First and Last Occurrence of Element in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C#

int FirstOccurrence(int[] arr, int target) {
    for (int i = 0; i < arr.Length; i++) {
        if (arr[i] == [1]) {
            return i;
        }
    }
    return -1;
}

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

ALength

Btarget

Carr

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using arr instead of target in the comparison.

Comparing with i instead of target.

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 array.

DSA C#

int LastOccurrence(int[] arr, int target) {
    for (int i = arr.Length - 1; i >= 0; i--) {
        if (arr[i] == [1]) {
            return i;
        }
    }
    return -1;
}

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

Aarr

Bi

Ctarget

DLength

Attempts:

3 left

💡 Hint

Common Mistakes

Using arr instead of target in the comparison.

Comparing with i instead of target.

We compare each element arr[i] with the target value starting from the end to find the last occurrence.

3fill in blank

hard

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

DSA C#

int FindFirst(int[] arr, int target) {
    int index = -1;
    for (int i = 0; i < arr.Length; i++) {
        if (arr[i] [1] target) {
            index = i;
            break;
        }
    }
    return index;
}

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

Atarget

Barr[i]

C!=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using '!=' which checks for inequality.

The if condition must use '==' to compare values, not '=' which assigns a value.

4fill in blank

hard

Fill both blanks to create a dictionary of element counts for the array.

DSA C#

Dictionary<int, int> counts = new Dictionary<int, int>();
foreach (int num in arr) {
    if (counts.ContainsKey([1])) {
        counts[[2]]++;
    } else {
        counts[num] = 1;
    }
}

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

Anum

Barr

Ci

Dtarget

Attempts:

3 left

💡 Hint

Common Mistakes

Using the array name instead of the current element variable.

Using an index variable that does not exist in the loop.

We check and update the count for the current number 'num' in the dictionary.

5fill in blank

hard

Fill all three blanks to create a dictionary of elements with their first occurrence index.

DSA C#

Dictionary<int, int> firstIndices = new Dictionary<int, int>();
for (int i = 0; i < arr.Length; i++) {
    if (!firstIndices.ContainsKey([1])) {
        firstIndices[[2]] = [3];
    }
}

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

Aarr[i]

Bi

DfirstIndices

Attempts:

3 left

💡 Hint

Common Mistakes

Using the dictionary name instead of the element as key.

Assigning the element instead of the index as value.

We check if the dictionary has the element arr[i], then store its index i as the first occurrence.

Practice

1. What does the "First and Last Occurrence of Element" problem ask you to find in an array?

easy

A. The number of unique elements in the array

B. The sum of all elements in the array

C. The maximum and minimum elements in the array

D. The first and last positions where a given element appears

Solution

1. Step 1: Understand the problem statement

   The problem asks to find the first and last positions of a specific element in an array.

2. Step 2: Differentiate from other array queries

   It is not about sums, max/min, or counting unique elements, but about positions of a target element.

3. Final Answer:

   The first and last positions where a given element appears -> Option D

4. Quick Check:

   Positions of element = first and last occurrence [OK]

Hint: Look for positions of target element, not values or counts [OK]

Common Mistakes:

• Confusing with finding max or min element
• Trying to sum elements instead of finding positions
• Counting unique elements instead of occurrences

2. Which of the following C# code snippets correctly initializes an array to find the first and last occurrence of an element?

easy

A. int arr = {1, 2, 3, 4, 5};

B. int arr = new int[5] {1, 2, 3, 4, 5};

C. int[] arr = new int[] {1, 2, 3, 4, 5};

D. int arr[] = new int[] {1, 2, 3, 4, 5};

Solution

1. Step 1: Check array initialization syntax in C#

   Correct syntax is: int[] arr = new int[] {1, 2, 3, 4, 5};

2. Step 2: Identify incorrect options

   int arr = new int[5] {1, 2, 3, 4, 5}; has wrong syntax (cannot assign array with size and initializer like that), A is missing 'new int[]', D uses C-style syntax invalid in C#.

