Dynamic Stack Using Resizable Array in DSA Python - Execution Trace

Concept Flow - Dynamic Stack Using Resizable Array

Start

↓

Push Element

↓

Check Capacity

↓

Add Element at Top

Yes↓

Resize Array (Double Size)

↓

Copy Old Elements to New Array

↓

Add Element at Top

↓

Update Top Index

↓

Pop Element

↓

Check if Array is Too Large

↓

Remove Top Element

Yes↓

Resize Array (Half Size)

↓

Copy Elements to Smaller Array

↓

Update Top Index

↓

End

The stack adds elements at the top, resizing the array when full by doubling its size, and shrinks the array by half when too empty after popping.

Execution Sample

DSA Python

stack = DynamicStack()
stack.push(10)
stack.push(20)
stack.push(30)
stack.pop()
stack.push(40)

This code pushes three elements, pops one, then pushes another, showing dynamic resizing and stack state changes.

Execution Table

Step	Operation	Top Index	Array Capacity	Nodes in Stack	Pointer Changes	Visual State
1	Initialize stack	-1	2	0	top = -1	[] (empty)
2	Push 10	0	2	1	top = 0	[10, _]
3	Push 20	1	2	2	top = 1	[10, 20]
4	Push 30 (resize)	2	4	3	Resize: capacity 2->4, top=2	[10, 20, 30, _]
5	Pop (remove 30)	1	4	2	top = 1	[10, 20, _, _]
6	Push 40	2	4	3	top = 2	[10, 20, 40, _]

💡 Execution stops after final push; stack has 3 elements with capacity 4.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
top	-1	0	1	2	1	2
capacity	2	2	2	4	4	4
stack array	[]	[10, _]	[10, 20]	[10, 20, 30, _]	[10, 20, _, _]	[10, 20, 40, _]
size	0	1	2	3	2	3

Key Moments - 3 Insights

Why does the stack resize when pushing the third element?

At step 4 in the execution_table, the array capacity is full (2), so it doubles to 4 before adding the new element.

Why does the top index decrease after popping an element?

At step 5, popping removes the top element, so top moves from 2 to 1 to point to the new top element.

Why don't we shrink the array immediately after popping one element?

The stack only shrinks when usage is low to avoid frequent resizing; here capacity remains 4 after pop at step 5.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What is the new capacity of the array after resizing?

A 4

B 3

C 2

D 6

💡 Hint

Check the 'Array Capacity' column at step 4 in the execution_table.

At which step does the top index become 1?

A Step 2

B Step 3

C Step 5

D Step 6

💡 Hint

Look at the 'Top Index' column in the execution_table and variable_tracker.

If we push one more element after step 6, what will happen to the array capacity?

A It will double from 4 to 8

B It will stay at 4

C It will shrink to 2

D It will increase by 1

💡 Hint

At step 6, capacity is 4 and size is 3; pushing one more element fills capacity and triggers resizing.

Concept Snapshot

Dynamic Stack Using Resizable Array:
- Uses an array to store stack elements
- top index tracks the last element
- Push adds element; if full, array doubles size
- Pop removes element; array can shrink if too empty
- Resizing copies elements to new array
- Efficient memory use with dynamic resizing

Full Transcript

This visualization shows a dynamic stack implemented with a resizable array. The stack starts empty with capacity 2. Each push adds an element at the top index. When pushing the third element, the array is full, so it resizes by doubling capacity to 4 and copies existing elements. Popping removes the top element and moves the top index down. The array does not shrink immediately after one pop to avoid frequent resizing. The visual state shows the array contents and top pointer after each operation, helping understand how the stack grows and shrinks dynamically.