Question 1

What will be the output of the following statements?

a) list1 = [12,32,65,26,80,10]
list1.sort()
print(list1)

b) list1 = [12,32,65,26,80,10]
sorted(list1)
print(list1)

c) list1 = [1,2,3,4,5,6,7,8,9,10]
list1[::-2]
list1[:3] + list1[3:]

d) list1 = [1,2,3,4,5]
list1[len(list1)-1]

Accepted Answer

a) list1.sort() sorts the list in place. So after sorting, list1 becomes [10, 12, 26, 32, 65, 80]. The print statement outputs:
[10, 12, 26, 32, 65, 80]

b) sorted(list1) returns a new sorted list but does not modify list1 itself. Since the returned sorted list is not assigned, list1 remains unchanged. So print(list1) outputs:
[12, 32, 65, 26, 80, 10]

c) list1[::-2] returns a slice starting from the end towards the start, stepping by 2. So it picks elements at indices 9,7,5,3,1 (0-based): [10,8,6,4,2]. However, this expression is not printed or assigned, so no output.
list1[:3] + list1[3:] returns the entire list (first 3 elements plus from 3rd index onwards), effectively the same list. Again, no print or assignment, so no output.

d) list1[len(list1)-1] accesses the last element of list1. For list1 = [1,2,3,4,5], len(list1) = 5, so index 4 is last element which is 5. However, no print statement, so no output.

Summary of outputs:
a) [10, 12, 26, 32, 65, 80]
b) [12, 32, 65, 26, 80, 10]
c) No output
d) No output — a) The sort() method sorts the list in place, modifying the original list.
b) The sorted() function returns a new sorted list but does not change the original list unless assigned.
c) Slicing operations without print or assignment produce no output.
d) Accessing an element without print produces no output.

Question 2

Consider the following list myList. What will be the elements of myList after each of the following operations?

myList = [10,20,30,40]
a) myList.append([50,60])
b) myList.extend([80,90])

Accepted Answer

Initial list: myList = [10, 20, 30, 40]

a) myList.append([50,60]) adds the entire list [50,60] as a single element at the end. So myList becomes:
[10, 20, 30, 40, [50, 60]]

b) myList.extend([80,90]) adds each element of the list [80,90] individually to the end of myList. So after extending, myList becomes:
[10, 20, 30, 40, [50, 60], 80, 90] — append() adds its argument as a single element at the end of the list.
extend() iterates over its argument and adds each element individually to the list.

Question 3

What will be the output of the following code segment?

myList = [1,2,3,4,5,6,7,8,9,10]
for i in range(0,len(myList)):
    if i%2 == 0:
        print(myList[i])

Accepted Answer

The code iterates over indices 0 to 9 of myList.
For each index i, if i is even (i%2 == 0), it prints myList[i].
Indices 0,2,4,6,8 are even.
Corresponding elements are: 1, 3, 5, 7, 9.
Hence, output will be:
1
3
5
7
9 — The for loop runs from 0 to length of list - 1.
The if condition filters even indices.
Elements at even indices are printed.

Question 4

What will be the output of the following code segment?

a) myList = [1,2,3,4,5,6,7,8,9,10]
del myList[3:]
print(myList)

b) myList = [1,2,3,4,5,6,7,8,9,10]
del myList[:5]
print(myList)

c) myList = [1,2,3,4,5,6,7,8,9,10]
del myList[::2]
print(myList)

Accepted Answer

a) del myList[3:] deletes elements from index 3 to end.
Original list: [1,2,3,4,5,6,7,8,9,10]
After deletion: [1,2,3]
Output:
[1, 2, 3]

b) del myList[:5] deletes elements from start to index 4.
Original list: [1,2,3,4,5,6,7,8,9,10]
After deletion: [6,7,8,9,10]
Output:
[6, 7, 8, 9, 10]

c) del myList[::2] deletes elements at indices 0,2,4,6,8.
Original list: [1,2,3,4,5,6,7,8,9,10]
Elements at indices 0,2,4,6,8 are 1,3,5,7,9.
After deletion, remaining elements are at indices 1,3,5,7,9: 2,4,6,8,10
Output:
[2, 4, 6, 8, 10] — del with slice removes specified elements from the list.
In (a), elements from index 3 onwards are removed.
In (b), elements from start to index 4 are removed.
In (c), elements at every second index starting from 0 are removed.

