List in Python

List in Python


A list in Python is known as a “sequence data type” like strings. It is an ordered collection of values enclosed within square brackets [ ]. Each value of a list is called as element. It can be of any type such as numbers, characters, strings and even the nested lists as well. Th e elements can be modifi ed or mutable which means the elements can be replaced, added or removed. Every element rests at some position in the list. Th e position of an element is indexed with numbers beginning with zero which is used to locate and access a particular element.


Create a List in Python

In python, a list is simply created by using square bracket. Th e elements of list should be specifi ed within square brackets. Th e following syntax explains the creation of list.

Syntax:

Variable = [element-1, element-2, element-3 …… element-n]

Example

Marks = [10, 23, 41, 75]
Fruits = [“Apple”, “Orange”, “Mango”, “Banana”]
MyList = [ ]

In the above example, the list Marks has four integer elements; second list Fruits has four string elements; third is an empty list. The elements of a list need not be homogenous type of data. The following list contains multiple type elements.

Mylist = [ “Welcome”, 3.14, 10, [2, 4, 6] ]

In the above example, Mylist contains another list as an element. This type of list is known as “Nested List”. 

Nested list is a list containing another list as an element.


Accessing List elements

Python assigns an automatic index value for each element of a list begins with zero. Index value can be used to access an element in a list. In python, index value is an integer number which can be positive or negative.

Positive value of index counts from the beginning of the list and negative value means counting backward from end of the list (i.e. in reverse order).

To access an element from a list, write the name of the list, followed by the index of the element enclosed within square brackets.

Syntax:

List_Variable = [E1, E2, E3 …… En]
print (List_Variable[index of a element])

Example (Accessing single element):

>>> Marks = [10, 23, 41, 75]
>>> print (Marks[0])
10

In the above example, print command prints 10 as output, as the index of 10 is zero.

Example: Accessing elements in revevrse order

>>> Marks = [10, 23, 41, 75]
>>> print (Marks[-1])
75

* A negative index can be used to access an element in reverse order.


(i) Accessing all elements of a list

Loops are used to access all elements from a list. The initial value of the loop must be zero. Zero is the beginning index value of a list.

Example

Marks = [10, 23, 41, 75]
i = 0
while i < 4:
print (Marks[i])
i = i + 1

Output

10

23

41

75

(ii) Reverse Indexing

Python enables reverse or negative indexing for the list elements. Thus, python lists index in opposite order. The python sets -1 as the index value for the last element in list and -2 for the preceding element and so on. This is called as Reverse Indexing.

Example

Marks = [10, 23, 41, 75]
i = -1
while i >= -4:
print (Marks[i])
i = i + -1

Output

75

41

23

10

List Length

The len( ) function in Python is used to find the length of a list. (i.e., the number of elements in a list). Usually, the len( ) function is used to set the upper limit in a loop to read all the elements of a list. If a list contains another list as an element, len( ) returns that inner list as a single element.

Example :Accessing single element

>>> MySubject = [“Tamil”, “English”, “Comp. Science”, “Maths”]
>>> len(MySubject)
4

Example : Program to display elements in a list using loop

MySubject = ["Tamil", "English", "Comp. Science", "Maths"]
i = 0
while i < len(MySubject):
print (MySubject[i])
i = i + 1

Output

Tamil

English

Comp. Science

Maths


Accessing elements using for loop

In Python, the for loop is used to access all the elements in a list one by one. This is just like the for keyword in other programming language such as C++.

Syntax:

for index_var in list:
print (index_var)

Here, index_var represents the index value of each element in the list. Python reads this “for” statement like English: “For (every) element in (the list of) list and print (the name of the) list items”

Example

Marks=[23, 45, 67, 78, 98]
for x in Marks:
print( x )

Output

23

45

67

78

98

In the above example, Marks list has 5 elements; each element is indexed from 0 to 4. The Python reads the for loop and print statements like English: “For (every) element (represented as x) in (the list of) Marks and print (the values of the) elements”.

