Array Access and Update at Index in DSA C++ - Build from Scratch

Array Access and Update at Index

📖 Scenario: You are managing a list of daily temperatures recorded over a week. Sometimes, you need to check the temperature of a specific day or update it if there was a mistake in recording.

🎯 Goal: Build a simple program that creates an array of temperatures, accesses a temperature at a given index, updates the temperature at another index, and then prints the updated array.

📋 What You'll Learn

Create an integer array called temperatures with exactly 7 elements: 23, 25, 22, 20, 24, 26, 21

Create an integer variable called index_to_access and set it to 3

Create an integer variable called index_to_update and set it to 5

Create an integer variable called new_temperature and set it to 28

Access the temperature at index_to_access and store it in a variable called accessed_temperature

Update the temperature at index_to_update in the temperatures array to new_temperature

Print the accessed_temperature

Print the updated temperatures array elements separated by spaces

💡 Why This Matters

🌍 Real World

Arrays are used to store collections of data like daily temperatures, stock prices, or sensor readings where you need quick access and updates by position.

💼 Career

Understanding array access and update is fundamental for software development, data processing, and working with any indexed data structures.

1

Create the temperatures array

Create an integer array called temperatures with exactly 7 elements: 23, 25, 22, 20, 24, 26, 21

DSA C++

# Your code here

Hint

Use C++ array syntax with curly braces to initialize the array.

2

Create index and new temperature variables

Create an integer variable called index_to_access and set it to 3. Create an integer variable called index_to_update and set it to 5. Create an integer variable called new_temperature and set it to 28.

DSA C++

int temperatures[7] = {23, 25, 22, 20, 24, 26, 21};
# Your code here

Hint

Declare each variable on its own line with the exact names and values.

3

Access and update the array

Access the temperature at index_to_access in the temperatures array and store it in an integer variable called accessed_temperature. Update the temperature at index_to_update in the temperatures array to new_temperature.

DSA C++

int temperatures[7] = {23, 25, 22, 20, 24, 26, 21};
int index_to_access = 3;
int index_to_update = 5;
int new_temperature = 28;
# Your code here

Hint

Use the array name with square brackets and the index variable to access and update elements.

4

Print the accessed temperature and updated array

Print the value of accessed_temperature on one line. Then print all elements of the temperatures array separated by spaces on the next line.

DSA C++

int temperatures[7] = {23, 25, 22, 20, 24, 26, 21};
int index_to_access = 3;
int index_to_update = 5;
int new_temperature = 28;
int accessed_temperature = temperatures[index_to_access];
temperatures[index_to_update] = new_temperature;
# Your code here

Hint

Use cout to print the accessed temperature and a loop to print the array elements separated by spaces.

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