Array Access and Update at Index in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the time complexity of accessing an element at a specific index in an array?

Accessing an element at a specific index in an array takes O(1) time because arrays allow direct access using the index.

beginner

How do you update the value at index 3 in an array named arr in C++?

You update the value by assigning a new value to that index:
arr[3] = newValue;

intermediate

What happens if you try to access an index outside the array bounds in C++?

Accessing an index outside the array bounds leads to undefined behavior. It may cause a crash or return garbage values.

intermediate

Explain why arrays provide fast access to elements compared to linked lists.

Arrays store elements in contiguous memory locations, so accessing an element by index is direct and fast. Linked lists require traversal from the head node to reach an element, which takes more time.

beginner

Show the C++ code snippet to print all elements of an integer array arr of size n.

for (int i = 0; i < n; i++) { std::cout << arr[i] << " "; } std::cout << std::endl;

What is the index of the first element in a C++ array?

A 0

B 1

C -1

D Depends on array size

In C++, arrays are zero-indexed, so the first element is at index 0.

Which of the following is the correct way to update the 5th element of array arr to 10?

A arr[10] = 5;

B arr[4] = 10;

C arr(5) = 10;

D arr[5] = 10;

Since arrays start at index 0, the 5th element is at index 4.

What will happen if you try to access arr[10] when arr has size 10?

A Access the last element

B Compile-time error

C Returns zero

D Undefined behavior

Accessing index 10 is out of bounds for size 10 (indices 0 to 9), causing undefined behavior.

What is the time complexity to update an element at a given index in an array?

A O(1)

B O(log n)

C O(n)

D O(n^2)

Updating an element at a specific index in an array is done in constant time O(1).

Which data structure allows faster access by index?

A Stack

B Linked List

C Array

D Queue

Arrays allow direct access by index, making them faster for this operation than linked lists.

Describe how to access and update an element at a specific index in an array.

Think about how you use the square brackets [] in C++.

Explain the risks of accessing an array index that is out of bounds.

What happens if you try to open a door that doesn't exist?

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