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:
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.

Practice

1. What does the expression arr[2] do in an array arr?

easy

A. Returns the length of the array

B. Adds a new element at index 2

C. Deletes the element at index 2

D. Accesses the element at index 2 in the array

Solution

1. Step 1: Understand array indexing

   Array elements are accessed using their index inside square brackets, starting from 0.

2. Step 2: Access element at index 2

   The expression arr[2] retrieves the third element in the array (index 2).

3. Final Answer:

   Accesses the element at index 2 in the array -> Option D

4. Quick Check:

   arr[2] means element at position 2 [OK]

Hint: Indexing with brackets gets element at that position [OK]

Common Mistakes:

• Thinking index starts at 1 instead of 0
• Confusing access with insertion or deletion
• Assuming arr[2] returns length

2. Which of the following is the correct syntax to update the element at index 1 of array arr to 10?

easy

A. arr[1] = 10

B. arr{1} = 10

C. arr(1) = 10

D. arr.1 = 10

Solution

1. Step 1: Identify correct syntax for array update

   In Python, arrays or lists use square brackets to access or update elements by index.

2. Step 2: Use square brackets for assignment

   To update element at index 1, write arr[1] = 10.

3. Final Answer:

   arr[1] = 10 -> Option A

4. Quick Check:

   Use square brackets for indexing and assignment [OK]

Hint: Use square brackets [] to access or update array elements [OK]

Common Mistakes:

• Using parentheses () instead of brackets []
• Using curly braces {} which are for sets or dicts
• Trying dot notation for indexing

3. What will be the output of the following code?

arr = [5, 10, 15, 20]
arr[2] = 25
print(arr)

medium

A. [5, 10, 25, 20]

B. [5, 10, 15, 20]

C. [25, 10, 15, 20]

D. Error

Solution

1. Step 1: Understand initial array and update

   Initial array is [5, 10, 15, 20]. The statement arr[2] = 25 updates the element at index 2 (third element) to 25.

2. Step 2: Print updated array

   After update, array becomes [5, 10, 25, 20]. Printing it shows this updated list.

3. Final Answer:

   [5, 10, 25, 20] -> Option A

4. Quick Check:

   arr[2] changed to 25, rest unchanged [OK]

Hint: Update changes element at index, print shows new array [OK]

Common Mistakes:

• Thinking arr[2] updates first element
• Expecting original array without change
• Assuming syntax error

4. Identify the error in the following code and choose the correct fix:

arr = [1, 2, 3]
arr[3] = 4
print(arr)

medium

A. TypeError because arr is immutable; fix by converting to list

B. SyntaxError due to wrong assignment; fix by using arr(3) = 4

C. IndexError because index 3 is out of range; fix by using arr.append(4)

D. No error; output is [1, 2, 3, 4]

Solution

1. Step 1: Analyze index used for update

   Array has indices 0,1,2. Trying to assign at index 3 causes IndexError (out of range).

2. Step 2: Correct way to add new element

   To add element at end, use arr.append(4) instead of assigning at invalid index.

3. Final Answer:

   IndexError because index 3 is out of range; fix by using arr.append(4) -> Option C

4. Quick Check:

   Index 3 invalid, append adds element safely [OK]

Hint: Use append() to add beyond last index, not assignment [OK]

Common Mistakes:

• Assigning to index beyond array length
• Using parentheses instead of brackets
• Assuming no error occurs

5. Given arr = [2, 4, 6, 8, 10], which code snippet correctly doubles the value at index 3 and then swaps it with the value at index 1?

hard

A. arr[3] *= 2
arr[1] = arr[3]
arr[3] = arr[1]

B. arr[3] = arr[3] * 2
arr[1], arr[3] = arr[3], arr[1]

C. arr[3] = arr[1]
arr[1] = arr[3] * 2

D. arr[1], arr[3] = arr[3] * 2, arr[1]

Solution

1. Step 1: Double value at index 3

   Original arr[3] is 8. Doubling: arr[3] = 8 * 2 = 16.

2. Step 2: Swap values at index 1 and 3

   Swap arr[1] (4) and arr[3] (16) using tuple unpacking: arr[1], arr[3] = arr[3], arr[1].

3. Final Answer:

   arr[3] = arr[3] * 2 arr[1], arr[3] = arr[3], arr[1] -> Option B

4. Quick Check:

   Double then swap using tuple unpacking [OK]

Hint: Double first, then swap with tuple unpacking [OK]

Common Mistakes:

• Overwriting values before swap
• Incorrect swap order or syntax
• Not doubling before swapping