Changing list elements

In Python, the lists are mutable, which means they can be changed. A list element or range of elements can be changed or altered by using simple assignment operator (=).

Syntax:

List_Variable [index of an element] = Value to be changed
List_Variable [index from : index to] = Values to changed

Where, index from is the beginning index of the range; index to is the upper limit of the range which is excluded in the range. For example, if you set the range [0:5] means, Python takes only 0 to 4 as element index. Thus, if you want to update the range of elements from 1 to 4, it should be specified as [1:5].

Example : Python program to update/change single value

MyList = [2, 4, 5, 8, 10]
print ("MyList elements before update... ")
for x in MyList:
print (x)
MyList[2] = 6
print ("MyList elements after updation... ")
for y in MyList:
print (y)

Output:

MyList elements before update...

2

4

5

8

10

MyList elements after updation...

2

4

6

8

10


Example : Python program to update/change range of values

MyList = [1, 3, 5, 7, 9]

print ("List Odd numbers... ")

for x in MyList:

print (x)

MyList[0:5] = 2,4,6,8,10

print ("List Even numbers... ")

for y in MyList:

print (y)

Output

List Odd numbers...

1

3

5

7

9

List Even numbers...

2

4

6

8

10


Adding more elements in a list

In Python, append( ) function is used to add a single element and extend( ) function is used to add more than one element to an existing list.

Syntax:

List.append (element to be added)
List.extend ( [elements to be added])

In extend( ) function, multiple elements should be specified within square bracket as arguments of the function.

Example

>>> Mylist=[34, 45, 48]
>>> Mylist.append(90)
>>> print(Mylist)
[34, 45, 48, 90]

In the above example, Mylist is created with three elements. Through >>> Mylist.append(90) statement, an additional value 90 is included with the existing list as last element, following print statement shows all the elements within the list MyList.

Example

>>> Mylist.extend([71, 32, 29])
>>> print(Mylist)
[34, 45, 48, 90, 71, 32, 29]

In the above code, extend( ) function is used to include multiple elements, the print statement shows all the elements of the list after the inclusion of additional elements.


Inserting elements in a list

As you learnt already, append( ) function in Python is used to add more elements in a list. But, it includes elements at the end of a list. If you want to include an element at your desired position, you can

use insert ( ) function. The insert( ) function is used to insert an element at any position of a list.

Syntax:

List.insert (position index, element)

Example

>>> MyList=[34,98,47,'Kannan', 'Gowrisankar', 'Lenin', 'Sreenivasan' ]
>>> print(MyList)
[34, 98, 47, 'Kannan', 'Gowrisankar', 'Lenin', 'Sreenivasan']
>>> MyList.insert(3, 'Ramakrishnan')
>>> print(MyList)
[34, 98, 47, 'Ramakrishnan', 'Kannan', 'Gowrisankar', 'Lenin', 'Sreenivasan']

In the above example, insert( ) function inserts a new element ‘Ramakrishnan’ at the index value 3, ie. at the 4th position. While inserting a new element in between the existing elements, at a particular location, the existing elements shifts one position to the right.

Deleting elements from a list

There are two ways to delete an element from a list viz. del statement and remove( ) function. del statement is used to delete known elements whereas remove( ) function is used to delete elements of a list if its index is unknown. The del statement can also be used to delete entire list.

Syntax:

del List [index of an element]
# to delete a particular element
del List [index from : index to]
# to delete multiple elements
del List
# to delete entire list

Example

>>> MySubjects = ['Tamil', 'Hindi', 'Telugu', 'Maths']
>>> print (MySubjects)
['Tamil', 'Hindi', 'Telugu', 'Maths']
>>> del MySubjects[1]
>>> print (MySubjects)
['Tamil', 'Telugu', 'Maths']

In the above example, the list MySubjects has been created with four elements. print statement shows all the elements of the list. In >>> del MySubjects[1] statement, deletes an element whose index value is 1 and the following print shows the remaining elements of the list.

