Push Operation on Stack in DSA Python - Execution Trace

Concept Flow - Push Operation on Stack

Create new element

↓

Check if stack is full?

Yes→Error: Stack Overflow

No↓

Place element on top

↓

Update top pointer

↓

Done

The push operation adds a new element on top of the stack if there is space, updating the top pointer.

Execution Sample

DSA Python

stack = []
stack.append(10)
stack.append(20)
print(stack)

This code pushes 10 and then 20 onto the stack and prints the stack.

Execution Table

Step	Operation	Element Added	Stack State	Top Pointer Index
1	Initialize empty stack	None	[]	None
2	Push 10	10	[10]	0
3	Push 20	20	[10, 20]	1
4	End of operations	None	[10, 20]	1

💡 No more push operations; stack contains two elements with top at index 1.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
stack	[]	[10]	[10, 20]	[10, 20]
top	None	0	1	1

Key Moments - 3 Insights

Why does the top pointer start at None and then become 0 after the first push?

Initially, the stack is empty so top has no valid index (None). After pushing the first element, top points to index 0 where the element is placed (see execution_table step 2).

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

The flow checks if the stack is full before pushing (concept_flow). If full, it stops with an error (stack overflow). This is not shown here because the stack is dynamic.

Why does the stack state show elements in order of insertion?

Because push adds elements on top, the last pushed element is at the end of the list (execution_table rows 2 and 3). The stack grows to the right.

Visual Quiz - 3 Questions

Look at the execution_table, what is the top pointer index after pushing 20?

A 0

B 1

C 2

D None

💡 Hint

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

At which step does the stack first contain two elements?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Stack State' column to see when two elements appear.

If we add a push operation for 30 after step 3, what will be the new top pointer index?

A 2

B 3

C 1

D 0

💡 Hint

Top pointer increases by 1 with each push (see variable_tracker for top).

Concept Snapshot

Push Operation on Stack:
- Adds new element on top
- Check if stack is full before push
- Place element at top + 1 index
- Update top pointer to new index
- Stack grows with last-in element on top

Full Transcript

The push operation on a stack adds a new element on top if there is space. We start with an empty stack and no top pointer. When we push the first element, it goes to index 0 and top points there. Each new push adds the element at the next index and updates top. If the stack is full, push cannot add more elements. The stack state shows elements in order of insertion, with the last pushed element on top. The top pointer tracks the index of the top element. This trace shows pushing 10 then 20, updating the stack and top pointer accordingly.