Question 5

Differentiate between append() and extend() methods of list.

Accepted Answer

append(): Adds its argument as a single element to the end of the list.
Example: list1.append([50,60]) adds the list [50,60] as one element.

extend(): Iterates over its argument and adds each element individually to the list.
Example: list1.extend([50,60]) adds 50 and 60 as separate elements.

Summary:
- append() increases list length by 1.
- extend() increases list length by number of elements in the argument. — append() treats the argument as a single element.
extend() treats the argument as an iterable and adds each element separately.

Question 6

Consider a list:
list1 = [6,7,8,9]
What is the difference between the following operations on list1:
a) list1 * 2
b) list1 *= 2
c) list1 = list1 * 2

Accepted Answer

Given list1 = [6,7,8,9]

a) list1 * 2 returns a new list which is list1 repeated twice:
[6,7,8,9,6,7,8,9]
But list1 itself remains unchanged.

b) list1 *= 2 modifies list1 in place by repeating its contents twice.
After this operation, list1 becomes:
[6,7,8,9,6,7,8,9]

c) list1 = list1 * 2 creates a new list by repeating list1 twice and assigns it back to list1.
Effectively same as (b), list1 becomes:
[6,7,8,9,6,7,8,9]

Summary:
- a) does not change list1, just returns a new list.
- b) modifies list1 in place.
- c) creates a new list and reassigns list1 to it. — The * operator on lists repeats the list.
The *= operator modifies the list in place.
Assignment with = replaces the reference with a new list.

Question 7

The record of a student (Name, Roll No, Marks in five subjects and percentage of marks) is stored in the following list:
stRecord = ['Raman','A-36',[56,98,99,72,69], 78.8]
Write Python statements to retrieve the following information from the list stRecord.
- a) Percentage of the student
- b) Marks in the fifth subject
- c) Maximum marks of the student
- d) Roll No. of the student
- e) Change the name of the student from 'Raman' to 'Raghav'

Accepted Answer

Given:
stRecord = ['Raman','A-36',[56,98,99,72,69], 78.8]

Solutions:
a) Percentage of the student:
print(stRecord[3])  # Output: 78.8

b) Marks in the fifth subject:
print(stRecord[2][4])  # Output: 69

c) Maximum marks of the student:
print(max(stRecord[2]))  # Output: 99

d) Roll No. of the student:
print(stRecord[1])  # Output: 'A-36'

e) Change the name from 'Raman' to 'Raghav':
stRecord[0] = 'Raghav'
print(stRecord[0])  # Output: 'Raghav' — List indexing:
- stRecord[3] accesses percentage.
- stRecord[2] is list of marks; stRecord[2][4] accesses fifth subject marks.
- max() function finds maximum in marks list.
- stRecord[1] is roll number.
- Assignment changes name at index 0.

Question 8

Consider the following dictionary stateCapital:
stateCapital = {"Assam":"Guwahati", "Bihar":"Patna","Maharashtra":"Mumbai", "Rajasthan":"Jaipur"}
Find the output of the following statements:
- a) print(stateCapital.get("Bihar"))
- b) print(stateCapital.keys())
- c) print(stateCapital.values())
- d) print(stateCapital.items())
- e) print(len(stateCapital))
- f) print("Maharashtra" in stateCapital)
- g) print(stateCapital.get("Assam"))
- h) del stateCapital["Assam"] print(stateCapital)

