Check if Stack is Empty or Full in DSA C

Mental Model

A stack is like a pile where you add or remove items only from the top. Checking if it is empty means seeing if there is nothing in the pile, and checking if it is full means seeing if the pile has reached its limit.

Analogy: Imagine a stack of plates. If there are no plates, the stack is empty. If the stack is as tall as the shelf allows, it is full and you cannot add more plates.

Top -> [ ] [ ] [ ] [ ] [ ]
Index 0  1   2   3   4
Top points to the last added plate or -1 if empty

Dry Run Walkthrough

Input: stack size 5, push 3 items, then check empty and full

Goal: To check if the stack is empty or full after some pushes

Step 1: Initialize stack with size 5 and top = -1

Top = -1, Stack: [ ] [ ] [ ] [ ] [ ]

Why: Start with an empty stack where no items are added yet

Step 2: Push item 10, increment top to 0

Top = 0, Stack: [10] [ ] [ ] [ ] [ ]

Why: Add first item at index 0, stack is no longer empty

Step 3: Push item 20, increment top to 1

Top = 1, Stack: [10] [20] [ ] [ ] [ ]

Why: Add second item at index 1

Step 4: Push item 30, increment top to 2

Top = 2, Stack: [10] [20] [30] [ ] [ ]

Why: Add third item at index 2

Step 5: Check if stack is empty (top == -1)

Top = 2, Stack: [10] [20] [30] [ ] [ ]

Why: Since top is 2, stack is not empty

Step 6: Check if stack is full (top == size-1)

Top = 2, Stack: [10] [20] [30] [ ] [ ]

Why: Top is 2 but size-1 is 4, so stack is not full

Result:

Top = 2, Stack: [10] [20] [30] [ ] [ ]
Empty? false
Full? false

Annotated Code

DSA C

#include <stdio.h>
#include <stdbool.h>
#define MAX 5

// Stack structure
typedef struct {
    int items[MAX];
    int top;
} Stack;

// Initialize stack
void init(Stack* s) {
    s->top = -1; // top = -1 means empty
}

// Check if stack is empty
bool isEmpty(Stack* s) {
    return s->top == -1; // true if no items
}

// Check if stack is full
bool isFull(Stack* s) {
    return s->top == MAX - 1; // true if top at last index
}

// Push item to stack
bool push(Stack* s, int value) {
    if (isFull(s)) {
        return false; // cannot push if full
    }
    s->top++;
    s->items[s->top] = value;
    return true;
}

// Print stack
void printStack(Stack* s) {
    printf("Stack: ");
    for (int i = 0; i <= s->top; i++) {
        printf("[%d] ", s->items[i]);
    }
    printf("\n");
}

int main() {
    Stack s;
    init(&s);

    push(&s, 10);
    push(&s, 20);
    push(&s, 30);

    printStack(&s);

    printf("Empty? %s\n", isEmpty(&s) ? "true" : "false");
    printf("Full? %s\n", isFull(&s) ? "true" : "false");

    return 0;
}

return s->top == -1; // true if no items

Check if stack has no elements by verifying top is -1

return s->top == MAX - 1; // true if top at last index

Check if stack reached max capacity by comparing top to max index

if (isFull(s)) { return false; }

Prevent pushing if stack is already full

s->top++; s->items[s->top] = value;

Add new item on top and update top pointer

Output

Stack: [10] [20] [30] 

Empty? false

Full? false

Complexity Analysis

Time: O(1) because checking top index is a direct comparison

Space: O(1) since no extra space is used beyond fixed stack size

vs Alternative: Compared to dynamic structures, fixed stack checks are faster but less flexible

Edge Cases

Empty stack

isEmpty returns true, isFull returns false

DSA C

return s->top == -1; // true if no items

Full stack

isFull returns true, push is prevented

DSA C

if (isFull(s)) { return false; }

Push on full stack

push returns false and does not add item

DSA C

if (isFull(s)) { return false; }

When to Use This Pattern

When you see a problem asking if a data structure can accept more items or is empty, check for stack empty/full conditions using top pointer comparisons.

Common Mistakes

Mistake: Checking if stack is empty by comparing top to 0 instead of -1
Fix: Use top == -1 to correctly identify empty stack

Mistake: Checking if stack is full by comparing top to size instead of size-1
Fix: Use top == MAX - 1 to correctly identify full stack

Summary

It checks if a stack has no items or is full by looking at the top index.

Use it when you need to know if you can add or remove items from a stack safely.

Remember: top == -1 means empty, top == size-1 means full.