First and Last Occurrence of Element in DSA Python - Build from Scratch

First and Last Occurrence of Element

📖 Scenario: You have a list of daily temperatures recorded in a week. You want to find the first and last day when a specific temperature was recorded.

🎯 Goal: Build a program that finds the first and last position of a given temperature in the list.

📋 What You'll Learn

Create a list called temperatures with exact values

Create a variable called target_temp with the temperature to find

Use a for loop to find the first occurrence index of target_temp

Use a for loop to find the last occurrence index of target_temp

Print the first and last occurrence indices in the format: First: X, Last: Y

💡 Why This Matters

🌍 Real World

Finding the first and last occurrence of a value in a list is useful in many real-world tasks like analyzing sensor data, logs, or user activity.

💼 Career

This skill helps in data processing, debugging, and building search features in software development.

1

Create the temperature list

Create a list called temperatures with these exact values: [23, 25, 23, 22, 25, 23, 24]

DSA Python

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

Hint

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

2

Set the target temperature

Create a variable called target_temp and set it to 23

DSA Python

temperatures = [23, 25, 23, 22, 25, 23, 24]
# Create the variable target_temp and set it to 23
# Your code here

Hint

Use the assignment operator = to set the value.

3

Find first and last occurrence

Use two for loops with variables i to find the first and last index of target_temp in temperatures. Store the first index in first_index and the last index in last_index. Initialize first_index to -1 and last_index to -1 before the loops.

DSA Python

temperatures = [23, 25, 23, 22, 25, 23, 24]
target_temp = 23
# Initialize first_index and last_index
# Use a for loop to find first_index
# Use a for loop to find last_index
# Your code here

Hint

Use range(len(temperatures)) to loop forward and range(len(temperatures) - 1, -1, -1) to loop backward.

4

Print the results

Print the first and last occurrence indices in this exact format: First: X, Last: Y where X is first_index and Y is last_index

DSA Python

temperatures = [23, 25, 23, 22, 25, 23, 24]
target_temp = 23
first_index = -1
last_index = -1
for i in range(len(temperatures)):
    if temperatures[i] == target_temp:
        first_index = i
        break
for i in range(len(temperatures) - 1, -1, -1):
    if temperatures[i] == target_temp:
        last_index = i
        break
# Print the first and last occurrence indices
# Your code here

Hint

Use an f-string to format the output exactly as shown.

Practice

1. What does the term first occurrence of an element in a list mean?

easy

A. The total count of the element in the list

B. The last index where the element appears in the list

C. The index where the element appears first in the list

D. The element with the smallest value in the list

Solution

1. Step 1: Understand the meaning of first occurrence

   The first occurrence means the earliest position (index) where the element appears in the list when reading from left to right.

2. Step 2: Compare with other options

   Other options describe last occurrence, count, or smallest value, which are different concepts.

3. Final Answer:

   The index where the element appears first in the list -> Option C

4. Quick Check:

   First occurrence = first index [OK]

Hint: First occurrence means earliest index found [OK]

Common Mistakes:

• Confusing first occurrence with last occurrence
• Thinking it means the count of the element
• Mixing occurrence with element value

2. Which of the following Python code snippets correctly finds the first occurrence index of element 5 in list arr?

easy

A. arr.find(5)

B. arr.index(5)

C. arr.search(5)

D. arr.locate(5)

Solution

1. Step 1: Recall Python list methods

   Python lists have the method index() to find the first occurrence index of an element.

2. Step 2: Check other options

   Methods find(), search(), and locate() do not exist for Python lists.

3. Final Answer:

   arr.index(5) -> Option B

4. Quick Check:

   Use list.index(element) to find first occurrence [OK]

Hint: Use list.index() to get first occurrence index [OK]

Common Mistakes:

• Using non-existent methods like find or search on lists
• Confusing string methods with list methods
• Trying to use index without handling exceptions

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

arr = [2, 4, 5, 4, 5, 6]
first = arr.index(4)
last = len(arr) - 1 - arr[::-1].index(4)
print([first, last])

medium

A. [1, 3]

B. [3, 1]

C. [1, 4]

D. [4, 1]

Solution

1. Step 1: Find first occurrence of 4

   Using arr.index(4) returns 1 because 4 first appears at index 1.

2. Step 2: Find last occurrence of 4

   arr[::-1] reverses the list to [6, 5, 4, 5, 4, 2]. The first index of 4 in reversed list is 2. So last occurrence index = len(arr) - 1 - 2 = 6 - 1 - 2 = 3.

3. Final Answer:

   [1, 3] -> Option A

4. Quick Check:

   First = 1, Last = 3 [OK]

Hint: Reverse list and use index for last occurrence [OK]

Common Mistakes:

• Mixing up reversed index with original index
• Using last = arr.index(4) instead of reversed search
• Confusing first and last occurrence indexes

4. The following code is intended to find the first and last occurrence of element 7 in list nums. What is the error?

nums = [1, 7, 3, 7, 5]
first = nums.index(7)
last = nums.index(7, -1)
print(first, last)

medium

A. Using negative start index in index() causes ValueError

B. index() cannot find element 7 in the list

C. No error; code works correctly

D. The second index() call should use a positive start index

Solution

1. Step 1: Understand index() with start parameter

   The index() method can take a start index. Using -1 as start means start searching from the last element only.

2. Step 2: Identify the error in last occurrence search

   Using nums.index(7, -1) starts searching from index -1 (last element), so it only checks last element which is 5, not 7, causing ValueError.

3. Final Answer:

   The second index() call should use a positive start index -> Option D

4. Quick Check:

   index(start=-1) limits search to last element [OK]

Hint: Use positive start index in index() for last occurrence [OK]

Common Mistakes:

• Using negative start index in list.index()
• Expecting index() to find last occurrence without correct start
• Not handling ValueError when element not found

5. Given a sorted list data = [1, 2, 2, 2, 3, 4, 5], which approach is best to find the first and last occurrence of 2 efficiently?

hard

A. Use binary search to find first and last occurrence

B. Use linear search from start and end to find occurrences

C. Use list.index() and reversed list index() methods

D. Use count() method to find number of occurrences

Solution

1. Step 1: Understand the list is sorted

   Since the list is sorted, binary search can be used to find positions efficiently in O(log n) time.

2. Step 2: Compare approaches

   Linear search (Use linear search from start and end to find occurrences) is slower (O(n)). Using list.index() and reversed index() (Use list.index() and reversed list index() methods) is also linear. count() (Use count() method to find number of occurrences) gives count but not positions.

3. Step 3: Best approach

   Binary search can be adapted to find first and last occurrence indexes quickly.

4. Final Answer:

   Use binary search to find first and last occurrence -> Option A

5. Quick Check:

   Sorted list + binary search = efficient occurrence search [OK]

Hint: Use binary search on sorted list for fast occurrence indexes [OK]

Common Mistakes:

• Using linear search on sorted lists
• Using count() instead of finding indexes
• Not leveraging sorted property for efficiency