Array Access and Update at Index in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the time complexity of accessing an element at a specific index in an array?

Accessing an element at a specific index in an array takes O(1) time because arrays allow direct access using the index.

beginner

How do you update the value at index 3 in an array named arr to 10 in Python?

You update the value by assigning the new value directly: <br>arr[3] = 10

beginner

What happens if you try to access an index that is out of the array's range?

Accessing an index outside the array's range causes an IndexError in Python, meaning the index is invalid.

intermediate

Explain why arrays are good for fast access but not always for fast insertion or deletion.

Arrays allow fast access because elements are stored contiguously and can be accessed by index directly. But insertion or deletion in the middle requires shifting elements, which takes O(n) time.

beginner

Show the Python code to access and update the 2nd element of an array arr and print the updated array.

arr = [5, 8, 12, 20] print("Before update:", arr) arr[1] = 15 # Update 2nd element (index 1) print("After update:", arr)

What is the index of the first element in a Python array (list)?

A -1

B 1

C 0

D Depends on the array size

Python arrays (lists) are zero-indexed, so the first element is at index 0.

What happens if you try to update an element at an index that does not exist in the array?

A An IndexError is raised

B The array automatically expands

C The update is ignored

D The last element is updated instead

Trying to update an element at an invalid index raises an IndexError in Python.

Which operation is fastest in an array?

A Accessing element by index

B Inserting element at the start

C Deleting element in the middle

D Searching for an element by value

Accessing by index is O(1), faster than insertion, deletion, or searching.

How do you update the 4th element in an array named arr to 100 in Python?

A arr.set(3, 100)

B arr[3] = 100

C arr.update(4, 100)

D arr[4] = 100

Indexing starts at 0, so the 4th element is at index 3.

If arr = [1, 2, 3], what is the output after arr[1] = 5?

A [5, 5, 3]

B [5, 2, 3]

C [1, 2, 5]

D [1, 5, 3]

The element at index 1 (second element) is updated to 5.

Describe how to access and update an element at a specific index in an array.

Think about how you pick and change a book on a shelf by its position.

Explain why accessing an element by index in an array is fast but inserting in the middle is slower.

Imagine a row of chairs where you can sit directly but adding a chair in the middle means moving others.