Python Sets

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For code examples see the [[Week2 Sets Lab]]

Python sets are a collection variable like lists and tuples and are capable of ingesting different data types.

Unlike lists and tuples, sets are:

• unordered, and
• contain only unique elements

Creating a set

You create a set much like a list only instead of square brackets you use curly brackets.

set1 = { "There", "can", "be", "only", "one"}

It is possible to create a set with non-unique variables, but once it is initialised, those non-unique elements will vanish, with only one of those elements remaining.

E.g.

marshall = { 
			"Slim Shady", 
			"The Real Shady", 
			"All You Other Slim Shadies", 
			"Slim Shady", 
			"Just Imatations", 
			"Marshall Mathers"
			}
>>> {"Slim Shady", "The Real Shady", "All You Other Slim Shadies", "Just Imatations", "Marshall Mathers"}

The second "Slim Shady" did not stand up, and was removed from the set.

Typecasting a list to a set to remove duplicates.

Say the previous set Marshall had been a list. We could use the set() function to remove dupiicates and typecast it as a set instead.

marshall = ["Slim Shady", "The Real Shady", "All You Other Slim Shadies", "Slim Shady", "Just Imatations", "Marshall Mathers"]
set(marshall)
>>> {"Slim Shady", "The Real Shady", "All You Other Slim Shadies", "Just Imatations", "Marshall Mathers"}

Set functions

Add an element to a set using the add() method. Note that adding an element that already exists will not work.

marshall.add("stan")

You can remove elements using the remove() method.

marshall.remove("stan")

You can verify if an element is in the set using the in command.

if "Slim Shady" in marshall:
	print("Please stand up.")

Mathematical Set operations

Finding the intersection of two sets

When you have two sets, you can find the intersection, that is, the common elements to both sets, using the ampersand & operator.

double_agent_set = special_circumstances_set & contact_set

Likewise, you can use the intersection() method.

double_agent_set = special_circumstances_set.intersection(contact_set)

Constructing the union of two sets

To construct the union of two sets, use the union() method.

high_level_involved = contact_set.union(special_circumstances_set)

Check if a Set is a Subset of another Set

Simply use the issubset() method of the smaller set and pass in the larger set.

32bool = m32_set.issubset(special_circumstances_set)

Check if a Set is a Superset of another Set

Simply use the issuperset() method of the larger set and pass in the smaller set.

SCbool = special_circumstances_set.issuperset(m32_set)

Find the difference between one set and another

To find how one set is different from another, call its difference() method on the second set.

difference = original_set.difference(other_set)