Array Access and Update at Index in DSA Python - Execution Trace

Concept Flow - Array Access and Update at Index

Start with array

↓

Access index i?

No→Stop

Yes↓

Read or Update value at index i

↓

Reflect change in array

↓

End operation

This flow shows how we start with an array, check the index, then either read or update the value at that index, and finally reflect the change in the array.

Execution Sample

DSA Python

arr = [10, 20, 30, 40, 50]
print(arr[2])
arr[2] = 35
print(arr)

This code accesses the element at index 2, prints it, updates it to 35, then prints the updated array.

Execution Table

Step	Operation	Index Accessed	Value Read/Updated	Array State
1	Initial array	-	-	[10, 20, 30, 40, 50]
2	Access element	2	Read 30	[10, 20, 30, 40, 50]
3	Update element	2	Updated to 35	[10, 20, 35, 40, 50]
4	Final array print	-	-	[10, 20, 35, 40, 50]
5	End	-	-	Operation complete

💡 Operation ends after accessing and updating the specified index.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
arr	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 35, 40, 50]	[10, 20, 35, 40, 50]
index	-	2	2	2
value	-	30 (read)	35 (updated)	35

Key Moments - 3 Insights

Why does the array state not change after just accessing an element?

Because reading an element at an index does not modify the array, as shown in step 2 where the array remains [10, 20, 30, 40, 50].

What happens if we update an index that is out of range?

The operation would cause an error and stop execution, but in this example, the index 2 is valid, so the update succeeds as shown in step 3.

Why is the updated value reflected immediately in the array?

Because arrays store values at fixed positions, updating index 2 changes the stored value directly, as seen in step 3 where the array changes to [10, 20, 35, 40, 50].

Visual Quiz - 3 Questions

Look at the execution table, what is the value read at step 2?

A 30

B 35

C 20

D 40

💡 Hint

Check the 'Value Read/Updated' column at step 2 in the execution table.

At which step does the array change its value at index 2?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Array State' column and see when the value at index 2 changes.

If we tried to access index 5 instead of 2, what would happen?

A It would read the value at index 5

B It would cause an error because index 5 is out of range

C It would update the value at index 5

D It would ignore the operation and continue

💡 Hint

Recall that the array length is 5, so valid indices are 0 to 4.

Concept Snapshot

Array Access and Update at Index:
- Use arr[i] to read value at index i
- Use arr[i] = value to update value at index i
- Index must be within array bounds
- Reading does not change array
- Updating changes the stored value immediately

Full Transcript

This lesson shows how to access and update elements in an array by index. We start with an array of five numbers. First, we read the value at index 2, which is 30, without changing the array. Then, we update the value at index 2 to 35, which changes the array immediately. The final array reflects this update. Accessing an index out of range would cause an error. Arrays store values at fixed positions, so updates directly change the stored value.

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