First and Last Occurrence of Element in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the first occurrence index of target in the list.

DSA Python

def first_occurrence(arr, target):
    for i in range(len(arr)):
        if arr[i] == [1]:
            return i
    return -1

Drag options to blanks, or click blank then click option'

Aarr

Bi

Clen(arr)

Dtarget

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing arr[i] with the wrong variable like i or arr.

Returning index without checking the element.

We compare each element arr[i] with the target value to find the first occurrence.

2fill in blank

medium

Complete the code to find the last occurrence index of target in the list.

DSA Python

def last_occurrence(arr, target):
    last_index = -1
    for i in range(len(arr)):
        if arr[i] == [1]:
            last_index = i
    return last_index

Drag options to blanks, or click blank then click option'

Ai

Btarget

Clast_index

Darr

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing arr[i] with i or last_index instead of target.

Not updating last_index correctly.

We update last_index whenever we find an element equal to target to get the last occurrence.

3fill in blank

hard

Fix the error in the code to correctly find the first occurrence of target.

DSA Python

def first_occurrence(arr, target):
    for i in range(len(arr)):
        if arr[i] = [1]:
            return i
    return -1

Drag options to blanks, or click blank then click option'

A==

B<=

C!=

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '==' in if condition.

Using '!=' which checks for inequality.

Use '==' to compare values, '=' is for assignment and causes a syntax error here.

4fill in blank

hard

Fill both blanks to create a dictionary with elements as keys and their first occurrence index as values.

DSA Python

def first_occurrence_dict(arr):
    result = {}
    for i, val in enumerate(arr):
        if val not in [1]:
            [2][val] = i
    return result

Drag options to blanks, or click blank then click option'

Aresult

Barr

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Checking in arr instead of result dictionary.

Assigning to wrong variable.

We check if val is not in result dictionary and then assign the index i to result[val].

5fill in blank

hard

Fill all three blanks to create a dictionary with elements as keys and their last occurrence index as values.

DSA Python

def last_occurrence_dict(arr):
    result = {}
    for i, val in enumerate(arr):
        [1][val] = [2]
    return [3]

Drag options to blanks, or click blank then click option'

Aresult

Bi

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning to wrong variable or key.

Returning wrong variable.

We update result[val] with the current index i to keep the last occurrence, then return result.

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