Accepted Answer

Given:
stateCapital = {"Assam":"Guwahati", "Bihar":"Patna", "Maharashtra":"Mumbai", "Rajasthan":"Jaipur"}

Outputs:
a) print(stateCapital.get("Bihar"))
Output: Patna

b) print(stateCapital.keys())
Output: dict_keys(['Assam', 'Bihar', 'Maharashtra', 'Rajasthan'])

c) print(stateCapital.values())
Output: dict_values(['Guwahati', 'Patna', 'Mumbai', 'Jaipur'])

d) print(stateCapital.items())
Output: dict_items([('Assam', 'Guwahati'), ('Bihar', 'Patna'), ('Maharashtra', 'Mumbai'), ('Rajasthan', 'Jaipur')])

e) print(len(stateCapital))
Output: 4

f) print("Maharashtra" in stateCapital)
Output: True

g) print(stateCapital.get("Assam"))
Output: Guwahati

h) del stateCapital["Assam"]
print(stateCapital)
Output: {'Bihar': 'Patna', 'Maharashtra': 'Mumbai', 'Rajasthan': 'Jaipur'} — get() returns value for given key.
keys() returns all keys.
values() returns all values.
items() returns key-value pairs.
len() returns number of key-value pairs.
'in' operator checks key presence.
del removes key-value pair.

Question 9

Write a program to find the number of times an element occurs in the list.

Accepted Answer

To find the number of times an element occurs in a list, we can use the count() method of list in Python. For example:

```python
# Sample list
lst = [1, 2, 3, 2, 4, 2, 5]

# Element to count
element = 2

# Count occurrences
count = lst.count(element)
print(f"Element {element} occurs {count} times in the list.")
```

This program will output:
Element 2 occurs 3 times in the list. — The list method count() returns the number of times the specified element appears in the list. We simply call lst.count(element) to get the count.

Question 10

Write a program to read a list of n integers (positive as well as negative). Create two new lists, one having all positive numbers and the other having all negative numbers from the given list. Print all three lists.

Accepted Answer

To solve this problem, we can read n integers from the user, store them in a list, then iterate through the list to separate positive and negative numbers into two different lists.

Example program:

```python
# Read number of elements
n = int(input("Enter number of elements: "))

# Read list elements
lst = []
for _ in range(n):
    num = int(input())
    lst.append(num)

# Separate positive and negative numbers
positive = [x for x in lst if x > 0]
negative = [x for x in lst if x < 0]

# Print all lists
print("Original list:", lst)
print("Positive numbers:", positive)
print("Negative numbers:", negative)
```

This program reads n integers, creates two lists positive and negative, and prints all three lists. — We use list comprehensions to filter positive and negative numbers from the original list. Zero is excluded from both lists as it is neither positive nor negative.

Question 11

Write a program to find the largest and the second largest elements in a given list of elements.

Accepted Answer

To find the largest and second largest elements in a list, we can use sorting or iterate through the list maintaining two variables.

Example using sorting:

```python
lst = [int(x) for x in input("Enter elements separated by space: ").split()]
lst = list(set(lst))  # Remove duplicates to handle cases where largest and second largest are same
lst.sort(reverse=True)
if len(lst) >= 2:
    print("Largest element:", lst[0])
    print("Second largest element:", lst[1])
elif len(lst) == 1:
    print("Only one element present:", lst[0])
else:
    print("List is empty.")
```

Alternatively, without sorting:

```python
def find_two_largest(lst):
    if len(lst) < 2:
        return None, None
    first = second = float('-inf')
    for num in lst:
        if num > first:
            second = first
            first = num
        elif first > num > second:
            second = num
    return first, second

lst = [int(x) for x in input("Enter elements separated by space: ").split()]
largest, second_largest = find_two_largest(lst)
if largest is not None and second_largest is not None:
    print("Largest element:", largest)
    print("Second largest element:", second_largest)
else:
    print("List does not have enough elements.")
```

This program finds and prints the largest and second largest elements. — Sorting the list in descending order places the largest elements at the front. The first two elements are then the largest and second largest. Alternatively, iterating through the list and updating two variables keeps track of the two largest values efficiently.

