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

First and Last Occurrence of Element

📖 Scenario: You have a list of numbers representing daily temperatures recorded over several days. You want to find the first and last day 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 vector called temperatures with the exact values: 23, 25, 23, 22, 25, 23, 24

Create an integer variable called target and set it to 23

Write a loop to find the first occurrence index of target in temperatures

Write a loop to find the last occurrence index of target in temperatures

Print the first and last occurrence indices separated by a space

💡 Why This Matters

🌍 Real World

Finding the first and last occurrence of an element is useful in analyzing time series data, logs, or any ordered data to know when an event started and ended.

💼 Career

This skill is important for software developers and data analysts who work with arrays or lists and need to locate specific data points efficiently.

1

Create the temperature list

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

DSA C++

#include 
#include 

int main() {
    // Create the vector temperatures with values 23, 25, 23, 22, 25, 23, 24
    // Your code here

    return 0;
}

Hint

Use std::vector<int> and initialize it with the given numbers inside curly braces.

2

Set the target temperature

Create an integer variable called target and set it to 23

DSA C++

#include 
#include 

int main() {
    std::vector temperatures = {23, 25, 23, 22, 25, 23, 24};
    // Create an integer variable target and set it to 23
    // Your code here

    return 0;
}

Hint

Use int target = 23; to create the variable.

3

Find first and last occurrence of target

Write a loop to find the first occurrence index of target in temperatures and store it in firstIndex. Then write a loop to find the last occurrence index of target in temperatures and store it in lastIndex. If target is not found, set both indices to -1.

DSA C++

#include 
#include 

int main() {
    std::vector temperatures = {23, 25, 23, 22, 25, 23, 24};
    int target = 23;

    // Initialize firstIndex and lastIndex to -1
    // Find first occurrence of target in temperatures
    // Find last occurrence of target in temperatures
    // Your code here

    return 0;
}

Hint

Use two separate loops: one from start to find firstIndex, one from end to find lastIndex. Initialize both to -1 before loops.

4

Print the first and last occurrence indices

Print the values of firstIndex and lastIndex separated by a space.

DSA C++

Hint

Use std::cout << firstIndex << " " << lastIndex << std::endl; 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 maximum and minimum elements in the array

B. The sum of all elements in the array

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

D. The number of unique elements in the array

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: Identify what is required

   We need to locate the earliest and latest index where the element appears.

3. Final Answer:

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

4. Quick Check:

   First and last occurrence = positions of element [OK]

Hint: Look for earliest and latest index of element [OK]

Common Mistakes:

• Confusing with sum or max/min of array
• Returning count instead of positions
• Ignoring if element is not present

2. Which of the following C++ code snippets correctly finds the first occurrence of element x in vector arr?

easy

A. int first = 0; while(arr[first] != x) { first++; }

B. int first = -1; for(int i=0; i

C. int first = arr.size(); for(int i=0; i

D. int first = -1; for(int i=arr.size()-1; i>=0; i--) { if(arr[i] == x) { first = i; break; } }

Solution

1. Step 1: Check each option for correctness

   int first = -1; for(int i=0; i

2. Step 2: Identify errors in other options

   int first = 0; while(arr[first] != x) { first++; } may cause out-of-bound error if x not found; int first = arr.size(); for(int i=0; i=0; i--) { if(arr[i] == x) { first = i; break; } } finds last occurrence due to reverse loop.

3. Final Answer:

   int first = -1; for(int i=0; i -> Option B

4. Quick Check:

   Loop from start, break on first match = first occurrence [OK]

Hint: Loop forward, break on first match for first occurrence [OK]

Common Mistakes:

• Looping backward for first occurrence
• Not handling element not found case
• Using wrong initial index values

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

std::vector<int> arr = {2, 3, 5, 3, 7};
int x = 3;
int first = -1, last = -1;
for(int i=0; i<arr.size(); i++) {
  if(arr[i] == x) {
    if(first == -1) first = i;
    last = i;
  }
}
std::cout << first << " " << last << std::endl;

medium

A. 2 3

B. 0 4

C. 1 1

D. 1 3

Solution

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

   Indices: 0->2, 1->3, 2->5, 3->3, 4->7. First 3 at index 1, last 3 at index 3.

2. Step 2: Confirm output printed

   Prints first and last indices: 1 3.

3. Final Answer:

   1 3 -> Option D

4. Quick Check:

   First=1, Last=3 for element 3 [OK]

Hint: Track first set once, update last on every match [OK]

Common Mistakes:

• Confusing indices with values
• Not updating last occurrence correctly
• Printing values instead of indices

4. Identify the bug in this code snippet that tries to find the last occurrence of x in arr:

int last = -1;
for(int i=0; i<arr.size(); i++) {
  if(arr[i] = x) {
    last = i;
  }
}

medium

A. Using assignment (=) instead of comparison (==) in if condition

B. Loop should run backwards from end to start

C. Variable last should be initialized to 0

D. Missing break statement after finding last occurrence

Solution

1. Step 1: Check the if condition

   The condition uses single '=' which assigns x to arr[i], not compares.

2. Step 2: Understand impact of assignment in condition

   Assignment returns value of x, which may cause wrong behavior or infinite loop.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in if condition -> Option A

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Use '==' for comparison, '=' is assignment [OK]

Common Mistakes:

• Confusing '=' and '==' in conditions
• Thinking loop direction causes bug here
• Assuming break needed for last occurrence

5. Given a sorted vector arr and an element x, which approach is the most efficient to find the first and last occurrence of x?

hard

A. Use binary search twice to find first and last positions

B. Use linear search from start to end

C. Sort the array again and then find occurrences

D. Use a hash map to store all indices of elements

Solution

1. Step 1: Understand the array is sorted

   Sorted array allows binary search to find elements efficiently.

2. Step 2: Use binary search twice

   One binary search finds first occurrence, another finds last occurrence in O(log n) time.

3. Step 3: Compare other options

   Linear search is O(n), sorting again is unnecessary, hash map adds extra space and complexity.

4. Final Answer:

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

5. Quick Check:

   Sorted array + binary search = efficient first/last occurrence [OK]

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

Common Mistakes:

• Using linear search on sorted array
• Sorting array again unnecessarily
• Ignoring binary search advantages