Example

>>> del MySubjects[1:3]
>>> print(MySubjects)
['Tamil']

In the above codes, >>> del MySubjects[1:3] deletes the second and third elements from the list. The upper limit of index is specified within square brackets, will be taken as -1 by the python.

Example

>>> del MySubjects
>>> print(MySubjects)
Traceback (most recent call last):
File "<pyshell#9>", line 1, in <module>
print(MySubjects)
NameError: name 'MySubjects' is not defined

List and range ( ) function

The range( ) is a function used to generate a series of values in Python. Using range( ) function, you can create list with series of values. The range( ) function has three arguments.

Syntax of range ( ) function:

range (start value, end value, step value)

where,

start value – beginning value of series. Zero is the default beginning value.

end value – upper limit of series. Python takes the ending value as upper limit – 1.

step value – It is an optional argument, which is used to generate different interval of values.

Example : Generating whole numbers upto 10

for x in range (1, 11):
print(x)

Output

1

2

3

4

5

6

7

8

9

10

Example : Generating first 10 even numbers

for x in range (2, 11, 2):
print(x)

Output

2

4

6

8

10

Creating a list with series of values

Using the range( ) function, you can create a list with series of values. To convert the result of range( ) function into list, we need one more function called list( ). The list( )

function makes the result of range( ) as a list.

Syntax:

List_Varibale = list ( range ( ) )

Example

>>> Even_List = list(range(2,11,2))
>>> print(Even_List)
[2, 4, 6, 8, 10]


Other important list funcion

Function
Description
Syntax
Example
copy ( )
Returns a copy of the list
List.copy( )

MyList=[12, 12, 36]
x = MyList.copy()
print(x)

Output:

[12, 12, 36]

count ( )
Returns the number of similar elements present in the last.
List.count(value)
MyList=[36 ,12 ,12]
x = MyList.count(12)
print(x)

Output:

2

index ( )
Returns the index value of the first recurring element
List.index(element)
MyList=[36 ,12 ,12]
x = MyList.index(12)
print(x)

Output:

0

reverse ( )
Reverses the order of the element in the list.
List.reverse( )
MyList=[36 ,23 ,12]
MyList.reverse()
print(MyList)

Output:

[12 ,23 ,36]

sort ( )
Sorts the element in list
List.sort(reverse=True|False, key=myFunc)

max( )
Returns the maximum value in a list.
max(list)
MyList=[21,76,98,23]
print(max(MyList))

Output:

98

min( )
Returns the minimum value in a list.
min(list)
MyList=[21,76,98,23]
print(min(MyList))

Output:

21

sum( )
Returns the sum of values in a list.
sum(list) 
MyList=[21,76,98,23]
print(sum(MyList))

Output:

218

Qus What do you understand by mutability?

Ans: Mutable means changeable. In Python, mutable types are those whose values can be changed in place. Only three types are mutable in python – Lists, Dictionaries and Sets.

Qus Start with the list[8,9,10]. Do the following using list functions

(a) Set the second entry (index 1) to 17

(b) Add 4, 5 and 6 to the end of the list.

(c) Remove the first entry from the list.

(d) Sort the list.

(e) Double the list.

(f) Insert 25 at index 3


Ans:

(a) list[1]=17

(b) list.append(4)

list.append(5)

list.append(6)

(c) list.pop(0)

(d) list.sort()

(e) list=list*2

(f) list.insert(3,25)


Qus.  If a is [1, 2, 3], what is the difference (if any) between a*3 and [a, a, a]?

Ans: a*3 will produce [1,2,3,1,2,3,1,2,3], means a list of integers and [a, a, a] will produce [[1,2,3],[1,2,3],[1,2,3]], means list of lists


Qus. If a is [1, 2, 3], is a *3 equivalent to a + a + a?

Ans: Yes, Both a*3 and a+a+a will produce same result.


