Stock Span Problem Using Stack in DSA Python - Build from Scratch

Stock Span Problem Using Stack

📖 Scenario: You are working as a stock analyst. You want to find out how many consecutive days before today the stock price was less than or equal to today's price. This helps in understanding the stock's performance trend.

🎯 Goal: Build a program that calculates the stock span for each day using a stack. The stock span is the number of consecutive days before the current day where the stock price was less than or equal to the current day's price.

📋 What You'll Learn

Create a list called prices with the exact values: [100, 80, 60, 70, 60, 75, 85]

Create an empty list called span to store the span values

Use a stack implemented as a list called stack to help calculate spans

Use a for loop with variable i to iterate over the indices of prices

Inside the loop, use a while loop to pop from stack while the top of the stack points to a price less than or equal to prices[i]

Calculate the span for each day and append it to span

Print the span list at the end

💡 Why This Matters

🌍 Real World

Stock analysts use the stock span problem to understand how long a stock price has been rising or stable, helping in making investment decisions.

💼 Career

This problem teaches stack usage, which is important in many software engineering tasks like parsing, undo operations, and managing function calls.

1

Create the list of stock prices

Create a list called prices with these exact values: [100, 80, 60, 70, 60, 75, 85]

DSA Python

# Create the list of stock prices
# Your code here

Hint

Use square brackets to create a list and separate values with commas.

2

Create the span list and stack

Create an empty list called span and an empty list called stack to use as a stack

DSA Python

prices = [100, 80, 60, 70, 60, 75, 85]
# Create empty lists called span and stack
# Your code here

Hint

Use [] to create empty lists.

3

Calculate the stock span using a stack

Use a for loop with variable i to iterate over the indices of prices. Inside the loop, use a while loop to pop from stack while stack is not empty and prices[stack[-1]] <= prices[i]. Then calculate the span for day i as i + 1 if stack is empty, else i - stack[-1]. Append the span to span and push i onto stack.

DSA Python

prices = [100, 80, 60, 70, 60, 75, 85]
span = []
stack = []
# Calculate the stock span using a stack
# Your code here

Hint

Use range(len(prices)) to loop over indices. Use stack[-1] to get the top of the stack.

4

Print the span list

Print the span list to display the stock span for each day

DSA Python

prices = [100, 80, 60, 70, 60, 75, 85]
span = []
stack = []
for i in range(len(prices)):
    while stack and prices[stack[-1]] <= prices[i]:
        stack.pop()
    if not stack:
        span.append(i + 1)
    else:
        span.append(i - stack[-1])
    stack.append(i)
# Print the span list
# Your code here

Hint

Use print(span) to show the result.