Python List Tutorial
There are a number of collection types in Python. While types such as int and str hold a single value, collection types hold multiple values
list data type:
Different Ways to Creating a list in Python
The list() function creates a list from an iterable object.
An iterable may be either a sequence, a container that supports iteration.
#Create an list by passing string as a sequence
Output:
Output:
Output:
zip returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument2. sequences or iterables:
The in Operator
cities=['agra','delhi','jaipur','kanpur','lucknow']
Selecting a
sublist from a list.
Python List Tutorial
There are a number of collection types in Python. While types such as int and str hold a single value, collection types hold multiple values
list data type:
§ If
we want to represent a group of values as a single entity where insertion order
required to preserve and duplicates are allowed then we should go for list data
type.
1. insertion
order is preserved
2. heterogeneous
objects are allowed
3. duplicates
are allowed
4. Growable
in nature
5. values
should be enclosed within square brackets.
§ A
list can be created by enclosing values, separated by commas, in square
brackets.
int_list = [1, 2, 3]
string_list = ['abc', 'defghi']
§ A list can be empty:
empty_list = []
§ The
elements of a list are not restricted to a single data type, which makes sense
given that Python is a dynamic language.
mixed_list = [1, 'abc', True, 2.34, None]
§ A
list can contain another list as its element:
nested_list = [['a', 'b', 'c'], [1, 2, 3]]
§ Lists
are mutable, so you can change the values in a list.
names = ['Allen', 'Scott', 'James', 'Mack', 'Eric']
print(names)# the output will be
['Allen', 'Scott', 'James', 'Mack', 'Eric']
names[0]='Adam'
print(names) # the output will be ['Adam', 'Scott',
'James', 'Mack', 'Eric']
§ list
is growable in nature. i.e. based on our requirement we can increase or
decrease the size.
players=['Virat','Rohit','Sachin']
print(players)
# the output will be ['Virat', 'Rohit', 'Sachin']
players.append('Shikhar')
print(players) #
the output will be ['Virat', 'Rohit', 'Sachin', 'Shikhar']
players.remove('Virat')
print(players) # the
output will be [ 'Rohit', 'Sachin', 'Shikhar']
Different Ways to Creating a list in Python
§ We can
create empty list object as follows.
list=[]
print(list)
print(type(list))
# The Output Will be
[]
<class 'list'>
list=[10,20,30,40,50]
print(list)
print(type(list))
# The Output Will be
[10, 20, 30, 40, 50]
<class 'list'>
§ We can
create list object with dynamic some element as follows.
list=eval(input("Enter
List:"))
print(list)
print(type(list))
# The Output Will be
Enter List:[10,20,30,40,50]
[10, 20, 30, 40, 50]
<class 'list'>
The list() function creates a list from an iterable object.
An iterable may be either a sequence, a container that supports iteration.
l=list()
print(l)#Create an list by passing string as a sequence
l=list('hello')
print(l)
Output:print(l)
['h',
'e', 'l', 'l', 'o']
§ Initializing
a List to a Fixed Number of Elements
my_list1 = [None] * 5
my_list2 = ['test'] * 5
my_list3=[0]*5
print('my_list1=',my_list1)
print('my_list2=',my_list2)
print('my_list3=',my_list3)
Output:
my_list1= [None, None, None, None, None]
my_list2= ['test', 'test', 'test', 'test', 'test']
my_list3= [0, 0, 0, 0, 0]
§ The
elements of a list can be accessed via an index, or numeric representation of
their position. Lists in Python are zero-indexed meaning that the first element
in the list is at index 0, the second element is at index 1 and so on:
names = ['Alice', 'Bob', 'Craig', 'Diana', 'Eric']
print(names[0]) # Alice
print(names[2]) # Craig
§ Indices
can also be negative which means counting from the end of the list (-1 being
the index of the last element).
names = ['Allen', 'Scott', 'James', 'Mack', 'Eric']
print(names[-1]) # Eric
print(names[-4]) # Bob
Iterating over a list
§ You
can iterate over the list elements using for loop like below:
for element in list:
do operation
my_list=[1,2,3,4,5,6]
for element in my_list:
print (element)
§ You
can also get the position of each item at the same time Using enumerate
function:
my_list=[1,2,3,4,5,6]
for (index, item) in enumerate(my_list):
print('The
item in position {} is: {}'.format(index, item))
Output:
The
item in position 0 is: 1
The
item in position 1 is: 2
The
item in position 2 is: 3
The
item in position 3 is: 4
The
item in position 4 is: 5
The
item in position 5 is: 6
my_list=[1,2,3,4,5,6]
for i in
range(0,len(my_list)):
print(i,'->',my_list[i])
Output:
0
-> 1
1
-> 2
2
-> 3
3
-> 4
4
-> 5
5
-> 6
The + Operator
§ The
+ operator is Used to concatenating to List.