Qus. If a is [1, 2, 3], what is the meaning of a [1:1] = 9?

Ans: This will generate an error "TypeError: can only assign an iterable'.


Qus If a is [1, 2, 3], what is the meaning of a [1:2] = 4 and a [1:1] = 4?

Ans: These will generate an error "TypeError: can only assign an iterable".

Qus. What are list slices?

Ans: List slices are the sub-part of a list extracted out. You can use indexes of the list elements to create list slices as per following format. Syntax is as follows –

Qus Does a slice operator always produce a new list?

Ans: Yes, this will create a new list.


Qus. How are lists different from strings when both are sequences?

Ans: Lists are similar to strings in many ways like indexing, slicing, and accessing individual elements but they are different in the sense that

(i) Lists are mutable while strings are immutable.

(ii) In consecutive locations, strings store the individual characters while list stores the references of its elements.

(iii) Strings store single type of elements-all characters while lists can store elements belonging to different types.


Qus. What are nested Lists?

Ans: A list can have an element in it, which itself is a list. Such a list is called nested list. e.g.

L = [1,2,3,4,[5,6,7],8]


Qus. Discuss the utility and significance of Lists.

Ans: The list is a most versatile datatype available in Python which can be written as a list of comma-separated values (items) between square brackets. Important thing about a list is that items in a list need not be of the same type. List is majorly used with dictionaries when there is large number of data.


Qus What is the purpose of the del operator and pop method? Try deleting a slice.

Ans: del operator is used to remove an individual item, or to remove all items identified by a slice. It is to be used as per syntax given below –

>>>del List[index]

>>>del List[start:stop]

pop method is used to remove single element, not list slices. The pop() method removes an individual item and returns it. Its syntax is –

>>>a=List.pop() #this will remove last item and deleted item will be assigned to a.

>>>a=List[10] # this will remove the ite at index 10 and deleted item will be assigned to a.


Qus. What are list slices?

Ans: List slices, like string slices are the sub part of a list extracted out. Indexes can be used to create list slices as per following format:

seq = L[start:stop]


Qus. What do you understand by true copy of a list? How is it different from shallow copy?

Ans: A shallow copy means constructing a new collection object and then populating it with references to the child objects found in the original. In essence, a shallow copy is only one level deep. The copying process does not recurse and therefore won‘t create copies of the child objects themselves.

True Copy means you can create a copy of a list using New_list=My_list. The assignment just copies the reference to the list, not the actual list, so both new_list and my_list refer to the same list after the assignment.


Qus What do you understand by immutability?

Ans: Immutability means not changeable. The immutable types are those that can never change their value in place. In python following types are immutable –

I. Integers

II. Floating point numbers

III. Booleans

IV. Strings

V. Tuples


Qus How are Tuples different from Lists when both are sequences?

Ans: Tuples are similar to lists in many ways like indexing, slicing, and accessing individual values but they are different in the sense that –

Tuples are immutable while lists are mutable.

List can grow or shrink while tuples cannot.


Qus Can tuples be nested?

Ans: Yes, tuples can be nested. e.g (1,2,3,(4,5,6))


Qus. Discuss the utility and significance of Tuples.

Ans: Tuples are great for holding static data that you don't plan on modifying and changing a lot. Tuples are four to seven times faster than lists. This means for every list item manipulated, four to seven tuples could be manipulated within that time frame. This has huge advantages for scientific work, as this allows for speedy data reading.


Qus. How can you say that a tuple is an ordered list of objects?

Ans: A tuple is an ordered list of objects. This is evidenced by the fact that the objects can be accessed through the use of an ordinal index amd for a given index, same element is returned every time.


Qus. Why can‟t List can be used as keys?

Ans: List is mutable datatype. And Keys of dictionary must be immutable type. This is the region that list cannot be used as keys.


Qus. What type of objects can be used as keys in dictionary?

Ans: Any immutable objects can be used as keys in dictionary.


