Priority Queue Introduction and Concept in DSA C - Execution Trace

Concept Flow - Priority Queue Introduction and Concept

Create empty priority queue

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Insert element with priority

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Place element based on priority

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Peek or remove highest priority element

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Update queue state

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Repeat insert/remove as needed

Shows how elements enter the priority queue, get placed by priority, and how the highest priority element is accessed or removed.

Execution Sample

DSA C

pq_insert(5, priority=2);
pq_insert(3, priority=1);
pq_insert(7, priority=3);
int top = pq_pop();
// top should be 7 (highest priority)

Inserts three elements with priorities and removes the highest priority element.

Execution Table

Step	Operation	Nodes in Queue (value:priority)	Pointer Changes	Visual State
1	Insert 5 with priority 2	5:2	head -> 5	[5:2] -> null
2	Insert 3 with priority 1	5:2 -> 3:1	3 placed after 5	[5:2] -> [3:1] -> null
3	Insert 7 with priority 3	7:3 -> 5:2 -> 3:1	7 placed before 5 (highest priority)	[7:3] -> [5:2] -> [3:1] -> null
4	Pop highest priority	5:2 -> 3:1	head moves from 7 to 5	[5:2] -> [3:1] -> null
5	Pop highest priority	3:1	head moves from 5 to 3	[3:1] -> null
6	Pop highest priority	empty	head moves from 3 to null	null
7	Pop on empty queue	empty	no change	null

💡 Queue is empty after all pops; no more elements to remove.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
head	null	points to 5:2	points to 5:2	points to 7:3	points to 5:2	points to 3:1	null	null
size	0	1	2	3	2	1	0	0

Key Moments - 3 Insights

Why does the highest priority element go to the front of the queue?

Because the priority queue always keeps the element with the highest priority at the front, as shown in step 3 where 7:3 is placed before 5:2 (execution_table row 3).

What happens when we pop from an empty priority queue?

No change occurs and the queue remains empty, as shown in step 7 where head stays null and size stays 0 (execution_table row 7).

Why do we update the head pointer when popping?

Because popping removes the front element, so the head pointer must move to the next element, shown in steps 4, 5, and 6 where head moves forward (execution_table rows 4-6).

Visual Quiz - 3 Questions

Look at the execution table, what is the queue state after step 3?

A [5:2] -> [3:1] -> [7:3] -> null

B [7:3] -> [5:2] -> [3:1] -> null

C [3:1] -> [5:2] -> [7:3] -> null

D [5:2] -> [7:3] -> [3:1] -> null

💡 Hint

Check the 'Nodes in Queue' and 'Visual State' columns at step 3 in the execution_table.

At which step does the head pointer move from 7:3 to 5:2?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Pointer Changes' column in the execution_table for when head changes after popping.

If we insert an element with priority 4 after step 3, where will it be placed?

A At the front, before 7:3

B Between 7:3 and 5:2

C Between 5:2 and 3:1

D At the end, after 3:1

💡 Hint

Higher priority means closer to the front; check how 7:3 was placed before 5:2 in step 3.

Concept Snapshot

Priority Queue stores elements with priorities.
Insert places elements so highest priority is at front.
Pop removes the front (highest priority) element.
Head pointer always points to highest priority element.
Empty queue has head = null and size = 0.

Full Transcript

A priority queue is a data structure where each element has a priority. Elements are inserted so that the one with the highest priority is always at the front. When we pop, we remove the front element, which is the highest priority. The head pointer tracks the front element. If the queue is empty, head is null. This example shows inserting elements with priorities 2, 1, and 3, and popping them in order of priority.