§ The
simplest way to concatenate list1 and list2
merged
= list1 + list2
list1=[1,2,3]
list2=[4,5,6]
merge_list=list1+list2
print('List1:- ',list1)
print('List2:- ',list2)
print('MergeList:- ',merge_list)
Output:
List1:- [1, 2, 3]
List2:- [4, 5, 6]
MergeList:- [1, 2, 3, 4, 5,
6]
v If
We want to merge List in such way as given below.
zip returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument2. sequences or iterables:
list1 = ['a1', 'a2', 'a3']
list2 = ['b1', 'b2', 'b3']
for a, b in
zip(list1, list2):
print(a, b)
§ If the lists have
different lengths then the result will include only as many elements as the
shortest one.
list1 = ['a1', 'a2', 'a3']
list2 = ['b1', 'b2', 'b3','b4']
for a, b in zip(list1, list2):
print(a, b)
§ The
* operator creates multiple copies of a string.
§ Replication
– multiplying an existing list by an integer will produce a larger list
consisting of that many copies11. of the original.
my_list1 = [1,2,3] * 3
my_list2 = ['a','b','c'] * 3
print('my_list1=',my_list1)
print('my_list2=',my_list2)
Output:
my_list1= [1, 2, 3, 1, 2, 3, 1, 2, 3]
my_list2= ['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c']
The in Operator
Checking Membership(List Contains):
§ By using in operator, we can check whether an item is in a list or not.
§ By using in operator, we can check whether an item is in a list or not.
cities=['agra','delhi','jaipur','kanpur','lucknow']
print('agra' in cities)#The Output will be True
print('london' in cities)#The Output will be False
Slicing of a List
(selecting parts of lists)
§ Slicing
of a List means selecting a part of a list.
s[beginindex:endindex:step]
§ from begin index to end-1 index and every
time update by step
§ default value for begin index is zero.
§ default value for end index is length of
string.
s[begin:end:step]
§ step value can be either +ve or -ve.
§ if +ve then it should be forward direction
(Left to Right).
§ If -ve then it should be backward
direction (Right to Left).
§ If +ve forward direction from begin to
end-1.
§ if -ve backward direction from begin to
end+1.
list1=[10,20,30,40,50,60,70]
print(list1[1:4])#The OutPut [20, 30, 40]
print(list1[3:7]);#The OutPut [40, 50, 60, 70]
v If you omit the first index, the slice starts at the
beginning of the list. Thus, s[:m] and s[0:m] are equivalent:
s[:endindex]
list1=[10,20,30,40,50,60,70]
print(list1[:2])#The OutPut [10, 20]
print(list1[:3]);#The OutPut [10, 20, 30]
v if you omit the second index as in s[n:], the slice
extends from the first index through the end of the list. This is a nice,
concise alternative to the more cumbersome s[n:len(s)]:
s[beginindex:]
list1=[10,20,30,40,50,60,70]
print(list1[5:])#The OutPut [60, 70]
print(list1[6:]);#The OutPut [70]
v If
you Omitting both indices return the original list.
list1[:]
list1=[10,20,30,40,50,60,70]
print(list1[:])#The OutPut[10, 20, 30, 40, 50, 60, 70]
v If
the first index in a slice is greater than or equal to the second index, Python
returns an empty list.
list1=[10,20,30,40,50,60,70]
print(list1[2:2])#The
OutPut []
print(list1[4:2]);#The
OutPut []
v
Reversing a list with
slicing
a = [10, 20, 30, 40, 50]
# steps through the list backwards (step=-1)
b = a[::-1]
print('Original List=',a)
print('Reverse List= ',b)
Python List Tutorial
