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

Array Access and Update at Index

📖 Scenario: You are managing a list of daily temperatures recorded in 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 a list of temperatures, sets a specific day index to a new temperature, and then prints the updated list.

📋 What You'll Learn

Create a list called temperatures with exactly these values: 23, 25, 22, 20, 24, 26, 21

Create a variable called day_index and set it to 3

Update the temperature at index day_index in temperatures to 19

Print the temperatures list after the update

💡 Why This Matters

🌍 Real World

Lists are used to store collections of data like daily temperatures, sales numbers, or scores. Being able to access and update specific items helps keep data accurate.

💼 Career

Many programming jobs require working with lists or arrays to manage data. Knowing how to access and change elements by index is a fundamental skill.

1

Create the temperatures list

Create a list called temperatures with these exact values in order: 23, 25, 22, 20, 24, 26, 21

DSA Python

# Create the temperatures list with the given values
# Your code here

Hint

Use square brackets [] to create a list and separate values with commas.

2

Set the day index

Create a variable called day_index and set it to the integer 3

DSA Python

temperatures = [23, 25, 22, 20, 24, 26, 21]
# Create the day_index variable and set it to 3
# Your code here

Hint

Use the assignment operator = to set day_index to 3.

3

Update the temperature at the given index

Update the value in the temperatures list at index day_index to 19

DSA Python

temperatures = [23, 25, 22, 20, 24, 26, 21]
day_index = 3
# Update the temperature at index day_index to 19
# Your code here

Hint

Use square brackets with the index variable to access and update the list element.

4

Print the updated temperatures list

Print the temperatures list to show the updated values

DSA Python

temperatures = [23, 25, 22, 20, 24, 26, 21]
day_index = 3
temperatures[day_index] = 19
# Print the temperatures list
# Your code here

Hint

Use the print() function to display the list.

Practice

1. What does the expression arr[2] do in an array arr?

easy

A. Returns the length of the array

B. Adds a new element at index 2

C. Deletes the element at index 2

D. Accesses the element at index 2 in the array

Solution

1. Step 1: Understand array indexing

   Array elements are accessed using their index inside square brackets, starting from 0.

2. Step 2: Access element at index 2

   The expression arr[2] retrieves the third element in the array (index 2).

3. Final Answer:

   Accesses the element at index 2 in the array -> Option D

4. Quick Check:

   arr[2] means element at position 2 [OK]

Hint: Indexing with brackets gets element at that position [OK]

Common Mistakes:

• Thinking index starts at 1 instead of 0
• Confusing access with insertion or deletion
• Assuming arr[2] returns length

2. Which of the following is the correct syntax to update the element at index 1 of array arr to 10?

easy

A. arr[1] = 10

B. arr{1} = 10

C. arr(1) = 10

D. arr.1 = 10

Solution

1. Step 1: Identify correct syntax for array update

   In Python, arrays or lists use square brackets to access or update elements by index.

2. Step 2: Use square brackets for assignment

   To update element at index 1, write arr[1] = 10.

3. Final Answer:

   arr[1] = 10 -> Option A

4. Quick Check:

   Use square brackets for indexing and assignment [OK]

Hint: Use square brackets [] to access or update array elements [OK]

Common Mistakes:

• Using parentheses () instead of brackets []
• Using curly braces {} which are for sets or dicts
• Trying dot notation for indexing

3. What will be the output of the following code?

arr = [5, 10, 15, 20]
arr[2] = 25
print(arr)

medium

A. [5, 10, 25, 20]

B. [5, 10, 15, 20]

C. [25, 10, 15, 20]

D. Error

Solution

1. Step 1: Understand initial array and update

   Initial array is [5, 10, 15, 20]. The statement arr[2] = 25 updates the element at index 2 (third element) to 25.

2. Step 2: Print updated array

   After update, array becomes [5, 10, 25, 20]. Printing it shows this updated list.

3. Final Answer:

   [5, 10, 25, 20] -> Option A

4. Quick Check:

   arr[2] changed to 25, rest unchanged [OK]

Hint: Update changes element at index, print shows new array [OK]

Common Mistakes:

• Thinking arr[2] updates first element
• Expecting original array without change
• Assuming syntax error

4. Identify the error in the following code and choose the correct fix:

arr = [1, 2, 3]
arr[3] = 4
print(arr)

medium

A. TypeError because arr is immutable; fix by converting to list

B. SyntaxError due to wrong assignment; fix by using arr(3) = 4

C. IndexError because index 3 is out of range; fix by using arr.append(4)

D. No error; output is [1, 2, 3, 4]

Solution

1. Step 1: Analyze index used for update

   Array has indices 0,1,2. Trying to assign at index 3 causes IndexError (out of range).

2. Step 2: Correct way to add new element

   To add element at end, use arr.append(4) instead of assigning at invalid index.

3. Final Answer:

   IndexError because index 3 is out of range; fix by using arr.append(4) -> Option C

4. Quick Check:

   Index 3 invalid, append adds element safely [OK]

Hint: Use append() to add beyond last index, not assignment [OK]

Common Mistakes:

• Assigning to index beyond array length
• Using parentheses instead of brackets
• Assuming no error occurs

5. Given arr = [2, 4, 6, 8, 10], which code snippet correctly doubles the value at index 3 and then swaps it with the value at index 1?

hard

A. arr[3] *= 2
arr[1] = arr[3]
arr[3] = arr[1]

B. arr[3] = arr[3] * 2
arr[1], arr[3] = arr[3], arr[1]

C. arr[3] = arr[1]
arr[1] = arr[3] * 2

D. arr[1], arr[3] = arr[3] * 2, arr[1]

Solution

1. Step 1: Double value at index 3

   Original arr[3] is 8. Doubling: arr[3] = 8 * 2 = 16.

2. Step 2: Swap values at index 1 and 3

   Swap arr[1] (4) and arr[3] (16) using tuple unpacking: arr[1], arr[3] = arr[3], arr[1].

3. Final Answer:

   arr[3] = arr[3] * 2 arr[1], arr[3] = arr[3], arr[1] -> Option B

4. Quick Check:

   Double then swap using tuple unpacking [OK]

Hint: Double first, then swap with tuple unpacking [OK]

Common Mistakes:

• Overwriting values before swap
• Incorrect swap order or syntax
• Not doubling before swapping