CBSE Class 11: Python List 3

Negative indexing

Python allows negative indexing for its sequences. The index of -1 refers to the last item, -2 to the second last item and so on.

Example:

Positive indexing (from left side index starts from 0(zero))

 0 1 2 3 4 5 6 7 c o m p u t e r -8 -7 -6 -5 -4 -3 -2 -1

Negative indexing (from right side index starts from -1)

n1=[‘c’ , ‘o’ , ‘m’ , ‘p’ , ‘u’ , ‘t’ , ‘e’ , ‘r’]

print(n1)
print(n1[-1])
print(n1[-2])
print(n1[-4])

output:
[‘c’, ‘o’, ‘m’, ‘p’, ‘u’, ‘t’, ‘e’, ‘r’]
r
e
u

```n1=['c','o','m','p','u','t','e','r']
print(n1)
print(n1[-1])
print(n1[-2])
print(n1[-4])
print(n1[-5])
print(n1[-7])```

Output:

[‘c’, ‘o’, ‘m’, ‘p’, ‘u’, ‘t’, ‘e’, ‘r’]
r
e
u
p
o

Example:

n2=[‘c’ , ‘a’ , ‘t’ , ‘a’ , ‘l’ , ‘y’ , ‘s’ , ‘t’]

Positive indexing (from left side index starts from 0(zero))

 0 1 2 3 4 5 6 7 c a t a l y s t -8 -7 -6 -5 -4 -3 -2 -1

Negative indexing (from right side index starts from -1)

n2=[‘c’ , ‘a’ , ‘t’ , ‘a’ , ‘l’ , ‘y’ , ‘s’ , ‘t’]

print(n2)
print(n2[-0]) # -0 is 0
print(n2[-2])
print(n2[-5])
print(n2[-7])
print(n2[-1])
print(n2[1])
print(n2[4])
print(n2[6])

output:
[‘c’, ‘a’, ‘t’, ‘a’, ‘l’, ‘y’, ‘s’, ‘t’]
c
s
a
a
t

```n2=['c','a','t','a','l','y','s','t']
print(n2)
print(n2[-0]) # -0 is 0
print(n2[-2])
print(n2[-5])
print(n2[-7])
print(n2[-1])
print(n2[1])
print(n2[4])
print(n2[6])```

Output:

[‘c’, ‘a’, ‘t’, ‘a’, ‘l’, ‘y’, ‘s’, ‘t’]
c
s
a
a
t
a
l
s

How to change/modify  elements to a list?

List are mutable, meaning, their elements can be changed unlike string or tuple.

We can use assignment operator (=) to change an item or a range of items.

n=[1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10]

Positive index from left to right (starts from 0)

 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10

print(n)

n[0]=10

print(n)

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

[10, 2, 3, 4, 5, 6, 7, 8, 9, 10]

 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10

n=[1,2,3,4,5,6,7,8,9,10]

print(n)

n[2]=300

n[6]=200

n[8]=80

print(n)

output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

[1, 2, 300, 4, 5, 6, 200, 8, 80, 10]

```n=[1,2,3,4,5,6,7,8,9,10]
print(n)
n[2]=300
n[6]=200
n[8]=80
print(n)```

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 300, 4, 5, 6, 200, 8, 80, 10]

```n=[10,20,30,40,50]
print(n)
n[2]=3000
n[3]=1200
n[3]=180
print(n)```

Output:

[10, 20, 30, 40, 50]
[10, 20, 3000, 180, 50]

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