Qus. Can you change the order of the dictionaries contents?

Ans: Yes, the order of dictionary contents may be changed.


Qus. Can you modify the keys in a dictionary?

Ans: Keys cannot be modified in dictionary while values are mutable in dictionary.


Qus. Can you modify the value in a dictionary?

Ans: Yes, Values can be modified in dictionaries.


Qus. Is dictionary Mutable? Why?

Ans: Dictionary is mutable type because we can change its values.


Qus How are dictionaries different from Lists?

Ans: The dictionary is similar to lists in the sense that it is also a collection of data-items but it is different from lists in the sense that lists are sequential collections(ordered) and dictionaries are non-sequential collections(unordered).

Elements in lists or tuples can be accessed by using indexes. But in dictionaries the values can be obtained using keys. By changing the sequence of key we can shuffle the order of elements of dictionary while this thing is not possible in list.


Qus. When are dictionaries more useful than lists?

Ans: Dictionaries can be much more useful than lists when we wanted to store all our friends cell-phone numbers. We could create a list of pairs,(name of friend, phone number), but once this list becomes long enough searching this list for a specific phone number will get-time consuming. Better would be if we could index the list by our friend‘s name. This is precisely what a dictionary does.


Qus. Discuss the utility and significance of Dictionaries.

Ans: Dictionaries can be much more useful than lists when we wanted to store all our friends cell-phone numbers. We could create a list of pairs,(name of friend, phone number), but once this list becomes long enough searching this list for a specific phone number will get-time consuming. Better would be if we could index the list by our friend‘s name. This is precisely what a dictionary does.


Qus. Why is a dictionary termed as an unordered collection of objects?

Ans: But in dictionaries the values can be obtained using keys. By changing the sequence of key we can shuffle the order of elements of dictionary while this thing is not possible in list. In dictionaries there is no index. It uses its keys as index which can be rearranged. That‘s why a dictionary termed as an unordered collection of objects


Qus. How is clear() function different from del <dict> Statement?

Ans: clear() removes all the elements of a dictionary and makes it empty dictionary while del statement removes the complete dictionary as an object. After del statement with a dictionary name, that dictionary object no longer exists, not even empty dictionary.


Programs

WAP to find the 2nd largest number from the list of the numbers entered through keyboard.


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Write a python program that creates a list of numbers from 1 to 20 that are divisible by 4


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Write a python program to define a list of countries that are a member of BRICS. Check whether a county is member of BRICS or not


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Python program to read marks of six subjects and to print the marks scored in each subject and show the total marks


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Python program to read prices of 5 items in a list and then display sum of all the prices, product of all the prices and find the average


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Python program to count the number of employees earning more than 1 lakh per annum. The monthly salaries of n number of employees are given


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Write a program to create a list of numbers in the range 1 to 10. Then delete all the even numbers from the list and print the final list.


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Write a program to generate in the Fibonacci series and store it in a list. Then find the sum of all values.


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Write a program to swap two values using tuple assignment


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Write a python program using a function that returns the area and circumference of a circle whose radius is passed as an argument.two values using tuple assignment


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Write a program that has a list of positive and negative numbers. Create a new tuple that has only positive numbers from the list


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Python Program that generate a set of prime numbers and another set of even numbers. Demonstrate the result of union, intersection, difference and symmetirc difference operations.


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WAP to find minimum element from a list of elements along with its index in the list.


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WAP to calculate mean of a given list of numbers.


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WAP to search for an element in a given list of numbers.


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WAP to count frequency of a given element in a list of numbers.


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WAP to calculate the sum of integers of the list


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WAP that reverses an array of integers (in place)


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WAP that creates a third list after adding two lists.


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WAP that repeatedly asks the user to enter product names and prices. Store all of them in a dictionary whose keys are product names and values are prices. And also write a code to search an item from the dictionary.


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WAP to create a dictionary named year whose keys are month names and values are their corresponding number of days.


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