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

Concept Flow - Array Access and Update at Index

Start with array

↓

Access element at index i

↓

Read or update value

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Store updated value if any

↓

End operation

The flow shows starting with an array, accessing an element by index, optionally updating it, and then storing the new value.

Execution Sample

DSA C

int arr[5] = {10, 20, 30, 40, 50};
int x = arr[2];
arr[3] = 100;

This code reads the element at index 2 and updates the element at index 3.

Execution Table

Step	Operation	Index Accessed	Value Read	Value Updated	Array State
1	Initialize array	-	-	-	[10, 20, 30, 40, 50]
2	Access element	2	30	-	[10, 20, 30, 40, 50]
3	Update element	3	40	100	[10, 20, 30, 100, 50]
4	End	-	-	-	[10, 20, 30, 100, 50]

💡 All operations complete; array updated at index 3.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
arr	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 100, 50]	[10, 20, 30, 100, 50]
x	undefined	30	30	30

Key Moments - 3 Insights

Why does accessing arr[2] give 30 and not 20 or 40?

Because array indexing starts at 0, so arr[2] is the third element, which is 30 as shown in step 2 of the execution table.

What happens to the array after updating arr[3] = 100?

The value at index 3 changes from 40 to 100, updating the array state as shown in step 3 of the execution table.

Can we access or update an index outside the array size?

No, accessing or updating outside the array bounds causes undefined behavior; this example only accesses valid indices as shown.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of x after step 2?

A 30

B 40

C 20

D 100

💡 Hint

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

At which step does the array change its state?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Array State' column changes in the execution table.

If we update arr[0] = 99 instead of arr[3], what would be the array state after update?

A [10, 20, 30, 40, 50]

B [10, 20, 30, 100, 50]

C [99, 20, 30, 40, 50]

D [10, 99, 30, 40, 50]

💡 Hint

Updating index 0 changes the first element; check how array state changes in variable_tracker.

Concept Snapshot

Array Access and Update:
- Use arr[index] to read or write.
- Index starts at 0.
- Reading returns value at index.
- Writing updates value at index.
- Accessing invalid index causes error.

Full Transcript

This visual trace shows how an array is accessed and updated at specific indices. We start with an array of five integers. First, we read the value at index 2, which is 30. Then, we update the value at index 3 from 40 to 100. The variable tracker shows how the array and variable x change after each step. Key moments clarify why indexing starts at zero and how updates affect the array. The quiz tests understanding of values at each step and the effect of updates.