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Python comprehension gives us shorter syntax to create a list , set and dictionary from an iterator .
List Comprehension in Python
List comprehension in Python gives us shorter syntax to create a list from an iterator .
Syntax of List Comprehension in Python
new_list =[expression for item in iterable [if conditional]]
OR
new_list =[expression [if conditional] for item in iterable ]
Why to use List Comprehension in Python?
Suppose we have a list of some numbers. Now we need to create a new list with numbers divisible by 3 from the given list. The general program for this is –
list_of_no=[2,7,4,9,11,6]
new_list=[]
for number in list_of_no:
if(number%3==0):
new_list.append(number)
print(new_list)[9, 6]
In the above code first an empty list has been created . Then each item in the old list is checked to see if it is divisible by 3. Those that are divisible by 3 are added to the new list.
We can solve this problem using list comprehension also –
list_of_no=[2,7,4,9,11,6]
#using list comprehension
new_list=[number for number in list_of_no if number%3==0]
print(new_list)[9, 6]
- In the above code the previous problem has been solved with the help of
listcomprehension. - Here list comprehension has three parts, they are –
number(1),for number in list_of_no(2),if number% 3 == 0(3) . - By
for number in list_of_no(2) each number in the oldlisthas been iterated, the iterated numbers have been checked byif number% 3 == 0(3). Numbers that returnTruehave been added to the newlistasnumber(1).
Now we will see how list comprehension solve the problem with the help of a table –
number | for number in list_of_no | if number%3==0 |
|---|---|---|
| 2 | 2%3==0(false) | |
| 7 | 7%3==0(false) | |
| 4 | 4%3==0(false) | |
| 9 | 9 | 9%3==0(true) |
| 11 | 11%3==0(false) | |
| 6 | 6 | 6%3==0(true) |
So we can easily see that program can be written in very few lines using list comprehension. This is why list comprehension is used.
List Comprehension in Python with if-else
Suppose, from the aforesaid list, we want to make a list so that the number divisible by 2 is replaced by even and the number not divisible by 2 is replaced by odd, then the program will be.-
list_of_no=[2,7,4,9,11,6]
new_list=["even" if number%2==0 else "odd" for number in list_of_no]
print(new_list)['even', 'odd', 'even', 'odd', 'odd', 'even']
In the above code each of the numbers in the given list is checked to see if it is divisible by 2. If it is divisible by 2 then it is replaced by even else it is replaced by odd. The program is presented in the form of a table for ease of understanding –
“even” if number%2==0 else “odd” | for number in list_of_no | if number%2==0 |
|---|---|---|
| “even” | 2 | 2%2==0(true) |
| “odd” | 7 | 7%2==0(false) |
| “even” | 4 | 4%2==0(true) |
| “odd” | 9 | 9%2==0(false) |
| “odd” | 11 | 11%2==0(false) |
| “even” | 6 | 6%2==0(true) |
Replace map function with List Comprehension in Python to take multiple integer input in a single line
We can easily solve the problem of receiving multiple integer inputs on the same line through map() function –
To learn about map() function click here
print(list(map(int,input().split())))2 5 4 8 [2, 5, 4, 8]
In the above code if we enter “2 5 4 8” as input and it will print [2, 5, 4, 8].
Now we will solve this problem through list comprehension –
print([int(number) for number in input().split()])2 5 4 8 [2, 5, 4, 8]
In the above code if we enter "2 5 4 8" as input . input().Split() will convert it to ["2", "5", "4", "8"]. The rest is shown through a table for ease of understanding –
int(number) | for number in input().split() |
|---|---|
| 2 | “2” |
| 5 | “5” |
| 4 | “4” |
| 8 | “8” |
Replace filter function with List Comprehension in Python
Suppose we have a list of names. Now we need to create a new list so that there will be names from old list that are 6 or less in length. We can easily solve this problem with the help of filter() function –
To learn about filter() function click here
list_of_name=["sourav","sudipta","partha","joy","gurudas","arup"]
print(list(filter(lambda x:len(x)<=6,list_of_name)))['sourav', 'partha', 'joy', 'arup']
In the above code, filter() function will check each name of list_of_name whether its length is 6 or less, if it is 6 or less then filter() function will add the name to the new list .
