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

First and Last Occurrence of Element

📖 Scenario: Imagine you have a list of daily temperatures recorded over several days. You want to find out on which days a specific temperature first and last appeared.

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

📋 What You'll Learn

Create a list of integers called temperatures with exact values

Create an integer variable called targetTemperature with a specific value

Write code to find the first and last occurrence index of targetTemperature in temperatures

Print the first and last occurrence indices in the specified format

💡 Why This Matters

🌍 Real World

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

💼 Career

This skill is important for software developers and data analysts who need to efficiently search and analyze data collections.

1

Create the list of temperatures

Create a list of integers called temperatures with these exact values: 23, 25, 23, 22, 25, 23, 24

DSA C#

# Your code here

Hint

Use List<int> and initialize it with the exact values inside curly braces.

2

Set the target temperature

Create an integer variable called targetTemperature and set it to 23

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List temperatures = new List { 23, 25, 23, 22, 25, 23, 24 };
        // Your code here
    }
}

Hint

Declare an integer variable and assign the value 23.

3

Find first and last occurrence indices

Write code to find the first occurrence index of targetTemperature in temperatures using IndexOf and the last occurrence index using LastIndexOf. Store them in variables called firstIndex and lastIndex respectively.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List temperatures = new List { 23, 25, 23, 22, 25, 23, 24 };
        int targetTemperature = 23;
        // Find first and last occurrence indices
        // Your code here
    }
}

Hint

Use IndexOf and LastIndexOf methods on the list.

4

Print the first and last occurrence indices

Write a print statement to display the first and last occurrence indices in this exact format: First occurrence: X, Last occurrence: Y where X and Y are the values of firstIndex and lastIndex.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List temperatures = new List { 23, 25, 23, 22, 25, 23, 24 };
        int targetTemperature = 23;
        int firstIndex = temperatures.IndexOf(targetTemperature);
        int lastIndex = temperatures.LastIndexOf(targetTemperature);
        // Print the result
        // Your code here
    }
}

Hint

Use Console.WriteLine with string interpolation to print the indices.

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