ListComprehension.md

List comprehension

Introduction

• List comprehension Help us to reduce the number of line code.
• List comprehension is generally more compact and faster than normal functions and loops for creating a list.
• List comprehensions are used for creating new lists from other iterables.
• List comprehension is an elegant way to define and create lists based on existing lists.

Syntax:

Newlist=[expression(item) for item in list]

Example:

Create a list which contains 1 to 10 number

Using normal for loop

#initilize empty list
lst=[]
for i in range(1, 11) :
      lst.append(i) 
print(lst) 

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Now using list comprehension

#using List comprehension
lst=[i for i in range(1,11)]
print(lst) 

output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

list comprehension using conditions

Syntax:

Newlist=[expression(item) for item in list conditions]

Example: Create a list which contains 2,4,6,8,10

Normal for loop

#blank list
lst=[]
for i  in range(11) :
    if i%2==0:
        lst.append(i) 
print(lst) 

Output:

[0, 2, 4, 6, 8, 10]

Using List comprehension

#using List comprehension
lst=[i for i in range(11)  if i%2==0]
print(lst) 

Output:

[0, 2, 4, 6, 8, 10]

You can achieve above sequence using range(2,11,2) also. But we need to understand conditional statement inside list comprehension so we use normal range function.

If else 💣 in list comprehension.

Example: Find out an even or odd number from 1 to 10

Using normal for loop

lst=[]
for i in range(1,11):
  if i%2==0:
    lst.append("Even")
  else:
    lst.append("odd") 
print(lst) 

Output:

['odd', 'Even', 'odd', 'Even', 'odd', 'Even', 'odd', 'Even', 'odd', 'Even']

Using list comprehension

lst=["Even" if i%2==0 else "odd" for i in range(1,11)]
print(lst) 

Output:

['odd', 'Even', 'odd', 'Even', 'odd', 'Even', 'odd', 'Even', 'odd', 'Even']

Error In List Comprehension

expression in a list comprehension can only accept single variable of any type(string, list, int, tuple) the object we can not add element more than one

Example

lst=[i,i*2 for i in range(5)]

Output:

lst=[i,i*2 for i in range(5)]
                 ^
SyntaxError: invalid syntax

This will always generate error Invalid syntax Correct way

#using Tuple
lst=[(i,i*2) for i in range(5)]
print(lst)
#Result:[(0, 0), (1, 2), (2, 4), (3, 6), (4, 8)]

#Using List
lst=[[i,i*2] for i in range(5)]
print(lst)
#Result:[[0, 0], [1, 2], [2, 4], [3, 6], [4, 8]]

Output:

[(0, 0), (1, 2), (2, 4), (3, 6), (4, 8)]
[[0, 0], [1, 2], [2, 4], [3, 6], [4, 8]]

Note every list comprehension can be rewritten in for loop, but every for loop can’t be rewritten in the form of list comprehension.