Refresher: List 1¶

Defining a List¶

Definition¶

A list is a built-in data type that represents an ordered collection of items. It is a mutable, dynamic array, meaning you can modify its elements and size after creation.

Syntax¶

my_list = [element1, element2, element3, ...]

Examples¶

The provided code snippets demonstrate various aspects of working with lists in Python.

Code 1:

a = [1, 2, 3]

Initializes a list a with three integer elements.

Code 2:

a = [1, "a"]

Initializes a list a with two elements: an integer 1 and a string "a".

Output 2:

# SyntaxError: closing parenthesis '}' does not match opening parenthesis '['
a = [1, "a"]

This part of the code seems incomplete and might be causing a syntax error. The specific error is related to unmatched parentheses.

Code 3:

a = ["a", 1, 3.14, True]  # Lists can be heterogeneous

Demonstrates that lists in Python can contain elements of different data types.

Code 4:

a = list(range(10))
print(a)

Output 4:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Uses the list() constructor to create a list containing elements from 0 to 9 (result of range(10)).

Code 5:

students = ['Kusum', 'Shubham', 'Pooja']
print(students)

Output 5:

['Kusum', 'Shubham', 'Pooja']

Prints the list of student names.

Indexing in a List¶

Code:

print(students[1])

Output:

Shubham

Code:

print(students[5])

Output:

# IndexError: list index out of range

Explanation:

Accessing elements in a list using indices.
An IndexError occurs when trying to access an index beyond the list's length.

Reverse Indexing¶

Code:

students = ['Kusum', 'Shubham', 'Pooja']
print(students[-1])

Output:

Pooja

Code:

students = ['Kusum', 'Shubham', 'Pooja']
print(students[-100])

Output:

# IndexError: list index out of range

Updating an Index in A List¶

students = ['Kusum', 'Shubham', 'Pooja']
print(students)

Output:

['Kusum', 'Shubham', 'Pooja']

Updating user at index 3

Code:

students[3] = 'Ruben'
print(students)

Output:

['Kusum', 'Shubham', 'Pooja', 'Ruben']

Code:

students[-100]

Output:

# IndexError: list index out of range

Code:

print(type(students))

Output:

<class 'list'>

Print even numbers till 10

Code:

a = list(range(0, 11, 2))
print(a)

Output:

[0, 2, 4, 6, 8, 10]

Print first 10 even numbers

Code:

a = list(range(0, 20, 2))
print(a)

Output:

[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

Explanation:

Lists can be modified by assigning new values to specific indices.
Negative indices count from the end of the list.

Iteration in a List¶

students = ['Kusum', 'Shubham', 'Pooja', 'Ruben', 'Aarushi', 'Vinoth', 'Veerendra']

len() gives you the number of elements in a list

Code:

print(len(students))

Output:

Code:

# Solution 1
n = len(students)
for i in range(0, n):
    print(students[i])

Output:

Kusum
Shubham
Pooja
Ruben
Aarushi
Vinoth
Veerendra

Code:

# Solution 2
for student in students:
    print(student)

Output:

Kusum
Shubham
Pooja
Ruben
Aarushi
Vinoth
Veerendra

Quiz 1¶

Code:

# quiz
li = [-1, 0, 4]
for i in li:
    if i > 0:
        print(i, end=' ')

Output:

Explanation:

Iterating through a list and printing positive numbers.

Functions in a List¶

len()¶

Returns the number of elements in a list.

Code:

a = list(range(2, 6))
print(len(a))

Output:

append()¶

Appends an object to the end of the list.

Code:

students = ['Kusum', 'Shubham', 'Pooja', 'Ruben', 'Aarushi', 'Vinoth', 'Veerendra']
students.append('Vicky')
print(students)

Output:

['Kusum', 'Shubham', 'Pooja', 'Ruben', 'Aarushi', 'Vinoth', 'Veerendra', 'Vicky']

Code:

a = []
a.append('Hi')
print(a)

Output:

['Hi']

insert()¶

The insert method is used to insert an element before the specified index in a list.

