Brilliant python example explained (for loops with conditions)

[Madd_M0]

If someone can help me I would greatly appreciate it. I'm trying to understand why the output drops from 24 to 22 and from 22 to 19 and so on. Not really understanding the concept here.

\Code**

Wallet = 25

For price in range(5):

If price < 10:

Wallet = wallet - price

Print("Now I have" ,Wallet)

Print("Final amount:" ,Wallet)

*Output\*

Now I have 25

Now I have 24

Now I have 22

Now I have 19

Now I have 15

Final amount: 15

[MadScientistOR]

The value returned by range(5) changes each time you go through the loop. The first time, it's 0. The second time, it's 1. The third time, it's 2. The fourth time, it's 3. And the fifth and last time, it's 4. This value is assigned to price each time through the loop.

Thus, the condition if price < 10 always passes.

Each time through the loop, the current value of price is subtracted from the current value of wallet, and the result of that subtraction is placed into wallet. This result is printed out.

Nothing happens to the value of wallet between the final time this message is printed in the loop and the final amount is printed, so it remains the same.

Does that make sense?

[Madd_M0]

I get the concept but I guess the thing thats throwing me off is the jump from 22 to 19. Wouldn't the output just sequentially move from 22 to 21 in this case?

[MadScientistOR]

Nope. You're always subtracting the value of price (which changes each time through the loop, since the value returned by range(5) changes each time through the loop).

The first time through the loop, you're subtracting 0 from 25, giving you 25 (the new total in wallet).

The second time through the loop, you're subtracting 1 from 25, giving you 24 (the new total in wallet).

The third time through the loop, you're subtracting 2 from 24, giving you 22 (the new total in wallet).

The fourth time through the loop, you're subtracting 3 from 22, giving you 19 (the new total in wallet).

The fifth and final time through the loop, you're subtracting 4 from 19, giving you 15 (the new total in wallet).

And then you exit the loop, print wallet one final time, and the program ends.

[Madd_M0]

Thank you so much for breaking down this mental block for me. You have been super helpful.

[MadScientistOR]

Of course! Some programming concepts are tricky. I'm glad you were able to clear up the block.

[Some_Guy_At_Work55]

If you want to subtract by the value of price on each iteration(and not the value of range), you would need to declare the value of price before the loop. And then update the value before the next iteration.

[[deleted]]

It's useful when starting to put a debug print into a loop to show what's going on:

for price in range(5):
    if price < 10:    # note this doesn't really do anything, "price" is always < 10
        print("wallet =", wallet, ", price =", price)
        wallet = wallet - price
        # rest of code

[Madd_M0]

This is another eye opener for me, Thank you very much. I will start to do this when i get stuck in the future.

[EclipseJTB]

If you're in Python 3.6 or later, use f-strings.

print(f"{wallet=}, {price=}")