List Datatype in Python
Python supports many simple and compound datatypes.
Few data types like list, dictionary are mutable and others like string datatype, tuple are immutable.
Mutable datatypes means changes can be done and are reflected in same variable.
Immutable datatypes means changes can be done but not in same variable.
List is a container for holding items or elements of same or different datatypes.
An empty list represents zero character or we can call it as null list.
A list datatype can be defined with use of opening and closing square brackets [ ].
The elements present in the list are separated by comma.
Initializing a List
We can initialize a list as empty list using [] or with values.
This values can be of different datatypes.
You can also initialize two or more lists in single line.
Example
>>> itv=[] #empty list
>>> itv1=[1,2.3,'python']
>>> itv
[]
>>> itv1
[1, 2.3, 'python']
>>> itv2=[1,2,3,4,5,6]
>>> itv2
[1, 2, 3, 4, 5, 6]
>>> itv3=['i','t','v','o','y','a','g','e','r','s']
>>> itv3
['i', 't', 'v', 'o', 'y', 'a', 'g', 'e', 'r', 's']
>>> F,O,G=[1,2],['h','i'],[1,5.3,'p']
>>> F
[1, 2]
>>> O
['h', 'i']
>>> G
[1, 5.3, 'p']
Accessing a list
A list can be accessed using square brackets [].
Example
>>> itv=['i','t','v','o','y','a','g','e','r','s']
>>> itv[5]
'a'
>>> itv[9]
's'
>>> print("First five elements", itv[0:6])
First five elements ['i', 't', 'v', 'o', 'y', 'a']
>>> print("last five elements in reverse", itv[10:4:-1])
last five elements in reverse ['s', 'r', 'e', 'g', 'a']
>>>
>>> itv[11]
Traceback (most recent call last):
File "<pyshell#12>", line 1, in <module>
itv[11]
IndexError: list index out of range
Above example consist of a list having 10 characters and index position starting from 0 till 9. To access a string we need to pass index value inside square brackets to retrieve character located on that index. As last index number is 9 we get and IndexError if we pass an index greater than 9. So first letter ‘i’ is at position 0, second letter ‘t’ is at position 1 and so on..
Example:
>>> itv=['i','t','v','o','y','a','g','e','r','s']
>>> itv[2.5]
Traceback (most recent call last):
File "<pyshell#16>", line 1, in <module>
itv[2.5]
TypeError: list indices must be integers, not float
From above example it is clear that we cannot use a float value as index. An index should always be an integer and last value of index will always be n-1 if total length is n.
len function or len()
Python provides a built-in function called as len() to calculate number of characters in a list or simply we can say length of a list.
Example:
>>> itv=[]
>>> len(itv)
0
>>> itv1=[1,2.3,'python']
>>> len(itv1)
3
>>> itv2=[1,2,3,4,5,6]
>>> len(itv2)
6
>>> itv3=['i','t','v','o','y','a','g','e','r','s']
>>> len(itv3)
10
If we wish to get last letter of the list we might script it as follows:
>>> itv=['i','t','v','o','y','a','g','e','r','s']
>>> h=len(itv)
>>> last=itv[h]
Traceback (most recent call last):
File "<pyshell#16>", line 1, in
last=itv[h]
IndexError: string index out of range
>>>
This raises an IndexError. So to over come this we can modify code using following script:
>>> itv=['i','t','v','o','y','a','g','e','r','s']
>>> h=len(itv)
>>> last=itv[h-1]
>>> last
's'
>>>
Negative Indexing
We can use negative indexing for list datatype which will help us to count backward from the end of the list.
If we use itv[-1] it will give output as ‘s’.
>>> itv=['i','t','v','o','y','a','g','e','r','s']
>>> itv[-1]
's'
>>> itv[-8]
'v'
>>> itv[-10]
'i'
>>> itv[-11]
Traceback (most recent call last):
File "<pyshell#37>", line 1, in <module>
itv[-11]
IndexError: list index out of range
In negative indices, index begins with -1 and not 0 so last value of indexing will be -10 in our example and not -9.
Traversing list using for loop
Traversing is a process of moving in an iterable and locating values from index position.
In this case our iterable is of type list and loop used is for loop.
>>> itv=['i','t','v','o','y','a','g','e','r','s']
>>> for i in itv:
print("Letter is", i * 2)
Output:
Letter is ii
Letter is tt
Letter is vv
Letter is oo
Letter is yy
Letter is aa
Letter is gg
Letter is ee
Letter is rr
Letter is ss List slicing
A list datatype can be sliced using [ : ] .
Syntax:
[start : end : step]
This provide start and end value.
Sometimes a step value is also mentioned after end value using colon separation.
In below example we will see multiple outputs or multiple ways to slice a list datatype.
Example:
#List slicing
demo_list = ['i','t','v','o','y','a','g','e','r','s','.','i','n']
print('demo_list[0:5]',demo_list[0:5])
print('demo_list[9:12]',demo_list[9:12])
print('demo_list[5:12]',demo_list[5:12])
print('demo_list[:3]',demo_list[:3])
print('demo_list[3:]',demo_list[3:])
print('demo_list[3:3]',demo_list[3:3])
print('demo_list[:]',demo_list[:])
print('demo_list[0]',demo_list[0])
print('demo_list[10]',demo_list[10])
print('demo_list[5]',demo_list[5])
print('demo_list[5:14:2]',demo_list[5:14:2])
print('demo_list[5::2]',demo_list[5::2])
print('demo_list[:14:2]',demo_list[:14:2])
print('demo_list[5:1:-2]',demo_list[5:1:-2])
print('demo_list[5:1:-1]',demo_list[5:1:-1])
print('demo_list[::]',demo_list[::])
Output:
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