Python Sets
Sets are unordered collection of
data items. They store multiple items in a single variable. Sets items are
separated by commas and enclosed within curly brackets {}. Sets are
unchangeable, meaning you cannot change items of the set once created. Sets do
not contain duplicate items.
Example:
info={"Carla",19,False,5.9,19}
print(info)
Output:
{False, 19, 5.9, 'Carla'}
Here we see that the items of set
occur in random order and hence they cannot be accessed using index numbers. Also
sets do not allow duplicate values.
Accessing
set items:
Using
a For loop
You can access items of set using
a for loop.
Example:
info={"Carla",19,False,5.9}
foritemininfo:
print(item)
Output:
FalseCarla195.9
Add/Remove Set Items
Add
items to set:
If you want to add a single item
to the set use the add() method.
Example:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities.add("Helsinki")
print(cities)
Output:
{'Tokyo', 'Helsinki', 'Madrid', 'Berlin', 'Delhi'}
If you want to add more than one
item, simply create another set or any other iterable object(list, tuple,
dictionary), and use the update() method to add it into the existing set.
Example:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Helsinki","Warsaw","Seoul"}
cities.update(cities2)
print(cities)
Output:
{'Seoul', 'Berlin', 'Delhi', 'Tokyo', 'Warsaw', 'Helsinki', 'Madrid'}
Remove
items from set:
We can use remove() and discard()
methods to remove items form list.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities.remove("Tokyo")
print(cities)
Output:
{'Delhi', 'Berlin', 'Madrid'}
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities.discard("Delhi")
print(cities)
Output:
{'Berlin', 'Tokyo', 'Madrid'}
The main difference between
remove and discard is that, if we try to delete an item which is not present in
set, then remove() raises an error, whereas discard() does not raise any error.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities.remove("Seoul")
print(cities)
Output:
KeyError: 'Seoul'
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities.discard("Seoul")
print(cities)
Output:
{'Madrid', 'Delhi', 'Tokyo', 'Berlin'}
There are various other methods to remove items from the set:
pop(), del(), clear().
pop():
This method removes the last item
of the set but the catch is that we don’t know which item gets popped as sets
are unordered. However, you can access the popped item if you assign the pop()
method to a variable.
Example:
cities={"Tokyo","Madrid","Berlin","Delhi"}
item=cities.pop()
print(cities)
print(item)
Output:
{'Tokyo', 'Delhi', 'Berlin'}Madrid
del:
del is not a method, rather it is
a keyword which deletes the set entirely.
Example:
cities={"Tokyo","Madrid","Berlin","Delhi"}
del citiesprint(cities)
Output:
NameError: name 'cities' is not defined
We get an error because our
entire set has been deleted and there is no variable called cities which
contains a set.
What if we don’t want to delete
the entire set, we just want to delete all items within that set?
clear():
This method clears all items in
the set and prints an empty set.
Example:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities.clear()
print(cities)
Output:
set()
Check
if item exists
You can also check if an item
exists in the set or not.
Example 1:
info={"Carla",19,False,5.9}
if"Carla"ininfo:
print("Carla is present.")
else:print("Carla is absent.")
Output:
Carla is present.
Example 2:
info={"Carla",19,False,5.9}
if"Carmen"ininfo:
print("Carmen is present.")
else:print("Carmen is absent.")
Output:
Carmen is absent.
Join Sets
Sets in python more or less work
in the same way as sets in mathematics. We can perform operations like union
and intersection on the sets just like in mathematics.
I.
union() and update():
The union() and update() methods
prints all items that are present in the two sets. The union() method returns a
new set whereas update() method adds item into the existing set from another
set.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Tokyo","Seoul","Kabul","Madrid"}
cities3=cities.union(cities2)
print(cities3)
Output:
{'Tokyo', 'Madrid', 'Kabul', 'Seoul', 'Berlin', 'Delhi'}
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Tokyo","Seoul","Kabul","Madrid"}
cities.update(cities2)
print(cities)
Output:
{'Berlin', 'Madrid', 'Tokyo', 'Delhi', 'Kabul', 'Seoul'}
II.
intersection and intersection_update():
The intersection() and
intersection_update() methods prints only items that are similar to both the
sets. The intersection() method returns a new set whereas intersection_update()
method updates into the existing set from another set.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Tokyo","Seoul","Kabul","Madrid"}
cities3=cities.intersection(cities2)
print(cities3)
Output:
{'Madrid', 'Tokyo'}
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Tokyo","Seoul","Kabul","Madrid"}
cities.intersection_update(cities2)
print(cities)
Output:
{'Tokyo', 'Madrid'}
III.
symmetric_difference and symmetric_difference_update():
The symmetric_difference() and
symmetric_difference_update() methods prints only items that are not similar to
both the sets. The symmetric_difference() method returns a new set whereas
symmetric_difference_update() method updates into the existing set from another
set.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Tokyo","Seoul","Kabul","Madrid"}
cities3=cities.symmetric_difference(cities2)
print(cities3)
Output:
{'Seoul', 'Kabul', 'Berlin', 'Delhi'}
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Tokyo","Seoul","Kabul","Madrid"}
cities.symmetric_difference_update(cities2)
print(cities)
Output:
{'Kabul', 'Delhi', 'Berlin', 'Seoul'}
IV.
difference() and difference_update():
The difference() and
difference_update() methods prints only items that are only present in the
original set and not in both the sets. The difference() method returns a new
set whereas difference_update() method updates into the existing set from
another set.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Seoul","Kabul","Delhi"}
cities3=cities.difference(cities2)
print(cities3)
Output:
{'Tokyo', 'Madrid', 'Berlin'}
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Seoul","Kabul","Delhi"}
print(cities.difference(cities2))
Output:
{'Tokyo', 'Berlin', 'Madrid'}
Set Methods
Apart from the methods we
discussed earlier in the chapter there are some more methods that we can use to
manipulate sets.
What if you want to check if
items of a particular set are present in another set?
There are a few methods to check
this.
•
isdisjoint():
The isdisjoint() method checks if
items of given set are present in another set. This method returns False if
items are present, else it returns True.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Tokyo","Seoul","Kabul","Madrid"}
print(cities.isdisjoint(cities2))
Output:
False
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Seoul","Kabul"}
print(cities.isdisjoint(cities2))
Output:
True
•
issuperset():
The issuperset() method checks if
all the items of a particular set are present in the original set. It returns
True if all the items are present, else it returns False.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Seoul","Kabul"}
print(cities.issuperset(cities2))
cities3={"Seoul","Madrid","Kabul"}
print(cities.issuperset(cities3))
Output:
FalseFalse
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Delhi","Madrid"}
print(cities.issuperset(cities2))
Output:
True
• issubset():
The issubset() method checks if
all the items of the original set are present in the particular set. It returns
True if all the items are present, else it returns False.
Example 1:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Delhi","Madrid"}
print(cities2.issubset(cities))
Output:
True
Example 2:
cities={"Tokyo","Madrid","Berlin","Delhi"}
cities2={"Seoul","Kabul"}
print(cities2.issubset(cities))
cities3={"Seoul","Madrid","Kabul"}
print(cities3.issubset(cities))
Output:
FalseFalse
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