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.