Stack Implementation Using Array in DSA C - Execution Trace

Concept Flow - Stack Implementation Using Array

Start

↓

Check if stack is full?

Yes→Cannot push, overflow

No↓

Increment top pointer

↓

Push element at top index

↓

Done

↓

Start

↓

Check if stack is empty?

Yes→Cannot pop, underflow

No↓

Return element at top index

↓

Decrement top pointer

↓

Done

The flow shows how push checks for overflow before adding an element and incrementing top, and pop checks for underflow before decrementing top and returning the element.

Execution Sample

DSA C

int stack[5];
int top = -1;

// Push 10
if(top < 4) stack[++top] = 10;

// Push 20
if(top < 4) stack[++top] = 20;

// Pop
if(top >= 0) top--;

This code pushes 10 and 20 onto the stack and then pops the top element.

Execution Table

Step	Operation	Top Index	Stack Array State	Pointer Changes	Visual State
1	Initialize stack	-1	[_, _, _, _, _]	top = -1	Empty stack
2	Push 10	-1 -> 0	[10, _, _, _, _]	top incremented to 0	10 -> _ -> _ -> _ -> _ -> null
3	Push 20	0 -> 1	[10, 20, _, _, _]	top incremented to 1	10 -> 20 -> _ -> _ -> _ -> null
4	Pop	1 -> 0	[10, 20, _, _, _]	top decremented to 0	10 -> _ -> _ -> _ -> _ -> null
5	End	0	[10, 20, _, _, _]	No change	10 -> _ -> _ -> _ -> _ -> null

💡 Execution stops after pop operation; top is 0 indicating one element left.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
top	-1	0	1	0	0
stack	[_, _, _, _, _]	[10, _, _, _, _]	[10, 20, _, _, _]	[10, 20, _, _, _]	[10, 20, _, _, _]

Key Moments - 3 Insights

Why does top start at -1 instead of 0?

Top starts at -1 to indicate the stack is empty. When we push, top increments first to 0, pointing to the first element. This is shown in execution_table rows 1 and 2.

Why do we increment top before assigning the new element in push?

Incrementing top first moves the pointer to the next free position. Then we assign the element there. This prevents overwriting existing elements. See execution_table rows 2 and 3.

After popping, why does the element remain in the array?

Pop only decrements top; it doesn't erase the value. The element is considered removed because top no longer points to it. This is visible in execution_table row 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of top?

A 0

B 1

C -1

D 2

💡 Hint

Check the 'Top Index' column at step 3 in the execution_table.

At which step does the stack have only one element?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Visual State' column and 'Top Index' to see how many elements are in the stack.

If we try to push when top is 4, what happens according to the concept_flow?

A Push succeeds and top becomes 5

B Push fails due to overflow

C Pop operation is performed instead

D Stack resets to empty

💡 Hint

Refer to the concept_flow where it checks if stack is full before pushing.

Concept Snapshot

Stack using array:
- Use an array and a 'top' index starting at -1.
- Push: increment top, then add element.
- Pop: return element at top, then decrement top.
- Check overflow (top == max size -1) before push.
- Check underflow (top == -1) before pop.

Full Transcript

This visualization shows how a stack is implemented using an array and a top pointer. The top starts at -1 to indicate the stack is empty. When pushing, top increments first to point to the next free slot, then the element is stored there. When popping, the element is not erased but top decrements to exclude it from the stack. Overflow and underflow checks prevent invalid operations. The execution table traces pushing 10 and 20, then popping once, showing how the stack array and top pointer change step-by-step.