Data Management {CLASS XI}

LIST (Class XI)

•It is an ordered set of values enclosed in square brackets [].

•Values in the list can be modified, i.e. it is mutable.

•As it is set of values, we can use index in square brackets [] to identify a value belonging to it.

•The values that make up a list are called its elements, and they can be of any type.

. Its syntax is:

Variable name [index] (variable name is name of the list).

Let's look at some example of simple list:

i) >>>L1 = [1, 2, 3, 4] # list of 4 integer elements.

ii) >>>L2 = [“Delhi”, “Chennai”, “Mumbai”] #list of 3 string elements.

iii) >>>L3 = [ ] # empty list i.e. list with no element

iv) >>>L4 = [“abc”, 10, 20] # list with different types of elements

v) >>>L5 = [1, 2, [6, 7, 8], 3] # A list containing another list known as

nested list

Creating and Accessing List

•To create a list, put a number of expressions in square brackets.

L=[]

L=[value1, value2………]

•The empty list

The empty list is []. It is equivalent to 0 or ‘ ‘.

For example

L=list()

It will generate an empty list and name that L

•Long list

If a list contains many elements, then to enter such long list, you can split it across several lines.

•Creating list from existing sequences

Syntax: L=list(<sequence>)

>>>l1=list(‘hello’)

>>>l1

[‘h’, ’e’, ’l’, ’l’, ’o’]

>>>a=“amit”

>>>l2=list(a)

•Creating list by input from keyboard during run time

For Example:

(1) To store data as String:

>>> l1=list(input("Enter elements"))

Enter elements12345

>>> l1

['1', '2', '3', '4', '5']

(2) To store data as integers:

>>> l2=eval(input("Enter elements"))

Enter elements12345

>>> l2

12345

>>>

a=[]

b=int(input("Enter total elements of list::"))

for i in range(b):

    c=int(input("Enter element::"))

    a.append(c)

print(a)

Accessing List

For accessing the elements of a list the bracket operator is used.

The expression inside the brackets specifies the index. The indices start at 0.

For example:

>>> l1=[1,2,3,4,5]

>>> l1[1]

2

>>> print(l1[1])

2

>>> l1[0]

1

>>> l1[4]

5

>>>

>>> l1=[1,2,3,4,5]

>>> l1[-1]

5

>>> l1[-2]

4

>>> l1[-5]

1

>>> l1[-6]

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

IndexError: list index out of range

>>>

Traversing List

•Traversing a list means, process or go through each element of the list sequentially. The most common way to traverse the elements of a list is with a for loop.

•The syntax is:

For<List_Item> in <List>:

    statement to process<List+Item>

OR

For<index>in range(len(List)):

    statement to process<List[Index]>

For Example:

Using for

(1) num=[11,12,13,14,15,16]

for i in num :

    print(i,end=" ")

(2) num=[11,12,13,14,15,16]

for i in range(0,5,1):

    print(num[i],end=" ")

Using While

num=[11,12,13,14,15,16]

i=0

n=len(num)

while i<n :

    print(num[i],end=" ")

    i=i+1

Print a list in reverse order

Using for loop

num=list(input("Enter the numbers:: "))

n=len(num)

for i in range(n-1,-1,-1):

    print(num[i],end=" ")

Using while loop

num=list(input("Enter the numbers:: "))

n=len(num)

i=n-1

while i>=0 :

    print(num[i],end=" ")

    i=i-1

Lists are Mutable :-

Lists are mutable, which means we can change their elements.

Using the bracket operator([ ]) on the left side of an assignment, we can update one of the elements.

For example:

>>> l1=[11,22,33,44,55]

>>> l1

[11, 22, 33, 44, 55]

>>> l1[2]=333

>>> l1

[11, 22, 333, 44, 55]

>>> l1[-1]=555

>>> l1

[11, 22, 333, 44, 555]

List Operations:-

Joining List:

>>> l1=[1,2,3]

>>> l2=[4,5,6]

>>> l1+l2

[1, 2, 3, 4, 5, 6]

>>> print(l1+l2)

[1, 2, 3, 4, 5, 6]

>>> l3=l1+l2

>>> print(l3)

[1, 2, 3, 4, 5, 6]

The + operator when used with lists require that both the operands must be of list Type.

Repeating or Replicating Lists:-

>>> l1=[1,2,3]

>>> l1*3

[1, 2, 3, 1, 2, 3, 1, 2, 3]

Comparing List:-

Lists can be compared by using relational operators.  Python compares individual elements of list.

