Why am I getting list elements < 0 ?

[Mark17]

Hi all!

Here's some code:

price_chg = np.arange(-10,11,2)

stock_prices = [100]

for i in range(10001):
    if (stock_prices[-1] + np.random.choice(price_chg)) < 0:
        stock_prices.append(0)
        continue
    else:
        stock_prices.append(stock_prices[-1] + np.random.choice(price_chg))

Why is the if block failing to keep negative integers out of the list?

[ibreeden]

Why is the if block failing to keep negative integers out of the list?

Because you are generating a new random price in the "else" block which can again be negative.
Apparently you mean to do this:

price_chg = np.arange(-10,11,2)
 
stock_prices = [100]
 
for i in range(10001):
    random_price = stock_prices.append(stock_prices[-1] + np.random.choice(price_chg))
    if random_price < 0:
        stock_prices.append(0)
    else:
        stock_prices.append(random_price)

[Mark17]

Why is the if block failing to keep negative integers out of the list?

Because you are generating a new random price in the "else" block which can again be negative.
Apparently you mean to do this:

price_chg = np.arange(-10,11,2)
 
stock_prices = [100]
 
for i in range(10001):
    random_price = stock_prices.append(stock_prices[-1] + np.random.choice(price_chg))
    if random_price < 0:
        stock_prices.append(0)
    else:
        stock_prices.append(random_price)

When will the "stupid mistakes" end? Rhetorical question. :)

Thank you!

[deanhystad]

max() may be a better choice.

import random
price = 10
stock_prices = [price]
for _ in range(11):
    price = max(0, price + random.randrange(-10, 11, 2))
    stock_prices.append(price)

print(stock_prices)

[naughtyCat]

One line code

import random
stock_prices = [10] + [max(0, 10+random.randrange(-10, 11, 2)) for _ in range(11)]

[deanhystad]

Not the same result. +/-10 is the range of daily fluctuation, not the range of stock_prices.

[naughtyCat]

Not the same result. +/-10 is the range of daily fluctuation, not the range of stock_prices.

two line

import random
price = 10
stock_prices = [price] + [max(0, price+random.randrange(-10, 11, 2)) for _ in range(11)]

[deanhystad]

Still not right. The price fluctuates from the previous day, not the first day. I don't think this can be done with a comprehension because there is no mechanism for remembering what the price was yesterday.

[naughtyCat]

Still not right. The price fluctuates from the previous day, not the first day. I don't think this can be done with a comprehension because there is no mechanism for remembering what the price was yesterday.

ya, you are right, I am learned, thanks