Priority Queue Introduction and Concept in DSA C - Build from Scratch

Priority Queue Introduction and Concept

📖 Scenario: Imagine you are managing a hospital emergency room. Patients arrive with different levels of urgency. You want to treat the most urgent patients first. This is a perfect use case for a priority queue.

🎯 Goal: You will build a simple priority queue using an array in C. You will insert patients with their urgency levels and then remove the patient with the highest urgency.

📋 What You'll Learn

Create an array to hold patient urgencies

Create a variable to track the number of patients

Write a function to insert a patient urgency into the queue

Write a function to remove and return the highest urgency patient

Print the queue after insertions and after removal

💡 Why This Matters

🌍 Real World

Priority queues are used in real life to manage tasks that need to be done in order of importance, like emergency rooms or printer job scheduling.

💼 Career

Understanding priority queues helps in software development roles that involve task scheduling, resource management, and real-time systems.

1

Create the initial data structure

Create an integer array called priorityQueue with size 5 and an integer variable called size initialized to 0.

DSA C

int priorityQueue[5];
int size = 0;
// Your code here

Hint

Use int priorityQueue[5]; to create the array and int size = 0; to track how many patients are in the queue.

2

Add a configuration variable

Create an integer variable called maxSize and set it to 5 to represent the maximum size of the priority queue.

DSA C

int priorityQueue[5];
int size = 0;
int maxSize = 5;
// Your code here

Hint

This variable helps us know the limit of the queue size.

3

Implement the insert function

Write a function called insert that takes an integer value and inserts it into priorityQueue at the end, then increments size. Assume size is less than maxSize.

DSA C

int priorityQueue[5];
int size = 0;
int maxSize = 5;

void insert(int value) {
    // Your code here
}

Hint

Insert the value at the current size index and then increase size by 1.

4

Print the queue after insertions

Write a main function that inserts these urgencies into priorityQueue using insert: 30, 50, 20, 40, 10. Then print all elements in priorityQueue separated by spaces.

DSA C

int priorityQueue[5];
int size = 0;
int maxSize = 5;

void insert(int value) {
    priorityQueue[size] = value;
    size++;
}

int main() {
    // Your code here
    return 0;
}

Hint

Use a for loop to print each element in the queue separated by spaces.

Practice

1. What is the main characteristic of a priority queue compared to a regular queue?

easy

A. It does not allow insertion of new elements after creation

B. Elements are always dequeued in the order they were enqueued

C. It only stores elements of the same value

D. Elements are dequeued based on their priority, not just arrival order

Solution

1. Step 1: Understand queue behavior

   A regular queue removes elements in the order they arrive (FIFO).

2. Step 2: Understand priority queue behavior

   A priority queue removes elements based on priority, not arrival order.

3. Final Answer:

   Elements are dequeued based on their priority, not just arrival order -> Option D

4. Quick Check:

   Priority queue = priority-based removal [OK]

Hint: Priority queue removes highest priority first, not first-in [OK]

Common Mistakes:

• Thinking priority queue is same as FIFO queue
• Assuming elements must be unique
• Believing insertion order is always preserved

2. Which of the following is the correct way to declare a priority queue in C using the standard library?

easy

A. There is no built-in priority queue in standard C

B. struct priority_queue pq;

C. PriorityQueue pq = new PriorityQueue();

D. priority_queue<int> pq;

Solution

1. Step 1: Recall C standard library features

   The C standard library does not include a built-in priority queue type.

2. Step 2: Analyze options

   The other options show declaration syntax resembling C++ (priority_queue or struct) or Java (PriorityQueue class), none of which are standard in C.

3. Final Answer:

   There is no built-in priority queue in standard C -> Option A

4. Quick Check:

   C standard library lacks priority queue [OK]

Hint: C standard library lacks priority queue type [OK]

Common Mistakes:

• Confusing C with C++ STL syntax
• Assuming priority queue is built-in in C
• Using Java or C++ syntax in C code

3. Consider the following pseudocode for a max-priority queue operations:

insert(5)
insert(3)
insert(10)
extract_max()
insert(1)
extract_max()

What is the sequence of extracted elements?

medium

A. [10, 3]

B. [5, 3]

C. [10, 5]

D. [3, 1]

Solution

1. Step 1: Insert elements into max-priority queue

   Insert 5, 3, 10. The max element is 10.

2. Step 2: Extract max elements in order

   First extract_max() returns 10. Then insert 1. Next extract_max() returns 5 (next highest).

3. Final Answer:

   [10, 5] -> Option C

4. Quick Check:

   Extracted max elements = [10, 5] [OK]

Hint: Extract max returns highest inserted so far [OK]

Common Mistakes:

• Extracting elements in insertion order
• Confusing min and max priority
• Ignoring new insertions before extraction

4. The following C-like pseudocode tries to implement a priority queue insertion but has a bug:

void insert(int pq[], int *size, int value) {
  pq[*size] = value;
  (*size)++;
  int i = *size - 1;
  while (i > 0 && pq[i] < pq[(i - 1) / 2]) {
    int temp = pq[i];
    pq[i] = pq[(i - 1) / 2];
    pq[(i - 1) / 2] = temp;
    i = (i - 1) / 2;
  }
}

What is the bug in this insertion logic?

medium

A. It uses < instead of > for max-heap property

B. It increments size before inserting value

C. It swaps with wrong parent index

D. It does not update the heap after insertion

Solution

1. Step 1: Understand heap property for max-heap

   Max-heap requires parent >= children, so swap if child > parent.

2. Step 2: Analyze condition in while loop

   Condition uses pq[i] < pq[parent], which is correct for min-heap, not max-heap.

3. Final Answer:

   It uses < instead of > for max-heap property -> Option A

4. Quick Check:

   Max-heap needs child > parent to swap [OK]

Hint: Max-heap swaps when child > parent, not < [OK]

Common Mistakes:

• Using min-heap condition for max-heap
• Incorrect parent index calculation
• Not incrementing size correctly

5. You have a priority queue that stores tasks with integer priorities. You want to process tasks in order of highest priority but also keep track of the order tasks with the same priority were added. Which data structure modification best supports this?

hard

A. Use a simple array sorted by priority only

B. Use a max-heap with a timestamp to break ties

C. Use a queue ignoring priorities

D. Use a stack to process tasks in reverse order

Solution

1. Step 1: Understand requirement for priority and insertion order

   We want highest priority first, but for equal priority, process by insertion order.

2. Step 2: Choose data structure supporting priority and tie-break

   A max-heap with timestamps can break ties by earlier insertion time, preserving order among equals.

3. Final Answer:

   Use a max-heap with a timestamp to break ties -> Option B

4. Quick Check:

   Max-heap + timestamp = priority + insertion order [OK]

Hint: Add timestamp to priority for tie-breaking [OK]

Common Mistakes:

• Ignoring insertion order for equal priorities
• Using simple array without tie-break
• Using stack or queue ignoring priority