Lists in Python
Lists
Another data type in Python is ‘List’. A list is a collection of items separated by commas and enclosed within square brackets [ ]. The values stored in a list can be accessed using the slice operator ([ ] and [:]) with index position starting at 0 from the beginning of the list and in case of reverse accessing of list then the index position is -1. The plus (+) sign is the list concatenation operator, and the asterisk (*) is the repetition operator.
Creating lists
We can create a list by inserting some items into a square braces, separated by commas.
Ex
>>> list=[ ] #Creating an empty list
>>> list [ ] >>> list=[“Hi”,5,1.3] #Appending elements into list >>> list [‘Hi’, 5, 1.3] |
Accessing elements in list
We can access the elements in a list by the following way.
>>> x=[“bcd”,8,2003,43.2]
>>> y=[“1″,2,”three”] >>> x [‘bcd’, 8, 2003, 43.2] >>> y [‘1’, 2, ‘three’] >>> x[2] 2003 >>> x[1:3] [8, 2003] >>> y[0] ‘1’ >>> y[0:1] [‘1’] |
Updating lists
We can update the existing list elements by mentioning the index position of that element and we can replace that element by reassigning a new element to that list. It is shown in below example.
>>> L=[“C++”,”Java”,”Python”]
>>> L [‘C++’, ‘Java’, ‘Python’] >>> L[0]=[“C Programming”] #Updating C++ with C Programming >>> L [[‘C Programming’], ‘Java’, ‘Python’] >>> L[2]=[“R Programming”] #Updating Python with R Programming >>> L [[‘C Programming’], ‘Java’, [‘R Programming’]] |
Deleting List elements
If you want to delete an element in a list we have to use del statement. It is shown in below:
>>> a=[“a”,”e”,”i”,”o”,”u”]
>>> del a[3] >>> a [‘a’, ‘e’, ‘i’, ‘u’] >>> del a[0:1] >>> a [‘e’, ‘i’, ‘u’]
>>> m=[[ ]]*4 >>> m [[ ], [ ], [ ], [ ]] >>> m[2].append(4) >>> m [[4], [4], [4], [4]] >>> del m[1] >>> m [[4], [4], [4]] |
Basic List operations
Some of the basic operations on list are shown in below:
>>> a=[“apple”,13,86] #Length of a list
>>> len(a) 3 >>> [1,2,3]+[“a”,”b”,”c”] #Concatenation of two lists [1, 2, 3, ‘a’, ‘b’, ‘c’] >>> [“Hey!”,”VMS”]*3 #Repetition of elements in a list [‘Hey!’, ‘VMS’, ‘Hey!’, ‘VMS’, ‘Hey!’, ‘VMS’] >>> for x in[1,2,3,4,5]: #Iteration print(x,end=” “)
1 2 3 4 5 >>> list=[“12″,”India”,3,40,12.6] #Indexing >>> list [’12’, ‘India’, 3, 40, 12.6] >>> list[-2] 40 >>> list[0] ’12’ >>> list[3] 40 >>> list=[“12″,”India”,3,40,12.6] #Slicing >>> list[1:-2] [‘India’, 3] >>> list[:1] [’12’] >>> list[-1:] [12.6] |
Built-in functions
Python includes the following built-in list functions.
1.List len( )
It returns the number of elements in the list.
Ex
>>> x=[“India”,”China”,”Nepal”,”Japan”,”London”]
>>> x [‘India’, ‘China’, ‘Nepal’, ‘Japan’, ‘London’] >>> len(x) 5 |
2.List max( ) and min( )
This returns the elements with maximum and minimum value from the list.
Ex
>>> x=[“India”,”China”,”Nepal”,”Japan”,”London”]
>>> max(x) ‘Nepal’ >>> y=[256,265,345,435,12.5] >>> max(y) 435 >>> min(x) ‘China’ >>> min(y) 12.5 >>> |
3.list( )
The list() method takes sequence types and converts them to lists. This is used to convert a given tuple into list.
Note: Tuple are very similar to lists. The only difference is that element values of a tuple cannot be changed and tuple elements are put in between parentheses instead of square bracket. This function also converts characters in a string into a list.
Ex:
>>> tuple=(123,”C++”,”Java”,”Python”)
>>> tuple (123, ‘C++’, ‘Java’, ‘Python’) >>> list(tuple) [123, ‘C++’, ‘Java’, ‘Python’] >>> str=”Hello World” >>> str ‘Hello World’ >>> list1=list(str) >>> list1 [‘H’, ‘e’, ‘l’, ‘l’, ‘o’, ‘ ‘, ‘W’, ‘o’, ‘r’, ‘l’, ‘d’] >>> |
List methods
The following are the methods of list.
1.append( )
The append( ) method appends an element into the existing list.
Ex
>>> a=[“Python”,3,15.6,2018]
>>> a [‘Python’, 3, 15.6, 2018] >>> a.append(‘Programming’) >>> a [‘Python’, 3, 15.6, 2018, ‘Programming’] |
2.count( )
This method returns the count of how many times an element occurs in the list.
Ex
>>> list=[1,2,3,4,1,2,3,5,2]
>>> list.count(2) 3 >>> list.count(5) 1 >>> list.count(7) 0 |
3.extend( )
This method appends the contents of one list to another list.
Ex
>>> m=[‘a’,’b’,’c’,’d’]
>>> n=[‘e’,’f’,’g’,’h’] >>> m.extend(n) >>> m [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’, ‘h’] |
4.index( )
This method returns the index position of an element in list.
Ex
>>> List
[‘A’, ‘Step’, ‘To’, ‘Learn’] >>> List.index(‘To’) 2 >>> List.index(‘Step’) 1 |
5.insert( ) and remove( )
This method inserts and removes an element into an existing list.
Ex
>>> k=[‘A’,’B’,’D’,’F’,’G’]
>>> k [‘A’, ‘B’, ‘D’, ‘F’, ‘G’] >>> k.insert(2,”C”) #Inserting an element >>> k [‘A’, ‘B’, ‘C’, ‘D’, ‘F’, ‘G’] >>> k.insert(4,”E”) >>> k [‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’, ‘G’] >>> k.remove(‘D’) #Removing an element >>> k [‘A’, ‘B’, ‘F’, ‘G’] >>> k.remove(“F”) |
6.reverse( )
This reverses the position of elements in a list.
Ex
>>> b=[‘1′,’2′,’3′,’4′,’5’]
>>> b [‘1’, ‘2’, ‘3’, ‘4’, ‘5’] >>> b.reverse( ) >>> b [‘5’, ‘4’, ‘3’, ‘2’, ‘1’] |
7.sort( )
This method sorts the elements of list.
Ex
>>> d=[“C”,”Java”,”.net”,”Python”]
>>> d [‘C’, ‘Java’, ‘.net’, ‘Python’] >>> d.sort( ) >>> d [‘.net’, ‘C’, ‘Java’, ‘Python’] |
Example programs on lists
1.A python program on lists to print the squares of numbers.
Output
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400] |
2.A python program on lists to store the list of different items in list and to display whether the list if full or not.
Output
list is full |