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

Concept Flow - Array Access and Update at Index

Start

↓

Access array at index i

↓

Read value at index i

↓

Update value at index i

↓

End

The flow shows starting with an array, accessing a specific index to read or update the value, then ending the operation.

Execution Sample

DSA C++

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

This code reads the value at index 2 and updates the value at index 3 in the array.

Execution Table

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

💡 All operations completed; array accessed and updated successfully.

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]
val	undefined	30	30	30

Key Moments - 3 Insights

Why does reading 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 execution_table step 2.

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

The value at index 3 changes from 40 to 100, reflected in execution_table step 3 and variable_tracker showing the updated array.

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

No, accessing an index outside 0 to 4 causes undefined behavior; this example only accesses valid indices as shown in the flow and table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value read at step 2?

A 30

B 20

C 40

D 100

💡 Hint

Check the 'Value Read' column at step 2 in the execution_table.

At which step does the array get updated?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Operation' and 'Value Updated' columns in the execution_table.

If we tried to read arr[5], what would happen?

A Read value 50

B Undefined behavior or error

C Read value 0

D Read value 100

💡 Hint

Array size is 5 with indices 0 to 4; accessing index 5 is invalid as explained in key_moments.

Concept Snapshot

Array Access and Update at Index:
- Arrays use zero-based indexing.
- Access with arr[i] reads value at index i.
- Update with arr[i] = val changes value at index i.
- Index must be within array bounds.
- Changes reflect immediately in the array state.

Full Transcript

This concept shows how to access and update values in an array by index. Arrays start at index 0. Reading arr[2] gets the third element. Updating arr[3] changes the fourth element. The execution table tracks each step, showing the array before and after changes. Variable tracker shows how 'arr' and 'val' change. Key moments clarify indexing and valid access. The quiz tests understanding of reading, updating, and invalid access.

Practice

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

easy

A. Accesses the element at index 2 of the array

B. Accesses the element at index 0 of the array

C. Updates the element at index 2 to zero

D. Deletes the element at index 2

Solution

1. Step 1: Understand array indexing

   Arrays use zero-based indexing, so arr[2] refers to the third element.

2. Step 2: Identify the operation

   The expression arr[2] accesses (reads) the element at index 2 without changing it.

3. Final Answer:

   Accesses the element at index 2 of the array -> Option A

4. Quick Check:

   arr[2] means read element at index 2 [OK]

Hint: Remember arrays start at index 0, so arr[2] is third element [OK]

Common Mistakes:

• Thinking arr[2] updates the element
• Confusing index 2 with index 0
• Assuming arr[2] deletes an element

2. Which of the following is the correct syntax to update the 4th element of an integer array nums to 10 in C++?

easy

A. nums(4) = 10;

B. nums[3] = 10;

C. nums[4] == 10;

D. nums{3} = 10;

Solution

1. Step 1: Recall zero-based indexing

   The 4th element is at index 3 because counting starts from 0.

2. Step 2: Check correct syntax for assignment

   Use square brackets for indexing and single equals sign for assignment: nums[3] = 10;.

3. Final Answer:

   nums[3] = 10; -> Option B

4. Quick Check:

   Use nums[3] = 10; to update 4th element [OK]

Hint: Use square brackets and single = to update array element [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using double equals (==) instead of single equals (=)
• Using curly braces for indexing

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

int arr[5] = {1, 2, 3, 4, 5};
arr[1] = arr[3] + 2;
std::cout << arr[1];

medium

A. 6

B. 4

C. 2

D. 5

Solution

1. Step 1: Identify initial array values

   arr = [1, 2, 3, 4, 5]

2. Step 2: Calculate new value for arr[1]

   arr[3] is 4, so arr[1] = 4 + 2 = 6

3. Step 3: Output the updated arr[1]

   std::cout prints 6

4. Final Answer:

   6 -> Option A

5. Quick Check:

   arr[1] = arr[3] + 2 = 6 [OK]

Hint: Remember arr[3] is 4, so arr[1] becomes 6 [OK]

Common Mistakes:

• Confusing arr[1] and arr[3] values
• Forgetting to add 2
• Printing old value of arr[1]

4. Find the error in the following code snippet that tries to update the 2nd element of array data to 100:

int data[3] = {10, 20, 30};
data(1) = 100;

medium

A. Cannot assign integer to array element

B. Array size is too small

C. Using parentheses instead of square brackets for indexing

D. Missing semicolon after array declaration

Solution

1. Step 1: Check array indexing syntax

   Arrays use square brackets [] for indexing, not parentheses ().

2. Step 2: Identify the incorrect line

   data(1) = 100; is invalid syntax; correct is data[1] = 100;.

3. Final Answer:

   Using parentheses instead of square brackets for indexing -> Option C

4. Quick Check:

   Use square brackets [] to access array elements [OK]

Hint: Use [] for array access, not () [OK]

Common Mistakes:

• Using parentheses for indexing
• Assuming array size is wrong
• Missing semicolon errors

5. Given an array int scores[5] = {10, 20, 30, 40, 50};, which code snippet correctly doubles the value at index 2 and then updates index 4 to be the sum of the new index 2 value and index 0 value?

hard

A. scores[2] *= 2;
scores[4] = scores[4] + scores[0];

B. scores[2] = scores[2] * 2;
scores[4] = scores[2] + scores[4];

C. scores[2] = scores[2] * scores[2];
scores[4] = scores[0] + scores[2];

D. scores[2] *= 2;
scores[4] = scores[0] + scores[2];

Solution

1. Step 1: Initial array

   scores = [10, 20, 30, 40, 50]

2. Step 2: Double index 2

   scores[2] = 30 * 2 = 60

3. Step 3: Update index 4

   scores[4] = 60 + 10 = 70

4. Step 4: Trace options

   • Snippet with *= 2 then [4] = [4] + [0]: [2]=60, [4]=50+10=60 (wrong)
   • Snippet with *= 2 then [4] = [0] + [2]: [2]=60, [4]=10+60=70 (correct)
   • Snippet with [2] = [2] * [2] then [4] = [0] + [2]: [2]=900, [4]=910 (wrong)
   • Snippet with [2] = [2] * 2 then [4] = [2] + [4]: [2]=60, [4]=60+50=110 (wrong)

5. Final Answer:

   scores[2] *= 2; scores[4] = scores[0] + scores[2]; -> Option D

6. Quick Check:

   scores[2] *= 2 (60); scores[4] = 10 + 60 (70) [OK]

Hint: Verify doubling uses *2 or equivalent, sum uses new [2] + [0] exactly [OK]

Common Mistakes:

• Adding to old index 4 value
• Squaring index 2 instead of doubling
• Summing new index 2 with index 4