>>>a=[1,2,3]

>>>b=[1,2,3]

>>>c=[‘1’,’2’,’3’]

>>>d=[1.0,2.0,3.0]

>>>a==b

True

>>>a==c

False

>>>a>b

False

>>>a==d

True

List Slice:-

An individual element of a list is called slice. Selecting a slice is similar to selecting an element of a list.

L[start:stop] creates list slice out of list L with the elements falling between start and stop, not included stop

>>> l1=[10,20,30,40,50,60,70]

>>> l1

[10, 20, 30, 40, 50, 60, 70]

>>> l1[2:5]

[30, 40, 50]

>>> print(l1[3:6])

[40, 50, 60]

L[start:stop:step] slice out L with elements falling between indexes start and stop, not including stop, skipping step-1 elements in between.

>>> l1=[11,22,33,44,55,66,77,88]

>>> l1[0:9:2]

[11, 33, 55, 77]

>>> l1[0:9:3]

[11, 44, 77]

>>> l1[::3]

[11, 44, 77]

Create a list and print the sum and average of all the elements of list.

a=[]

sum=avg=0

b=int(input("Enter total elements of list::"))

for i in range(b):                                                             #for i in range(0,b,1):

    c=int(input("Enter element::"))

    a.append(c)

for j in range(0,b+1,1):

    sum=sum+j

avg=sum/b

print(sum)

print(avg)

Slice the list and print sum & average

sum=avg=count=0

l1=[1,2,3,4,5,6,7,8,9,10]

s1=l1[1:11:2]

print(s1)

for j in s1:

    sum=sum+j

    count=count+1

avg=sum/count

print(sum)

print(avg)

List Functions and Methods:-

Python also offers many built-in functions and methods for list manipulation. Every list object that

 we create in Python is actually an instance of list class.

1. index: List.index(<item>)

>>>L1=[12,14,16,18]

>>>L1.index(14)

>>>1

2. append:

>>>Colors=[‘red’, ‘blue’, ‘green’]

>>>Colors.append(‘yellow’)

3.extend:

>>>a1=[‘a’, ‘e’, ‘i’]

>>>a2=[‘o’,’u’]

>>>a1.extend(a2)

>>>a1

[‘a’, ‘e’, ‘i’, ‘o’,’u’]

Difference between append() and extend()

append() adds one element to the list, extend() can add multiple elements from the list supplied to it as argument.

>>>a1=[1,2,3]

>>>a1.append(6,7)  # Produce error

>>>a1.append([6,7])# List can be appended but length will increase by 1

>>>len(a1)

4

>>>a1

[1,2,3,[6,7]]

>>>a1=[1,2,3]

>>>a2=[4,5]

>>>a1.extend(a2)    or    >>>a1.extend([4,5])

>>>a1

[1,2,3,4,5]

>>>len(a1)

5

After append() the length of list will increase by 1 only, after extend() length of list will increase by length of inserted list.

4. insert(): list.insert(<pos>, <item>)

>>>a1=[1,2,4,5]

>>>a1.insert(2,3)

>>>a1

[1,2,3,4,5]

Other formats

List.insert(0,x)

List.insert(len(a1),x) # will place element at last of list

5.pop(): It removes an element from the list.

List.pop(<index>)

>>>a1=[‘a’, ‘b’, ‘c’, ‘d’]

>>>x=a1.pop(0)

>>>x

‘a’

>>>y=a1.pop(2)

>>>y

‘d’

>>>a1

[‘b’, ‘c’]

6.remove(): It removes the first occurrence of given item from list, it takes element as an argument(where as pop() takes index as argument)

List.remove(<value>)

>>>a1=[1,2,3,4,5,6,7]

>>>a1.remove(2)

>>>a1

[1,3,4,5,6,7]

7. clear(): It removes all the items of the list.

List.clear()

>>>a1=[1,2,3,4,5]

>>>a1.clear()

>>>a1

[]

8.count():It returns the count of item passed as argument.

List.count(<item>)

>>>a1.count(2)

1

9.reverse(): It reverse the items of the list.

List.reverse()

>>>a1=[1,2,3,4,5,6,7]

>>>a1.reverse()

>>>a1

[7,6,5,4,3,2,1]

10.sort():It sort the items of list, by default in increasing order.

List.sort()

>>>a1=[2,1,6,3,8,4]

>>>a1.sort()

>>>a1

[1,2,3,4,6,8]

>>>a1.sort(reverse=True) # for sorting in decreasing order.