Code:

a = [1, 2, 3, 4]
a.insert(1, 'Aakar')
print(a)

Output:

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

Explanation:

The element 'Aakar' is inserted at index 1, shifting the original elements to the right.

Code:

a = [1, 2, 3, 4]
a.insert(-3, 'Aakar')
print(a)

Output:

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

Explanation:

The negative index -3 is interpreted as counting from the end of the list, so 'Aakar' is inserted at index 2 from the end.

Code:

a = [1, 2, 3, 4]
a.insert(100, 'Aakar')
print(a)

Output 3:

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

Explanation:

If the specified index is greater than the length of the list, the element is inserted at the end.

Code:

a = [1, 2, 3, 4]
a.insert(-100, 'Aakar')
print(a)

Output:

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

Explanation:

The negative index -100 is interpreted as counting from the end of the list, so 'Aakar' is inserted at the beginning of the list.

pop()¶

Removes and returns an item at the specified index (default last).
Raises IndexError if the list is empty or the index is out of range.

Code:

students = ['Kusum', 'Shubham', 'Pooja', 'Ruben', 'Aarushi', 'Vinoth', 'Veerendra']
print(students.pop())
print(students)

Output:

Veerendra
['Kusum', 'Shubham', 'Pooja', 'Ruben', 'Aarushi', 'Vinoth']

Code:

a = [1, 2, 3, 4]
print(a.pop(5))
print(a)

Output:

# IndexError: pop index out of range

Code:

a = []
print(a.pop())
print(a)

Output:

# IndexError: pop from an empty list

Explanation:

The pop() function removes and returns an item at a specified index.
Raises an IndexError if the index is out of range.

remove()¶

Removes the first occurrence of a value.
Raises ValueError if the value is not present.

Code:

students = ['Kusum', 'Shubham', 'Pooja', 'Ruben', 'Aarushi', 'Vinoth', 'Veerendra']
students.remove('Shubham')
print(students)

Output:

['Kusum', 'Pooja', 'Ruben', 'Aarushi', 'Vinoth', 'Veerendra']

Code:

a = [1, 2, 3, 2, 4]
a.remove(2)
print(a)

Output:

[1, 3, 2, 4]

Code:

a = [1, 2, 3, 4]
a.remove(5)
print(a)

Output:

# ValueError: list.remove(x): x not in the list

Explanation:

The remove() function removes the first occurrence of a specified value.
Raises a ValueError if the value is not present.

Quiz¶

Quiz 1¶

li = [1, 2, 3]
li.append('4')
print(li)

Answer: [1, 2, 3, '4']

Quiz 2¶

li = []
print(len(li))

Answer: 0

Quiz 3¶

li = [1, 2]
print(li.pop())

Answer: 2

Quiz 4¶

li = [1, 3, 4]
li.insert(0, 2)
print(li)

Answer: [2, 1, 3, 4]

Quiz 5¶

li = [1, 2]
print(li.remove(2))

Answer: None

Reverse¶

Code:

a = [1, 2, 3]
print(a.reverse())
print(a)

Output:

[3, 2, 1]

Code:

a = []
print(a.reverse())
print(a)

Output:

None
[]

Explanation:

The reverse() method reverses the elements of a list in place.

+ operator¶

Concatenating two lists.
Creates a new list.

Code:

a = [1, 2, 3]
b = [4, 5, 6]
c = a + b
print(c)
print(a)
print(b)

Output:

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

extend()¶

Extend list by appending elements from the iterable.

Code:

a = [1,2,3]
b = [4,5,6]
a.append(b)
print(a)
print(a[-1]) # not what we want

Output:

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

Explanation:

The append() method is used, but it appends the entire list b as a single element at the end of list a.

Code:

a = [1,2,3]
b = [4,5,6]
a.extend(b)
print(a)
print(b)
print(a[-1]) # not what we want

Output:

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

Explanation:

The extend() method is used, adding each element from list b individually to the end of list a.