3. Final Answer:

   int[] arr = new int[] {1, 2, 3, 4, 5}; -> Option C

4. Quick Check:

   Correct C# array syntax = int[] arr = new int[] {1, 2, 3, 4, 5}; [OK]

Hint: Use 'int[] arr = new int[] { ... }' for array initialization [OK]

Common Mistakes:

• Using C/C++ style array declarations
• Omitting 'new int[]' keyword
• Assigning array size with initializer incorrectly

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

int[] arr = {5, 3, 7, 3, 9};
int target = 3;
int first = -1, last = -1;
for (int i = 0; i < arr.Length; i++) {
  if (arr[i] == target) {
    if (first == -1) first = i;
    last = i;
  }
}
Console.WriteLine($"{first} {last}");

medium

A. 3 1

B. 1 3

C. 0 4

D. -1 -1

Solution

1. Step 1: Trace the loop to find first and last occurrence of 3

   Array: [5, 3, 7, 3, 9]. Target = 3. Indices: 0 to 4.
   i=0: arr[0]=5 !=3 skip
   i=1: arr[1]=3 == target, first=-1 so first=1, last=1
   i=2: arr[2]=7 !=3 skip
   i=3: arr[3]=3 == target, first=1 (unchanged), last=3
   i=4: arr[4]=9 !=3 skip

2. Step 2: Print first and last

   first=1, last=3

3. Final Answer:

   1 3 -> Option B

4. Quick Check:

   First and last index of 3 = 1 and 3 [OK]

Hint: Track first when found, update last every time [OK]

Common Mistakes:

• Swapping first and last indices
• Not updating last occurrence correctly
• Initializing first and last with wrong values

4. Identify the error in the following code that tries to find the first and last occurrence of an element:

int[] arr = {2, 4, 2, 5, 2};
int target = 2;
int first = 0, last = 0;
for (int i = 0; i < arr.Length; i++) {
  if (arr[i] == target) {
    first = i;
    last = i;
  }
}
Console.WriteLine($"{first} {last}");

medium

A. first and last are initialized incorrectly, causing wrong output

B. Loop condition should be i <= arr.Length

C. Target value is not assigned properly

D. Console.WriteLine syntax is incorrect

Solution

1. Step 1: Analyze initialization of first and last

   Both first and last start at 0. Inside loop, first and last are set to i every time target is found, so first will be overwritten and end up as last occurrence index.

2. Step 2: Identify correct approach

   First should be set only once when first occurrence is found (e.g., if first == -1). Last should update every time target is found.

3. Final Answer:

   first and last are initialized incorrectly, causing wrong output -> Option A

4. Quick Check:

   Initialize first to -1 and update only once [OK]

Hint: Initialize first to -1 and update only once [OK]

Common Mistakes:

• Overwriting first occurrence index
• Using wrong loop boundary
• Incorrect Console.WriteLine format

5. Given a sorted array with possible duplicates, how can you efficiently find the first and last occurrence of a target element using C#?

hard

A. Use two binary searches: one for first occurrence, one for last occurrence

B. Traverse the array twice: once from start, once from end

C. Use a dictionary to store element counts

D. Sort the array again and find the target

Solution

1. Step 1: Understand the array is sorted with duplicates

   Sorted array allows binary search to find positions efficiently.

2. Step 2: Use binary search twice

   One binary search finds the first occurrence by moving left when target found, another finds last occurrence by moving right.

3. Step 3: Avoid full traversal or extra sorting

   Traversing twice or sorting again is inefficient. Dictionary counts do not give positions.

4. Final Answer:

   Use two binary searches: one for first occurrence, one for last occurrence -> Option A

5. Quick Check:

   Binary search twice = efficient first and last occurrence [OK]

Hint: Binary search twice for first and last occurrence in sorted array [OK]

Common Mistakes:

• Using linear search on large sorted arrays
• Sorting array again unnecessarily
• Using dictionary counts instead of positions