Now we will solve this problem through list comprehension –
list_of_name=["sourav","sudipta","partha","joy","gurudas","arup"]
print([name for name in list_of_name if len(name)<=6])['sourav', 'partha', 'joy', 'arup']
In the above code, each name of list_of_name is checked whether its length is 6 or less, if the length is 6 or less then it will be in the new list else it will be omitted.
Set Comprehension in Python
Set comprehension in Python gives us shorter syntax to create a set from an iterator .
Syntax of Set Comprehension in Python
new_set ={expression for item in iterable [if conditional]}
OR
new_set ={expression [if conditional] for item in iterable }
Set Comprehension in Python Examples
Example 1: Set comprehension in Python with if condition
list_of_name=["sourav","sudipta","partha","joy","gurudas","arup"]
#using set comprehension
print({name for name in list_of_name if len(name)<=6}){'arup', 'sourav', 'partha', 'joy'}
The above code is the solution to the previous problem which we have solved through set comprehension with if condition.
Example 2: Set comprehension in Python with if else
list_of_name=["sourav","sudipta","partha","joy","gurudas","arup"]
# using set comprehension
# if length<=6 then yes otherwise no
print({"yes" if len(name)<=6 else "no" for name in list_of_name}){'yes', 'no'}
In the above code we have used set comprehension with if else . As set does not allow duplicate values so only 1 “yes” and 1 “no” is seen in the output .
Dictionary Comprehension in Python
Dictionary comprehension in Python gives us shorter syntax to create a dictionary from an iterator .
Syntax of Dictionary Comprehension in Python
new_dictionary ={key:value for item in iterable [if conditional]}
OR
new_dictionary ={key:(value1 [if conditional] value2) for item in iterable }
Dictionary Comprehension in Python Examples
Example 1: Dictionary comprehension in Python with if condition
list_of_name=["sourav","sudipta","partha","joy","gurudas","arup"]
# using dict comprehension with if condition
print({name:len(name) for name in list_of_name if len(name)<=6}){'sourav': 6, 'partha': 6, 'joy': 3, 'arup': 4}
We have created a dictionary by dictionary comprehension with if condition taking name as “key” and len(name) as “value” –
Example 2 : Dictionary comprehension in Python with if else
list_of_name=["sourav","sudipta","partha","joy","gurudas","arup"]
# using dict comprehension with if else
print({name:("yes" if len(name)<=6 else "no") for name in list_of_name }){'sourav': 'yes', 'sudipta': 'no', 'partha': 'yes', 'joy': 'yes', 'gurudas': 'no', 'arup': 'yes'}
In the above code we have used dictionary comprehension in python with if else .
Nested Python Comprehension
With nested comprehension python we can easily create list of list, dictionary of list, list of dictionary, list of set etc.
Suppose a school has five classes and each class has ten students, now the correct data structure for storing the results of all of them is a dictionary contains a list –
Nested Comprehension Python Examples
list_of_class=["class-v","class-vi","class-vii","class-viii","class-ix"]
#dictionary comprehension nested by list comprehension
print({class_name:["roll"+str(number) for number in range(1,11)] for class_name in list_of_class}){'class-v': ['roll1', 'roll2', 'roll3', 'roll4', 'roll5', 'roll6', 'roll7', 'roll8', 'roll9', 'roll10'], 'class-vi': ['roll1', 'roll2', 'roll3', 'roll4', 'roll5', 'roll6', 'roll7', 'roll8', 'roll9', 'roll10'], 'class-vii': ['roll1', 'roll2', 'roll3', 'roll4', 'roll5', 'roll6', 'roll7', 'roll8', 'roll9', 'roll10'], 'class-viii': ['roll1', 'roll2', 'roll3', 'roll4', 'roll5', 'roll6', 'roll7', 'roll8', 'roll9', 'roll10'], 'class-ix': ['roll1', 'roll2', 'roll3', 'roll4', 'roll5', 'roll6', 'roll7', 'roll8', 'roll9', 'roll10']}
In the above code, a dictionary has been created by taking the name of each class as key and the list of roll number of ten students of each class as value.
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