Code:

a = [1,2,3]
a.extend(a)
print(a)

Output:

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

Explanation:

The extend() method is used to add elements of list a to the end of list a, effectively doubling its content.

in Operator¶

return True or False after searching list

Code:

students = ['Kusum', 'Shubham', 'Pooja', 'Ruben', 'Aarushi', 'Aakar', 'Veerendra']
print('Aakar' in students)
print('Kusum' in students)

Output:

False
True

Explanation:

The in operator is used to check if an element is present in a list.
The first print statement checks if 'Aakar' is in the list of students, resulting in False.
The second print statement checks if 'Kusum' is in the list of students, resulting in True.

How to take List as Input?¶

Code:

n = int(input())
a = []
for i in range(n):
    item = input()
    a.append(item)
print(a)

Output:

5
a
b
c
d
e
['a', 'b', 'c', 'd', 'e']

Explanation:

The code takes an integer n as input, then uses a loop to take n input items and appends them to a list a, resulting in a list of items.

Split¶

Code:

s = 'I-love-bananas'
li = s.split('-')
print(li)
print(type(li))

Output:

['I', 'love', 'bananas']
<class 'list'>

Explanation: - The split method is used to split a string s into a list of substrings based on the specified delimiter ('-'), creating a list li.

Code:

s = 'I--love--bananas'
li = s.split('--')
print(li)
print(type(li))

Output:

['I', 'love', 'bananas']
<class 'list'>

Explanation: - Even if there are multiple consecutive delimiters, split correctly handles them and produces the desired list.

Code:

n = int(input())
s = input()  # always returns string "a b c d e"
li = s.split(' ')
print(li)

Output:

5
a b c d e
['a', 'b', 'c', 'd', 'e']

Explanation: - The code takes an integer n and a space-separated string as input, then uses split to create a list li of individual items.

Code:

# INPUT
# 5
# 12 14 15 16 17
n = int(input())
s = input()  # always returns string "a b c d e"
li = s.split(' ')
new_li = []
for item in li:
    new_li.append(int(item))
print(new_li)

Output:

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

Explanation:

The code converts a space-separated string of numbers into a list of integers, demonstrating the use of split and conversion to integers.

Code:

# INPUT
# 5
# 12 14 15 16 17
n = int(input())
s = input()  # always returns string "a b c d e"
li = s.split(' ')
for index in range(len(li)):
    li[index] = int(li[index])
print(li)

Output:

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

Explanation: - Similar to the previous example, this code converts a space-separated string of numbers into a list of integers using a loop.Explanation:

Problem¶

Given a List of Student Marks, Count the Number of Student Who Failed

Info

Please take some time to think about the solution approach on your own before reading further.....

Code:

n = int(input())
s = input() # always returns string "a b c d e"
marks = s.split(' ')
for index in range(len(marks)):
 marks[index] = float(marks[index])
print(marks)
# ------------------------
count = 0
for index in range(len(marks)):
 if marks[index] <= 30:
 count += 1
print(count)

Output:

5
10 20 30 40 50
[10.0, 20.0, 30.0, 40.0, 50.0]
3

Explanation:

The code first takes an integer n as input, representing the number of students. Then, it takes a string s as input, which contains space-separated marks of students in the form of "a b c d e".
The string of marks is split into a list of strings using the split method, and then each element in the list is converted to a floating-point number using a loop.
The list of marks is printed as output.
The code initializes a variable count to 0 and then iterates through the list of marks. For each mark, if it is less than or equal to 30, the count is incremented.
Finally, the count of students who failed (marks <= 30) is printed as output.

Output Explanation:

For the given input "5" and "10 20 30 40 50", the list of marks after conversion to float is [10.0, 20.0, 30.0, 40.0, 50.0].
Among these marks, three students have marks less than or equal to 30 (10.0, 20.0, and 30.0). Therefore, the count of students who failed is printed as 3.