Check if Stack is Empty or Full in DSA Python - Execution Trace

Concept Flow - Check if Stack is Empty or Full

Start

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Check if stack pointer == -1?

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Stack is Empty

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Stack is Full

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End

We check the stack pointer to see if it is -1 (empty) or max_size-1 (full) to determine stack state.

Execution Sample

DSA Python

stack = []
max_size = 3
pointer = -1

# Check empty
is_empty = (pointer == -1)

# Check full
is_full = (pointer == max_size - 1)

This code checks if the stack is empty or full by comparing the pointer with -1 and max_size-1.

Execution Table

Step	Operation	Pointer Value	Condition Checked	Result	Stack Visual
1	Initial state	-1	pointer == -1	True (Empty)	[]
2	Push element 'A'	0	pointer == -1	False	[A]
3	Check if full	0	pointer == max_size-1 (2)	False	[A]
4	Push element 'B'	1	pointer == max_size-1 (2)	False	[A, B]
5	Push element 'C'	2	pointer == max_size-1 (2)	True (Full)	[A, B, C]
6	Pop element	1	pointer == max_size-1 (2)	False	[A, B]
7	Pop element	0	pointer == -1	False	[A]
8	Pop element	-1	pointer == -1	True (Empty)	[]
9	Check if full	-1	pointer == max_size-1 (2)	False	[]

💡 Operations stop after stack pointer returns to -1 (empty) and no more pushes are done.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	Final
pointer	-1	0	1	2	1	0	-1	-1
stack	[]	[A]	[A, B]	[A, B, C]	[A, B]	[A]	[]	[]

Key Moments - 3 Insights

Why does the pointer start at -1 instead of 0?

Pointer at -1 means the stack is empty before any push. See Step 1 in execution_table where pointer == -1 means empty stack.

Why do we check pointer == max_size - 1 to know if stack is full?

Because pointer starts at -1 and increments with each push, max index is max_size - 1. Step 5 shows pointer == 2 (max_size-1) means full.

What happens if we try to push when stack is full?

Pointer won't increase beyond max_size - 1, so push should be blocked. Step 5 shows stack full at pointer 2, no further push allowed.

Visual Quiz - 3 Questions

Look at the execution_table, what is the pointer value after pushing element 'B'?

A 0

B 1

C 2

D -1

💡 Hint

Check Step 4 in execution_table where 'Push element B' updates pointer.

At which step does the stack become full according to the execution_table?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look for 'True (Full)' result in the Result column.

If the pointer was initialized to 0 instead of -1, what would change in the empty check?

A Empty check would be pointer == 0

B Empty check would be pointer == -1

C Empty check would be pointer == max_size - 1

D Empty check would not be needed

💡 Hint

Refer to variable_tracker and concept_flow about pointer initialization and empty condition.

Concept Snapshot

Stack pointer starts at -1 meaning empty stack.
Push increments pointer, pop decrements.
Empty if pointer == -1.
Full if pointer == max_size - 1.
Check these conditions to know stack state.
Prevent push if full, pop if empty.

Full Transcript

This concept shows how to check if a stack is empty or full by using a pointer that tracks the top element index. The pointer starts at -1 when the stack is empty. When we push elements, the pointer increases by one each time. When the pointer reaches max_size - 1, the stack is full. We check if the pointer equals -1 to know if the stack is empty, and if it equals max_size - 1 to know if it is full. The execution table traces these pointer changes and stack states step by step, showing pushes and pops. Key moments clarify why pointer starts at -1 and why full check uses max_size - 1. The visual quiz tests understanding of pointer values and conditions. This helps beginners see how stack fullness and emptiness are tracked simply by pointer comparisons.