With comprehensions you can construct python sequences. In this article we will see how to create and access such sequences.
Below are the different types of comprehensions in python.
- List Comprehensions
- Dictionary Comprehensions
- Set Comprehensions
- Generator Comprehensions
List comprehension
There are various ways we can create a list and access the elements in it.
Using for loop
Example
# Cretae an empty list
listA = []
# Append elements to the list
for n in range(4, 9):
listA.append(n ** 3)
print("List using for loop:\n", listA)Output
Running the above code gives us the following result −
List using for loop: [64, 125, 216, 343, 512]
From another list
We can also create a list form another list by directly assigning elements to the new list.
Example
# Take a list
listA = [12,9,32,45]
#Given list
print("Given list:\n ",listA)
new_list = [n for n in listA if n % 3 == 0]
print("New List:\n", new_list)Output
Running the above code gives us the following result −
Given list: [12, 9, 32, 45] New List: [12, 9, 45]
Dictionary Comprehensions
A dictionary contains elements in form of pairs known as key-value pairs. In this article we will see how to create such dictionaries.
Using for loop and zip
We can take in two lists which can have keys and values. Then join them through comprehension to create a new dictionary.
Example
Day = ['Mon', 'Tue', 'Wed',]
Time= ['2pm','10am','11am']
# Create an empty dict
dictA = {}
# Use for loop
for (key, value) in zip(Day, Time):
dictA[key] = value
print("Dictionary using for loop:\n",dictA)Output
Running the above code gives us the following result −
Dictionary using for loop:
{'Mon': '2pm', 'Tue': '10am', 'Wed': '11am'}Using key as index
In this case we take a Python list and uses elements as key for the dictionary. Also derive the values for each key using an expression.
Example
listA = [3,5,4,8,9,2]
dictA = {}
# Using for loop
for key in listA:
if key % 2 != 0:
dictA[key] = key ** 3
print("Dictionary using for loop:\n",dictA)Output
Running the above code gives us the following result −
Dictionary using for loop:
{3: 27, 5: 125, 9: 729}Set Comprehension
Python Set contains unique elements. It can be created in a similar manner as list.
With for loop
Here we take a list and apply a condition to fetch some elements from it. Then put those elements into to an empty set by using add method.
Example
listA = [12, 4, 25, 12,4,9]
setA = set()
# Using for loop
for x in listA:
if x % 3 == 0:
setA.add(x)
print("Set using for loop:", setA)Output
Running the above code gives us the following result −
Set using for loop: {9, 12}With for and in
In this method we directly use a for loop within {}. And assign the result into a set. Inside the follow we use elements from a list.
Example
listA = [12, 4, 25, 12,4,9]
# Using for loop
setA = {x for x in listA if x % 2 == 0}
print("Set using for loop:", setA)Output
Running the above code gives us the following result −
Set using for loop: {12, 4}Generator Comprehensions
In this approach we take the elements from a list and apply certain conditions on them and then put them into a for loop. The result is assigned to sequence. This method is called generator comprehension.
Example
listA = [12, 4, 25, 12,4,9]
genrtr = (var for var in listA if var % 2 == 0)
print("Values using generator comprehensions:\n")
for x in genrtr:
print(x, end=',')Output
Running the above code gives us the following result −
Values using generator comprehensions: 12,4,12,4,