Array Access and Update at Index in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after updating an array element

What is the output of the following C++ code snippet?

DSA C++

int arr[5] = {10, 20, 30, 40, 50};
arr[2] = 100;
for(int i = 0; i < 5; i++) {
    std::cout << arr[i] << " ";
}

A 10 20 100 40 50

B 10 20 30 40 50

C 10 100 30 40 50

D 100 20 30 40 50

💡 Hint

Remember that array indexing starts at 0.

The element at index 2 (third element) is updated from 30 to 100. The rest remain unchanged.

❓ Predict Output

intermediate

Value of array element after multiple updates

What is the value of arr[1] after executing this code?

DSA C++

int arr[3] = {5, 10, 15};
arr[1] = arr[1] + 20;
arr[1] = arr[1] - 5;

A 15

B 30

C 10

D 25

💡 Hint

Add and subtract carefully step by step.

Initially arr[1] is 10. After adding 20, it becomes 30. Then subtracting 5 results in 25.

🔧 Debug

advanced

Identify the error in array update

What error will this code produce when compiled or run?

DSA C++

int arr[4] = {1, 2, 3, 4};
arr[4] = 10;
std::cout << arr[4];

A Runtime error: out-of-bounds access

B No error, output is 10

C Compilation error: index out of range

D Logical error: arr[4] remains unchanged

💡 Hint

Array indices start at 0 and go up to size-1.

Index 4 is out of bounds for an array of size 4 (valid indices 0 to 3). Accessing arr[4] causes undefined behavior, often a runtime error.

❓ Predict Output

advanced

Final array state after multiple updates

What is the printed output after running this code?

DSA C++

int arr[6] = {1, 2, 3, 4, 5, 6};
for(int i = 0; i < 6; i++) {
    if(i % 2 == 0) {
        arr[i] = arr[i] * 2;
    } else {
        arr[i] = arr[i] + 3;
    }
}
for(int i = 0; i < 6; i++) {
    std::cout << arr[i] << " ";
}

A 1 5 3 7 5 9

B 2 2 6 4 10 6

C 2 5 6 7 10 9

D 2 5 6 4 10 9

💡 Hint

Check even and odd indices separately.

Even indices (0,2,4) are doubled: 1*2=2, 3*2=6, 5*2=10. Odd indices (1,3,5) add 3: 2+3=5, 4+3=7, 6+3=9.

🧠 Conceptual

expert

Effect of pointer arithmetic on array update

Given the code below, what is the output after execution?

DSA C++

int arr[3] = {10, 20, 30};
int *p = arr;
*(p + 1) = 50;
std::cout << arr[0] << " " << arr[1] << " " << arr[2] << " ";

A 50 20 30

B 10 50 30

C 10 20 50

D 10 20 30

💡 Hint

Pointer arithmetic moves the pointer by element size.

Pointer p points to arr[0]. *(p + 1) accesses arr[1], which is updated to 50. Others remain unchanged.

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