Check if Stack is Empty or Full in DSA Python

Mental Model

A stack is like a pile where you add or remove items only from the top. Checking if it is empty means no items are there, and checking if it is full means no more items can fit.

Analogy: Imagine a stack of plates in a cupboard. If there are no plates, the stack is empty. If the cupboard is full and can't hold more plates, the stack is full.

Top -> [ ] ← Bottom (empty stack)
Top -> [5] -> [3] -> [1] ← Bottom (partially filled stack)
Top -> [9] -> [8] -> [7] -> [6] -> [5] ← Bottom (full stack)

Dry Run Walkthrough

Input: stack capacity 3, current stack elements: [5, 3], check empty and full

Goal: Determine if the stack is empty or full at this moment

Step 1: Check if stack size is 0 to see if empty

Stack: 5 -> 3 -> null, size=2, capacity=3

Why: If size is 0, stack is empty; here size is 2, so not empty

Step 2: Check if stack size equals capacity to see if full

Stack: 5 -> 3 -> null, size=2, capacity=3

Why: If size equals capacity, stack is full; here size is 2, capacity 3, so not full

Result:

Stack: 5 -> 3 -> null
Empty? False
Full? False

Annotated Code

DSA Python

class Stack:
    def __init__(self, capacity):
        self.capacity = capacity
        self.items = []

    def is_empty(self):
        return len(self.items) == 0

    def is_full(self):
        return len(self.items) == self.capacity

    def push(self, item):
        if self.is_full():
            print("Stack is full, cannot push")
            return
        self.items.append(item)

    def __str__(self):
        return ' -> '.join(map(str, reversed(self.items))) + ' -> null'

# Driver code
stack = Stack(3)
stack.push(5)
stack.push(3)
print(stack)
print("Empty?", stack.is_empty())
print("Full?", stack.is_full())

return len(self.items) == 0

Check if stack has zero elements to determine emptiness

return len(self.items) == self.capacity

Check if stack size reached capacity to determine fullness

if self.is_full():

Prevent pushing if stack is already full

self.items.append(item)

Add new item to top of stack

Output

5 -> 3 -> null

Empty? False

Full? False

Complexity Analysis

Time: O(1) because checking length and comparing is a constant time operation

Space: O(n) where n is capacity, because stack stores up to n items

vs Alternative: Using a linked list stack would also have O(1) checks for empty but no fixed capacity, so no full check

Edge Cases

Empty stack

is_empty returns True, is_full returns False

DSA Python

return len(self.items) == 0

Stack with one element

is_empty returns False, is_full depends on capacity

DSA Python

return len(self.items) == self.capacity

Stack at full capacity

is_full returns True, push is blocked

DSA Python

if self.is_full():

When to Use This Pattern

When you need to control adding or removing items safely in a fixed-size container, check if the stack is empty or full to avoid errors.

Common Mistakes

Mistake: Checking if stack is empty by comparing to None instead of length zero
Fix: Use len(self.items) == 0 to check emptiness

Mistake: Not checking if stack is full before pushing, causing overflow
Fix: Always check is_full() before push to prevent overflow

Summary

Checks if a stack has no items or is at its maximum capacity.

Use when managing a stack with limited size to avoid errors.

Remember: empty means zero items, full means size equals capacity.