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

Concept Flow - Check if Stack is Empty or Full

Start

↓

Check if top == -1?

Yes→Stack is Empty

|No↓

Check if top == max_size-1?

Yes→Stack is Full

|No↓

Stack has space

↓

End

The flow checks the top pointer to decide if the stack is empty or full, then concludes the stack state.

Execution Sample

DSA C

int isEmpty() {
  return top == -1;
}

int isFull() {
  return top == max_size - 1;
}

This code checks if the stack is empty or full by comparing the top index with boundary values.

Execution Table

Step	Operation	top Value	max_size	Condition Checked	Result	Stack Visual State
1	Initial state	-1	5	top == -1	True	[] (empty)
2	Check isEmpty()	-1	5	top == -1	True	[] (empty)
3	Push 10	0	5	top == -1?	No	[10]
4	Check isEmpty()	0	5	top == -1	False	[10]
5	Push 20	1	5	top == max_size-1 (4)?	No	[10, 20]
6	Push 30	2	5	top == max_size-1 (4)?	No	[10, 20, 30]
7	Push 40	3	5	top == max_size-1 (4)?	No	[10, 20, 30, 40]
8	Push 50	4	5	top == max_size-1 (4)?	Yes	[10, 20, 30, 40, 50]
9	Check isFull()	4	5	top == max_size-1 (4)	True	[10, 20, 30, 40, 50]
10	Attempt Push 60	4	5	top == max_size-1 (4)?	Yes - Cannot push	[10, 20, 30, 40, 50]
11	Pop	3	5	top == -1?	No	[10, 20, 30, 40]
12	Check isFull()	3	5	top == max_size-1 (4)?	No	[10, 20, 30, 40]
13	Check isEmpty()	3	5	top == -1?	No	[10, 20, 30, 40]
14	Pop all to empty	-1	5	top == -1?	Yes	[] (empty)
15	Check isEmpty()	-1	5	top == -1?	True	[] (empty)

💡 Execution stops after stack is empty and checks confirm empty state.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9	After Step 11	After Step 14
top	-1	0	1	2	3	4	4	3	-1
max_size	5	5	5	5	5	5	5	5	5
Stack	[]	[10]	[10, 20]	[10, 20, 30]	[10, 20, 30, 40]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40]	[]

Key Moments - 3 Insights

Why does the stack start with top = -1 instead of 0?

Because top = -1 means the stack is empty (no elements). This is shown in execution_table step 1 where top == -1 means empty stack.

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

Because top points to the last filled index. When top equals max_size - 1, the stack has no space left, as shown in step 8 and 9.

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

The push operation is blocked and top does not change, as shown in step 10 where top remains 4 and stack state is unchanged.

Visual Quiz - 3 Questions

Look at the execution_table, what is the top value after pushing 30 (step 6)?

A 1

B 2

C 3

D 4

💡 Hint

Check the 'top Value' column at step 6 in execution_table.

At which step does the stack become full?

A Step 8

B Step 7

C Step 9

D Step 10

💡 Hint

Look for when 'top == max_size-1' condition becomes True in execution_table.

If max_size was 3 instead of 5, at which step would the stack be full?

A Step 7

B Step 5

C Step 6

D Step 8

💡 Hint

max_size-1 would be 2, so top == 2 means full; check when top reaches 2 in variable_tracker.

Concept Snapshot

Stack uses 'top' index to track elements.
Empty stack: top == -1.
Full stack: top == max_size - 1.
Check empty/full by comparing top.
Push increments top if not full.
Pop decrements top if not empty.

Full Transcript

This concept shows how to check if a stack is empty or full by using the 'top' index. The stack starts empty with top = -1. When top equals max_size - 1, the stack is full. The code checks these conditions to prevent invalid push or pop operations. The execution table traces pushing elements until full, checking empty and full states, and popping elements back to empty. Variable tracker shows how 'top' and stack contents change step-by-step. Key moments clarify why top starts at -1 and how full condition is detected. The quiz tests understanding of top values and full condition steps.