients in a cheese pizza are:") for ingredient in cheesePizza : print(ingredient)

• Note that the order in which the elements of the set are visited depends on how they are stored internally. For example, the loop above displays the following: The ingredients in a Cheese pizza are: Toasted Parmesan Creamy garlic Parmesan sauce Cheese

• Note that the order of the elements in the output can be different from the order in which the set was created.

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Sets & Order

• The fact that sets do not retain the initial ordering is not a problem when working with sets. In fact, the lack of an ordering makes it possible to implement set operations very efficiently.

• However, you usually want to display the elements in sorted order. Use the sorted function, which returns a list (not a set) of the elements in sorted order. The following loop prints the pizza ingredients in alphabetical sorted order:

for ingredient in sorted(cheesePizza) : print(ingredient)

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Adding Elements To A Set

• Like lists, sets are collections, so you can add and remove elements.

• For example, suppose we need to add more ingredient to a meat lover’s pizza. Use the add method to add elements: meatLoversPizza.add("Mushrooms")

• A set cannot contain duplicate elements. If you attempt to add an element that is already in the set, there is no effect and the set is not changed.

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Removing Elements From

A Set• There are two methods that can be used to remove individual

elements from a set. The discard method removes an element if the element exists: meatLoversPizza.discard("Italian sausage")

• The remove method, on the other hand, removes an element if it exists, but raises an exception if the given element is not a member of the set. meatLoversPizza.remove("Italian sausage")

• Finally, the clear method removes all elements of a set, leaving the empty set: meatLoversPizza.clear() # no ingredients

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Subsets• A set is a subset of another set if and only if every element of the first set

is also an element of the second set.

• The issubset method returns True or False to report whether one set is a subset of another

Cheese Pizza Pepperoni Pizza

• Marinara sauce

• Cheese

• Pepperoni

• Marinara sauce

• Cheese

Image Credit: www.vectorious.net

cheesePizza.issubset(pepperoniPizza)

Comparing Subsets

• You can also test for equality between two sets using the == and != operators.

• Two sets are equal if and only if they have exactly the same elements.

cheesePizza != pepperoniPizza

cheesePizza1 == cheesePizza2

Set Union, Intersection, and Difference

• The union of two sets contains all of the elements from both sets, with duplicates removed.

• The intersection of two sets contains all of the elements that are in both sets.

• The difference of two sets results in a new set that contains those elements in the first set that are not in the second set.

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cheesePizza.union(pepperoniPizza) = {“Marinara sauce”, “Cheese”, “Pepperoni”}

cheesePizza.intersection(pepperoniPizza) = {“Marinara sauce”, “Cheese”}

cheesePizza.difference(pepperoniPizza) = {“Pepperoni”}

What’s In Your Python Toolbox?

print() math strings I/O IF/Else elif While For

Lists And/Or/Not Functions Files Exception Sets

What We Covered Today

1. Sets

2. Sets & Order

3. Adding to a set

4. Removing from a set

5. Subsets

6. Union, Intersection, Difference

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What We’ll Be Covering Next Time

1. Dictionaries

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