Question 12

Write a program to read a list of n integers and find their median.

Note: The median value of a list of values is the middle one when they are arranged in order. If there are two middle values then take their average.

Hint: Use an inbuilt function to sort the list.

Accepted Answer

To find the median of a list of integers:

1. Sort the list.
2. If the number of elements n is odd, median is the middle element.
3. If n is even, median is the average of the two middle elements.

Example program:

```python
n = int(input("Enter number of elements: "))
lst = []
for _ in range(n):
    lst.append(int(input()))
lst.sort()

if n % 2 != 0:
    median = lst[n // 2]
else:
    median = (lst[(n // 2) - 1] + lst[n // 2]) / 2

print("Median is:", median)
```

This program reads n integers, sorts them, and calculates the median accordingly. — Sorting arranges the elements in order. For odd n, the middle element at index n//2 is the median. For even n, the median is the average of elements at indices (n//2 - 1) and (n//2).

Question 13

Write a program to read a list of elements. Modify this list so that it does not contain any duplicate elements i.e. all elements occurring multiple times in the list should appear only once.

Accepted Answer

To remove duplicates from a list while preserving order, we can use a set to track seen elements.

Example program:

```python
lst = input("Enter elements separated by space: ").split()
seen = set()
unique_list = []
for item in lst:
    if item not in seen:
        unique_list.append(item)
        seen.add(item)
print("List after removing duplicates:", unique_list)
```

This program reads elements as strings, removes duplicates preserving order, and prints the modified list. — Using a set to track elements already encountered allows us to avoid adding duplicates to the new list. This preserves the original order of first occurrences.

Question 14

Write a program to create a list of elements. Input an element from the user that has to be inserted in the list. Also input the position at which it is to be inserted.

Accepted Answer

To insert an element at a specified position in a list:

Example program:

```python
lst = input("Enter elements separated by space: ").split()
element = input("Enter element to insert: ")
position = int(input("Enter position to insert at (0-based index): "))

if 0 <= position <= len(lst):
    lst.insert(position, element)
    print("List after insertion:", lst)
else:
    print("Invalid position")
```

This program reads a list, an element, and a position, then inserts the element at the given position if valid. — The list method insert(pos, element) inserts the element at the specified index pos, shifting subsequent elements to the right.

Question 15

Write a program to read elements of a list and do the following.

a) The program should ask for the position of the element to be deleted from the list and delete the element at the desired position in the list.

b) The program should ask for the value of the element to be deleted from the list and delete this value from the list.

Accepted Answer

To delete an element by position and by value:

Example program:

```python
lst = input("Enter elements separated by space: ").split()

# a) Delete by position
pos = int(input("Enter position to delete (0-based index): "))
if 0 <= pos < len(lst):
    deleted_element = lst.pop(pos)
    print(f"Deleted element at position {pos}: {deleted_element}")
    print("List after deletion:", lst)
else:
    print("Invalid position")

# b) Delete by value
value = input("Enter value to delete: ")
if value in lst:
    lst.remove(value)
    print(f"Deleted element with value {value}")
    print("List after deletion:", lst)
else:
    print("Value not found in list")
```

This program first deletes element at given position, then deletes element by value if present. — The list method pop(pos) removes and returns element at index pos. The remove(value) method removes the first occurrence of value from the list.

Working with Chapter Lists and Dictionaries

Working with Chapter Lists and Dictionaries — Study Notes

4.1 INTRODUCTION TO LIST

4.1.1 Accessing Elements in a List

4.1.2 Lists are Mutable

Practice Questions — Working with Chapter Lists and Dictionaries

All 8 Chapters in Informatics Practices