SEQUENCES
Python List
python offers a range of compound
datatypes often referred to as sequences. list is one of the most frequenty
used and very versatile datatypes used in python.
J List is Mutable
J Mutable means we can
change and reassign value of variable index
J List is like Array
J but Array store only
similar datatype
J but list stores
different datatype
J list index starts
with 0
how to create a list?
- In python programming , a
list is create by placing all the item (elements) inside
a square bracket [ ],separated by commas.
- It can have any
number of item and they may be different types (integer, float,
string etc.. ).
#empty list
list1= []
# list of integers
list1 = [20 , 12 , 43]
# list with mixed datatypes
List1 = [2 , 4 , "hai" , 15.2
, 2+3j]
Also , a list can even have another
list as an item . this is called nested list
# nested list
list1 = ["hai" , [4,3,96],['a','e','i','o','u']]
LIST INDEX
list1 = ['a','e','i','o','u']
print(List1 [0])
#output: a
print(list1[2])
#output:i
print(List1[4])
#output: u
#List1[4.0]
#Error! only integer can be used for
indexing
#nested list
n_list = ["happy",[2,0,1,5]]
#Nested indexing
#output: a
print(n_list[0][1])
#output : 5
print(n_list[1][3])
Negative indexing
The index of -1 refers to thelast
item, -2 to the second last item and so on.
list=['a','e','i','o','u']
print(list[-1])
print(list[-5])
OUTPUT
u
a
slice lists
list1=['p','y','t','h','o','n']
#element 3rd to 4th
print(list1[2:4])
Output: ['t', 'h']
#elements begining to 4th
print(list1[:-5])
Output: ['p']
#elements 5th to end
print(list1[4:])
Output: ['o', 'n']
#elements begining to end
print(list1[:])
Output: ['p', 'y', 't',
'h', 'o', 'n']
HOW TO CHANGE OR ADD
ELEMENTS TO A LIST ?
num = [2,4,6,8]
#change the 1st item
num[0]=1
print(num)
#output: [1,4,6,8]
#change 2nd to 4th items
num[1:4] = [3,5,7]
print(num)
#Output: [1,3,5,7]
Append() and extend() Method
oddnum=[1,3,5]
oddnum append(7)
print(oddnum)
oddnum.extend([9,11,13])
print(oddnum)
output
[1,3,5,7]
[1,3,5,7,9,11,13]
List Concatenation
oddnum = [1,3,5]
print(oddnum + [9,7,5])
print(["yes"]*3)
output
[1,3,5,9,7,5]
['yes','yes','yes']
Insert() Method
oddnum= [1,9]
oddnum.insert(1,3)
print(oddnum)
OUTPUT
[1,3,9]
oddnum[2:2]= [5,7]
print(oddnum)
OUTPUT
[1,3,5,7,9]
HOW TO DELETE OR REMOVE ELEMENTS FROM
A LIST :
we can delete one or more items using
del keyword.
list1=['p','r','o','b','l','e','m']
del list1[2]
print(list1)
Output :
['p','r','b','l','e','m']
del list1[1:5]
print(list1)
Output: ['p','m']
print(list1)
output
['p','r','b','l','e','m']
['p','m']
traceback (most
recent call last):
file"c:/python33/ss.py",line
17 , in<module>
print(list 1)
list1 = ['p','r','o','b','l','e','m']
list1.remove('p')
print(list1)
print(list1.pop(1))
print(list1)
print(list1.pop())
print(list1)
list1.clear()
print(list1)
output
['r','o','b','l','e','m']
o
['r','b','l','e','m']
m
['r','b','l','e']
[]
>>>list1 =
['p','r','o','b','l','e','m']
>>>list1[2:3] =[]
>>>list1
['p','r','b','l','e','m']
>>>list1[2:5]=[]
>>>list1
['p','r','m']
Python list methods
|
append() |
Add an element to the end of
the list |
|
extend() |
Add all elements of a list to
the another list |
|
insert() |
Insert an item at the defined
index |
|
remove() |
Removes an item from the list |
|
pop() |
Removes and returns an element
at the given index |
|
clear() |
Removes all items from the list |
|
index() |
Returns the index of the first
matched item |
|
count() |
Returns the count of number of
items passed |
|
sort() |
Sort items in a list in
ascending order |
|
reverse() |
Reverse the order of items in
the list |
|
copy() |
Returns a shallow copy of the
list |
list1=[34,56,12,56,78,0,6,7,12]
print(list1.index(12))
print(list1.count(12))
list1.sort()
print(list1)
list1.reverse()
print(list1)
OUTPUT
2
2
[0,6,7,12,12,34,56,56,78]
[78,56,56,34,12,12,7,6,0]
List comprehension:Elegant way to
create new list
list comprehension is an elegant and
concise way to create new list from an existing list in python
list comprehension consists of an
expression followed by for statement inside square brakets.
>>>odd=[x for x in range(20)
if x % 2 ==1]
>>>odd
[1,3,5,7,9,11,13,15,17,19]
Other list operations in Python
List Membership Test
we can test if an item exists in a list
or not,using the keyword in.
list1=['p','y','t,'h','o','n']
print('p' in
list1) #output:true
print('a' in
list1) #output:false
print('c' not in list1)
#output:true
Iterating through a List
using a for loop we can iterate though
each item in a list.
for fruit in['apple','banana','mango']:
print("i
like",fruit)
OUTPUT
i like apple
i like banana
i like mango
Built-in functions with list
|
function |
Description |
|
all() |
Return true if all elements of
the list are true (or if the list is empty) |
|
any() |
Return true if any
elements of the list is true.if the list is empty,return false |
|
enumerate() |
Return an enumerate object.it
contains the index and value of all the items of list as a tuple |
|
len() |
Return the length(the number of
items)in the list |
|
list() |
Convert an iterable
(tuple,string,set,dictionary)to a list |
|
max() |
Return the largest item in the
list |
|
min() |
Return the smallest item in the
list |
|
sorted() |
Return a new sorted list(does
not sort the list itself) |
|
sum() |
Return the sum of all elements
in the list |
#empty list 'a'
>>>a=[]
#all() function return true if all
elements are true or list is empty
>>>all(a)
True
>>>a.append(3)
>>>a.appened(4)
>>>a
[3,4]
>>>all(a)
True
>>>a.append(0)
#return false because one zero
element added
>>>all(a)
false
#any() return true if any element of
a list is true
#if list is empty return false
>>>a
[3,4,0]
>>>any(a)
True
>>>b=[]
>>any(b)
False
#example len() to count no of element
in the list
>>>len(a)
3
>>>max(a)
4
>>>min(a)
0
#sorted return a new sorted list(does
not sort the list itself)
>>>sorted (a)
[0,3,4]
#list remain unchanged
>>>d.sorted(a)
>>>d
[0,3,4]
>>>sum(a)
7
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