Array Access and Update at Index in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the element at index 2 in the array.

DSA C++

int arr[] = {10, 20, 30, 40, 50};
int value = arr[[1]];
std::cout << value << std::endl;

Drag options to blanks, or click blank then click option'

A4

B1

C3

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 3 or 4 which accesses wrong elements.

Forgetting that indexing starts at 0.

The index 2 accesses the third element in the array, which is 30.

2fill in blank

medium

Complete the code to update the element at index 1 to 100.

DSA C++

int arr[] = {5, 15, 25, 35};
arr[[1]] = 100;
std::cout << arr[1] << std::endl;

Drag options to blanks, or click blank then click option'

A1

B0

C2

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Updating index 0 or 2 instead of 1.

Using an index out of array bounds.

Index 1 is the second element, which we update to 100.

3fill in blank

hard

Fix the error in the code to correctly update the last element of the array.

DSA C++

int arr[] = {7, 14, 21, 28};
arr[[1]] = 35;
std::cout << arr[3] << std::endl;

Drag options to blanks, or click blank then click option'

A3

B4

C2

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 4 which is outside the array.

Using index 2 which updates the third element, not last.

The last element is at index 3, so we must use arr[3] to update it.

4fill in blank

hard

Fill both blanks to print the updated array elements after changing index 0 and index 2.

DSA C++

int arr[] = {1, 2, 3, 4};
arr[[1]] = 10;
arr[[2]] = 30;
for(int i = 0; i < 4; i++) {
    std::cout << arr[i] << " ";
}
std::cout << std::endl;

Drag options to blanks, or click blank then click option'

A0

B1

C2

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping indexes or using wrong indexes for updates.

Using indexes out of array bounds.

Index 0 is updated to 10 and index 2 to 30, so output is 10 2 30 4.

5fill in blank

hard

Fill all three blanks to update elements and print the sum of updated elements at index 1, 2, and 3.

DSA C++

int arr[] = {2, 4, 6, 8};
arr[[1]] = 20;
arr[[2]] = 30;
arr[[3]] = 40;
int sum = arr[1] + arr[2] + arr[3];
std::cout << sum << std::endl;

Drag options to blanks, or click blank then click option'

A0

B1

C2

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Updating wrong indexes not included in sum.

Forgetting to update all three indexes.

We update indexes 1, 2, and 3 to 20, 30, and 40 respectively. Sum is 20 + 30 + 40 = 90.

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