Array Access and Update at Index in DSA C++

Mental Model

An array is like a row of boxes where each box holds a value and has a number. You can quickly find or change the value in any box by using its number.

Analogy: Imagine a row of mailboxes numbered from 0 to n-1. To get or put a letter, you just go to the mailbox number directly without checking others.

[0] 10 -> [1] 20 -> [2] 30 -> [3] 40 -> [4] 50

Dry Run Walkthrough

Input: array: [10, 20, 30, 40, 50], update index 2 with value 35

Goal: Change the value at index 2 from 30 to 35 and show the updated array

Step 1: Access the element at index 2

[0] 10 -> [1] 20 -> [2] 30 ↑ -> [3] 40 -> [4] 50

Why: We need to find the exact box to update

Step 2: Update the element at index 2 to 35

[0] 10 -> [1] 20 -> [2] 35 ↑ -> [3] 40 -> [4] 50

Why: Replace old value with new value at the correct position

Step 3: Finish update and show final array

[0] 10 -> [1] 20 -> [2] 35 -> [3] 40 -> [4] 50

Why: Update is complete, array reflects new value

Result:

[0] 10 -> [1] 20 -> [2] 35 -> [3] 40 -> [4] 50

Annotated Code

DSA C++

#include <iostream>
#include <vector>

class Array {
public:
    std::vector<int> data;

    Array(std::vector<int> init) : data(init) {}

    void updateAtIndex(int index, int value) {
        if (index < 0 || index >= (int)data.size()) {
            std::cout << "Index out of bounds\n";
            return;
        }
        data[index] = value; // update value at given index
    }

    void print() {
        for (int i = 0; i < (int)data.size(); i++) {
            std::cout << "[" << i << "] " << data[i];
            if (i != (int)data.size() - 1) std::cout << " -> ";
        }
        std::cout << "\n";
    }
};

int main() {
    Array arr({10, 20, 30, 40, 50});
    arr.updateAtIndex(2, 35);
    arr.print();
    return 0;
}

if (index < 0 || index >= (int)data.size()) {

check if index is valid to avoid errors

data[index] = value; // update value at given index

directly replace the value at the specified index

for (int i = 0; i < (int)data.size(); i++) {

iterate over array to print all elements

Output

[0] 10 -> [1] 20 -> [2] 35 -> [3] 40 -> [4] 50

Complexity Analysis

Time: O(1) because accessing or updating an element by index in an array is done directly without looping

Space: O(n) because the array stores n elements in memory

vs Alternative: Compared to linked lists where access by index is O(n), arrays provide instant access and update at any index

Edge Cases

index is negative

prints 'Index out of bounds' and does not update

DSA C++

if (index < 0 || index >= (int)data.size()) {

index is equal to or greater than array size

prints 'Index out of bounds' and does not update

DSA C++

if (index < 0 || index >= (int)data.size()) {

array is empty

any index is out of bounds, so update is rejected

DSA C++

if (index < 0 || index >= (int)data.size()) {

When to Use This Pattern

When you need to quickly find or change a value at a specific position, use array access or update by index because it is direct and fast.

Common Mistakes

Mistake: Trying to update an index outside the array bounds causing runtime errors
Fix: Always check if the index is within valid range before updating

Summary

Allows direct reading or changing of a value at a specific position in an array.

Use when you know the exact position and want fast access or update.

The key is that arrays let you jump straight to any position without checking others.

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