Elementary operations on lists in python

Daidalos November 20, 2019


Examples of elementary operations on lists in python

Create a list

Create an empty list

>>> l = []
>>> type(myfirstlist)
<type 'list'>

Create a list with 3 elements

>>> l = ['a','b','c']

Note: in a list the elements can have different typrs

>>> l = ['hello',1,2]

Get the number of elements in a list

To get the number of elements in a list a solution is to use len()

>>> l = ['Pierre', 'Paul', 'Mathieu']
>>> len(l)
3

here the list has 3 elements

Extract element from a list

To get an element stored in a list:

>>> l = ['Pierre', 'Paul', 'Mathieu']
>>> l[0]
'Pierre'
>>> l[1]
'Paul'
>>> l[2]
'Mathieu'

Create a loop

>>> l = ['Pierre', 'Paul', 'Mathieu']
>>> for i in l:
...     print( i )
... 
Pierre
Paul
Mathieu

Get the frequency of an element

To get a count of an element a solution is to use count():

>>> l = ['Pierre', 'Paul', 'Mathieu','Paul']
>>> l.count('Pierre')
1
>>> l.count('Paul')
2

here 'Paul' appeared two times in the list

Get element index

To get element index a solution is to use index()

>>> l = ['Pierre', 'Paul', 'Mathieu']
>>> l.index('Paul')
1

If the element does not exist it returns an error

>>> l.index('Ben')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: 'Ben' is not in list

Add an element in a list

To add an element in a list a solution is to use append()

>>> l = ['Pierre', 'Paul', 'Mathieu']
>>> l.append('toto')
>>> l
['Pierre', 'Paul', 'Mathieu', 'toto']

Note: append() add the new element at the end of the list.

To add an element in first position a solution is to do for example:

>>> l = ['Pierre', 'Paul', 'Mathieu']
>>> l = ['Ben'] + l
>>> l
['Ben', 'Pierre', 'Paul', 'Mathieu']

Another solution is to use insert() to specify the position in the list of a new element:

>>> l = ['Pierre', 'Paul', 'Mathieu']
>>> l.insert(1, 'toto')
>>> l 
['Pierre', 'toto', 'Paul', 'Mathieu']

Merge two lists

Two lists can be merged using extend():

>>> l1 = ['Pierre', 'Paul', 'Mathieu']
>>> l2 = ['Toto', 'George']
>>> l1.extend(l2)
>>> l1 
['Pierre', 'Paul', 'Mathieu', 'Toto', 'George']

Another solution is to use the + operator

>>> l1 = ['Pierre', 'Paul', 'Mathieu']
>>> l2 = ['Toto', 'George']
>>> l3 = l1 + l2
>>> l3 
['Pierre', 'Paul', 'Mathieu', 'Toto', 'George']

Remove an element

To remove an element a solution is to use remove():

>>> l = ['Pierre', 'Paul', 'Mathieu']
>>> l.remove('Paul')
>>> l 
['Pierre', 'Mathieu']
>>> l.remove('Ben')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: list.remove(x): x not in list

Another solution is to use pop()

>>> l = ['Pierre', 'Paul', 'Mathieu','Paul']
>>> l.pop(3)
'Paul'
>>> l 
['Pierre', 'Paul', 'Mathieu']

Slice a list

Example of how to slice a list

>>> l1 = ['Pierre', 'Paul', 'Mathieu','Paul']
>>> l2 = l1[0:2]
>>> l2
['Pierre', 'Paul']
>>> l3 = l1[1:3]
>>> l3
['Paul', 'Mathieu']

Sort a list

To sort a list a solution is to use sort()

>>> l = [4,3,2,1]
>>> l.sort()
>>> l
[1, 2, 3, 4]

Reverse a list

To reverse a list a solution is to use reverse()

>>> l1 = ['Pierre', 'Paul', 'Mathieu','Paul']
>>> l1.reverse()
>>> l1
['Paul', 'Mathieu', 'Paul', 'Pierre']

Check if an element exist

>>> l = ['Pierre', 'Paul', 'Mathieu','Paul']
>>> 'George' in l
False
>>> 'Pierre' in l
True

Check if an element is not in a list

>>> l = ['Pierre', 'Paul', 'Mathieu','Paul']
>>> 'George' not in l
True
>>> 'Pierre' not in l
False

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