Stacks and Queues
Understand the stack (LIFO) and queue (FIFO) data structures and their applications.
Stacks and Queues is a free C Academy lesson on CoddyKit — lesson 2 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
1
Stacks and Queues in C
Stacks and queues are linear data structures used for storing and managing data.
In this lesson, you will learn:
- How stacks work (LIFO - Last In, First Out).
- How queues work (FIFO - First In, First Out).
- How to implement stacks and queues in C.
2
What is a Stack?
A stack follows the Last In, First Out (LIFO) principle.
Operations:
- Push - Add an element to the top.
- Pop - Remove the top element.
- Peek - View the top element without removing it.
3
Example: Implementing a Stack
This program demonstrates stack operations using an array.
#include <stdio.h>
#define MAX 5
int stack[MAX], top = -1;
void push(int value) {
if (top == MAX - 1) {
printf("Stack Overflow\n");
} else {
stack[++top] = value;
}
}
int pop() {
if (top == -1) {
printf("Stack Underflow\n");
return -1;
} else {
return stack[top--];
}
}
int main() {
push(10);
push(20);
printf("Popped: %d\n", pop());
return 0;
}4
What is a Queue?
A queue follows the First In, First Out (FIFO) principle.
Operations:
- Enqueue - Add an element to the rear.
- Dequeue - Remove an element from the front.
- Peek - View the front element without removing it.
5
Example: Implementing a Queue
This program demonstrates queue operations using an array.
#include <stdio.h>
#define MAX 5
int queue[MAX], front = -1, rear = -1;
void enqueue(int value) {
if (rear == MAX - 1) {
printf("Queue Overflow\n");
} else {
if (front == -1) front = 0;
queue[++rear] = value;
}
}
int dequeue() {
if (front == -1 || front > rear) {
printf("Queue Underflow\n");
return -1;
} else {
return queue[front++];
}
}
int main() {
enqueue(10);
enqueue(20);
printf("Dequeued: %d\n", dequeue());
return 0;
}6
Circular Queue
A circular queue overcomes the limitations of a normal queue by reusing empty spaces.
Operations remain the same but wrap around the array.
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Example: Circular Queue
This program demonstrates a circular queue implementation.
#include <stdio.h>
#define MAX 5
int queue[MAX], front = -1, rear = -1;
void enqueue(int value) {
if ((rear + 1) % MAX == front) {
printf("Queue Overflow\n");
} else {
if (front == -1) front = 0;
rear = (rear + 1) % MAX;
queue[rear] = value;
}
}
int dequeue() {
if (front == -1) {
printf("Queue Underflow\n");
return -1;
}
int data = queue[front];
if (front == rear) {
front = rear = -1;
} else {
front = (front + 1) % MAX;
}
return data;
}
int main() {
enqueue(10);
enqueue(20);
printf("Dequeued: %d\n", dequeue());
return 0;
}8
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Applications of Stacks and Queues
Stacks are used in:
- Function call management (recursion).
- Undo/redo operations.
- Expression evaluation.
Queues are used in:
- Scheduling tasks in operating systems.
- Managing requests in web servers.
- Data transfer in networking.
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Summary
In this lesson, you learned:
- How stacks and queues work.
- How to implement stack and queue operations.
- The applications of stacks and queues.
Next, we will explore trees and graphs in C!
Frequently asked questions
Is the “Stacks and Queues” lesson free?
Yes — the full text of “Stacks and Queues” is free to read here on the web, and the C Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO.
What will I learn in “Stacks and Queues”?
Understand the stack (LIFO) and queue (FIFO) data structures and their applications. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start C Academy?
No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “Stacks and Queues” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this C Academy lesson?
Yes. Every C Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Linked Lists
- Stacks and Queues
- Trees and Graphs