Sets in Python

A set is a collection of elements which are unique. A set is created by using built-in set( ) function or by using curly braces which has elements separated by ‘comma’.

The following program shows a sets which are created by using curly braces and set( ) function .

setItems = {"pen", "pencil", "eraser"}
print(setItems)

# Using set() function
setItems = set(("pen", "pencil", "eraser"))
print(setItems)

Output:

{'pencil', 'eraser', 'pen'}
{'pencil', 'eraser', 'pen'}

Length of a set can be found by using the len( ) method. The following program shows the use of len( ) method inorder to determine its length.

setItems = {"pen", "pencil", "eraser"}
print(len(setItems))

Output:

A set item cannot be accessed by referring to its index number since they are unordered. Items in a set can be printed by using a for loop.

setItems = {"pen", "pencil", "eraser"}
for x in setItems:
    print(x)

Output:

pencil
eraser
pen

An item can be added in a set by using the add( ) method. Multiple items and items of lists, strings, tuples and other sets can be added by using the update( ) method.

The following program shows add( ) and update( ) method which is applied to set.

# Adding an element
setItems = {"pen", "pencil", "eraser"}
setItems.add("sharpener")
print(setItems)

# Update() method
setItems1 = {"pen", "pencil", "eraser"}
setItems2 = {"sharpener", "notebook"}

setItems1.update(setItems2)
print(setItems1)

Output:

{'eraser', 'pen', 'sharpener', 'pencil'}
{'eraser', 'sharpener', 'pen', 'pencil', 'notebook'}

An item in a set can be removed by using the remove( ) method or discard( ) method. remove( ) method only removes when that item is present otherwise it throws an error while discard( ) method does not throws that error.

An item can also be removed by using the pop( ) method. This method removes the last item but since the items are unordered in set, any of the items will be deleted. All the items of a set can be removed by using the clear( ) method.

# Use of remove() method
setItems = {"pen", "pencil", "eraser"}
setItems.remove("pencil")
print("After removing \"pencil\" set items are", setItems)

# Use of discard() method
setItems = {"pen", "pencil", "eraser"}
setItems.discard("eraser")
print(setItems)

# Use of pop() method
setItems = {"pen", "pencil", "eraser"}
removedItem = setItems.pop()
print(setItems)
print("Removed item is", removedItem)

# Use of clear() method 
setItems = {"pen", "pencil", "eraser"}
setItems.clear()
print(setItems)

Output:

After removing "pencil" set items are {'pen', 'eraser'}
{'pencil', 'pen'}
{'pen', 'eraser'}
Removed item is pencil
set()

The elements of sets can be added together to form a new set using union( ) method or by using ‘|’ operator. Also the new set does not contain the duplicate items. The following program shows union( ) method and ‘|’ operator to have the elements in a new set( ).

setItems1 = {"pen", "pencil", "eraser"}
setItems2 = {"sharpener", "notebook"}

setItems3 = setItems1.union(setItems2)

print(setItems3)
print(setItems1 | setItems2)

Output:

{'notebook', 'pen', 'sharpener', 'pencil', 'eraser'}
{'notebook', 'pen', 'sharpener', 'pencil', 'eraser'}

Items which are common in sets can be found by using the intersection( ) method or by ‘&’ operator. The following program shows the use of intersection( ) method and ‘&’ operator to print the elements which are common in two sets( ).

setItems1 = {"pen", "pencil", "eraser"}
setItems2 = {"pencil", "notebook"}

setItems3 = setItems1.intersection(setItems2)

print(setItems3)
print(setItems1 & setItems2)

Output:

{'pencil'}
{'pencil'}

Difference of set can be printed by using difference( ) method or by using ‘-‘ operator. Difference of set A and set B, which is (A – B) are the elements which are only present in set A. The following program shows the use of difference( ) method and ‘‘-‘ operator on two sets.

setItems1 = {"pen", "pencil", "eraser"}
setItems2 = {"pencil", "notebook"}

setItems3 = setItems1.difference(setItems2)

print(setItems3)
print(setItems1 - setItems2)

Output:

{'pen', 'eraser'}
{'pen', 'eraser'}

Frozen sets are immutable, that the elements cannot be changed once it is assigned. A frozen set is made by using the frozenset( ) method. The following program shows a frozen set.

setItems1 = frozenset({"pen", "pencil", "eraser"})