It is a collections of items and each item has its own index value.
Index of first item is 0 and the last item is n-1.Here n is number of items in a list.
Creating a list
Lists are enclosed in square brackets [ ] and each item is separated by a comma.
e.g.
list1 = [‘English', ‘Hindi', 1997, 2000]
list2 = [11, 22, 33, 44, 55 ]
list3 = ["a", "b", "c", "d"]
Access Items From A List
List items can be accessed using its index position.
e.g.
list =[3,5,9]
print(list[0])
print(list[1])
print(list[2])
print('Negative indexing')
print(list[-1])
print(list[-2])
print(list[-3])
Do your self: Find the output of the above example
Iterating Through A List
List elements can be accessed using looping statement.
e.g.
list =[3,5,9]
for i in range(0, len(list)): print(list[i])
Output
3
5
9
Slicing of A List
List elements can be accessed in subparts.
e.g.
list =['I','N','D','I','A']
print(list[0:3])
print(list[3:])
print(list[:])
Output
['I', 'N', 'D'] ['I', 'A']
['I', 'N', 'D', 'I', 'A']
Updating Lists
We can update single or multiple elements of lists by giving the slice on the left-hand side of the assignment
operator.
e.g.
list = ['English', 'Hindi', 1997, 2000]
print ("Value available at index 2 : ", list[2])
list[2:3] = 2001,2002 #list[2]=2001 for single item update print ("New value available at index 2 : ",
list[2])
print ("New value available at index 3 : ", list[3])
Output
('Value available at index 2 : ', 1997) ('New value available at index 2 : ', 2001) ('New value available at index 3 : ', 2002)
Add Item to A List
append() method is used to add an Item to a List.
e.g. list=[1,2]
print('list before append', list) list.append(3)
print('list after append', list)
Output
('list before append', [1, 2])
('list after append', [1, 2, 3])
NOTE :- extend() method can be used to add multiple item at a time in list.eg - list.extend([3,4])
Add Two Lists
e.g.
list = [1,2]
list2 = [3,4]
list3 = list + list2 print(list3)
OUTPUT [1,2,3,4]
Delete Item From A List
e.g.
list=[1,2,3]
print('list before delete', list) del list [1]
print('list after delete', list)
Output
('list before delete', [1, 2, 3])
('list after delete', [1, 3])
e.g.
del list[0:2] # delete first two items del list # delete entire list
Basic List Operations
|
Python Expression |
Results |
Description |
|
len([4, 2, 3]) |
3 |
Length |
|
[4, 2, 3] + [1, 5, 6] |
[4, 2, 3, 1, 5, 6] |
Concatenation |
|
[‘cs!'] * 4 |
['cs!', 'cs!', 'cs!', 'cs!'] |
Repetition |
|
3 in [4, 2, 3] |
True |
Membership |
|
for x in [4,2,3] : print (x,end = ' ') |
4 2 3 |
Iteration |
Important methods and functions of List
|
Function |
Description |
|
list.append() |
Add
an Item at end of a list |
|
list.extend() |
Add
multiple Items at end of a list |
|
list.insert() |
insert
an Item at a defined index |
|
list.remove() |
remove an
Item from a list |
|
del list[index] |
Delete
an Item from a list |
|
list.clear() |
empty all
the list |
|
list.pop() |
Remove
an Item at a defined index |
|
list.index() |
Return index
of first matched item |
|
list.sort() |
Sort
the items of a list in ascending or descending order |
|
list.reverse() |
Reverse the
items of a list |
|
len(list) |
Return
total length of the list. |
|
max(list) |
Return item
with maximum value in the list. |
|
min(list) |
Return
item with min value in the list. |
|
list(seq) |
Converts a
tuple, string, set, dictionary into list. |
Programs on List
* find the largest number in a list
#Using sort a=[]
n=int(input("Enter number of elements:")) for i in range(1,n+1):
b=int(input("Enter element:")) a.append(b)
a.sort()
print("Largest element is:",a[n-1])
#using function definition
def max_num_in_list( list ): max = list[ 0 ]
for a in list: if a >
max:
max = a return max
print(max_num_in_list([1, 2, -8, 0]))
Find the Maximum Value
list1, list2 = [123, 'xyz', 'zara', 'abc'], [456, 700, 200]
print "Max value element : ", max(list1)
print "Max value element : ", max(list2)
Output
Max value element : zara Max value element : 700
Programs on List
* find the mean of a list
def Average(lst):
return sum(lst) / len(lst)
# Driver Code
lst = [15, 9, 55, 41, 35, 20, 62, 49]
average = Average(lst)
# Printing average of the list
print("Average of the list =", round(average, 2))
Output
Average of the list = 35.75
Note : The inbuilt function mean() can be used to calculate the
mean(
average ) of the list.e.g. mean(list)
Programs on List
* Linear Search
list_of_elements = [4, 2, 8, 9, 3, 7]
x = int(input("Enter number to search: "))
found = False
for i in range(len(list_of_elements)):
if(list_of_elements[i] == x):
found = True
print("%d found at %dth position"%(x,i))
break
if(found == False):
print("%d is not in list"%x)
Programs on List
* Frequency of an element in list import collections
my_list = [101,101,101,101,201,201,201,201]
print("Original List : ",my_list)
ctr = collections.Counter(my_list) print("Frequency of the elements in the List
: ",ctr)
OUTPUT
Original List : [101,
101,101, 101, 201, 201, 201, 201]
Frequency of the elements in the List : Counter({101: 4, 201:4})
Note: Same can be done using count and other functions.
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