🐍2.Python Lists for Interviews_details

Here’s a detailed Python List Guide for Interviews covering key concepts, functions, and coding examples:

1. List Basics

A list is an ordered, mutable collection that can hold different data types.

Syntax:

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

Properties of Lists:

  • Ordered (maintains insertion order)
  • Mutable (can be modified)
  • Allows Duplicates
  • Dynamic (can grow/shrink)

2. Creating Lists

1. Using Square Brackets

numbers = [10, 20, 30]

2. Using list() Constructor

numbers = list((10, 20, 30))

3. Empty List

empty_list = []

4. List with Mixed Data Types

mixed_list = [10, "Hello", 3.14, True]

5. List of Lists (Nested List)

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

3. Accessing List Elements

1. Indexing

nums = [10, 20, 30, 40]
print(nums[1])  # Output: 20
print(nums[-1])  # Output: 40 (Last Element)

2. Slicing

nums = [10, 20, 30, 40, 50]
print(nums[1:4])  # Output: [20, 30, 40]
print(nums[:3])   # Output: [10, 20, 30]
print(nums[::2])  # Output: [10, 30, 50] (Step size 2)

4. Modifying Lists

1. Changing Elements

nums = [10, 20, 30]
nums[1] = 50
print(nums)  # Output: [10, 50, 30]

2. Adding Elements

  • append() → Adds at the end
  • insert() → Adds at a specific index
  • extend() → Adds multiple elements
nums = [10, 20, 30]
nums.append(40)        # [10, 20, 30, 40]
nums.insert(1, 15)     # [10, 15, 20, 30, 40]
nums.extend([50, 60])  # [10, 15, 20, 30, 40, 50, 60]

5. Removing Elements

  • remove(value) → Removes first occurrence of value
  • pop(index) → Removes and returns element at index
  • del list[index] → Deletes element at index
  • clear() → Removes all elements
nums = [10, 20, 30, 40]
nums.remove(20)  # [10, 30, 40]
nums.pop(1)      # [10, 40]
del nums[0]      # [40]
nums.clear()     # []

6. List Operations

1. Concatenation

list1 = [1, 2]
list2 = [3, 4]
print(list1 + list2)  # [1, 2, 3, 4]

2. Repetition

nums = [1, 2] * 3
print(nums)  # [1, 2, 1, 2, 1, 2]

3. Membership (in, not in)

nums = [10, 20, 30]
print(20 in nums)   # True
print(50 not in nums)  # True

7. Iterating Over Lists

1. Using for Loop

nums = [10, 20, 30]
for num in nums:
    print(num)

2. Using while Loop

nums = [10, 20, 30]
i = 0
while i < len(nums):
    print(nums[i])
    i += 1

3. Using enumerate()

nums = [10, 20, 30]
for index, value in enumerate(nums):
    print(index, value)

8. List Comprehensions

squares = [x**2 for x in range(5)]
print(squares)  # [0, 1, 4, 9, 16]

With Condition:

even_nums = [x for x in range(10) if x % 2 == 0]
print(even_nums)  # [0, 2, 4, 6, 8]

9. Sorting and Reversing

1. sort() (Modifies List)

nums = [3, 1, 4, 1, 5]
nums.sort()
print(nums)  # [1, 1, 3, 4, 5]

2. sorted() (Returns New List)

nums = [3, 1, 4, 1, 5]
print(sorted(nums))  # [1, 1, 3, 4, 5]

3. reverse()

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

10. List Functions

Function Description
len(lst) Returns length of the list
max(lst) Returns max value
min(lst) Returns min value
sum(lst) Returns sum of elements
list.count(value) Counts occurrences of value
list.index(value) Returns index of first occurrence of value 
nums = [10, 20, 30, 20]
print(len(nums))      # 4
print(max(nums))      # 30
print(min(nums))      # 10
print(sum(nums))      # 80
print(nums.count(20)) # 2
print(nums.index(30)) # 2

11. Copying Lists (Avoiding Pitfalls)

Incorrect Way (Shallow Copy)

list1 = [1, 2, 3]
list2 = list1  # Both point to the same list!
list2[0] = 99
print(list1)  # [99, 2, 3]

Correct Way (copy() or [:])

list1 = [1, 2, 3]
list2 = list1[:]  # Creates a new copy
list2[0] = 99
print(list1)  # [1, 2, 3]

list1 = [1, 2, 3]
list2 = list1.copy()

12. Nested List Processing

matrix = [[1, 2, 3], [4, 5, 6]]
for row in matrix:
    for item in row:
        print(item, end=" ")
# Output: 1 2 3 4 5 6

Common Interview Questions

  1. Find second largest number in a list.
  2. Remove duplicates from a list.
  3. Flatten a nested list.
  4. Rotate a list left or right.
  5. Find pairs in a list that sum to a given number.

Do you want solutions for any of these? 🚀

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