Python 'For' Loop Iterate Array

Question

I am new to python and I am looking for some help with loop structures, specifically how to use a 'For' loop or alternative loop to solve a problem. I need to figure a discounted price based on the previous days' discounted price in an array so it will iterate through the array an apply the 10% discount. Here is what I have so far:

opnDays = ["mon", "tue", "wed", "thr", "fri"]

price = 10

def discount(array):
    for day in array:
        disPrice = price - (price * .1)
        print(day, disPrice)
discount(opnDays)
>>>
mon 9.0
tue 9.0
wed 9.0
thr 9.0
fri 9.0

I want to get:

>>>
mon 9.0
tue 8.1
wed 7.29
thr 6.561
fri 5.9109

Any help with how to organize this loop to get a the 10% discount from the previous day would be helpful. Thank you

"I need to figure a discounted price based on the previous days' discounted price in an array". Ok, but that code just uses an array (actually, it's a list) for the day names, but it sounds like you're also supposed to use one for the discounted prices as well. As the answers below show, you don't actually need that, but maybe the point of this exercise is to create a list that contains the discounted price for each day. — PM 2Ring
– PM 2Ring, Commented Sep 21, 2018 at 20:54
@JoranBeasley: Your answer will work if either price is defined inside the function OR price is passed to the function in a variable also called price OR you will have to define global price inside the function. — Sheldore
– Sheldore, Commented Sep 21, 2018 at 21:02
Why on earth was this downvoted? Jay asked a clear & simple question, and showed us his code, along with expected output and actual output. — PM 2Ring
– PM 2Ring, Commented Sep 21, 2018 at 21:21

Woody1193 · Accepted Answer · 2018-09-21 20:49:34Z

4

Your code, as written, is close:

opnDays = ["mon", "tue", "wed", "thr", "fri"]

price = 10

def discount(array):
    disPrice = price
    for day in array:
        disPrice *= 0.9
        print(day, disPrice)

What I did here was change how disPrice was set on your loop. You were setting it to the same value (0.9 * price) on every iteration. All I did was set it to price in the beginning and multiply it by 0.9 every iteration, resulting in the desired behavior.

answered Sep 21, 2018 at 20:49

Woody1193

8,1929 gold badges65 silver badges129 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Jay Over a year ago

All very helpful beta. Appreciate the insight.

nosklo · Accepted Answer · 2018-09-21 20:48:09Z

0

You're not changing the value of price so the same result is calculated on every iteration of the loop.

This should work:

for day in array:
    price = price - (price * .1)
    print(day, price)

answered Sep 21, 2018 at 20:48

nosklo

224k58 gold badges299 silver badges299 bronze badges

3 Comments

T Burgis Over a year ago

It would be better to calculate the price using price = price * 0.9

Sheldore Over a year ago

Gives UnboundLocalError: local variable 'price' referenced before assignment. If I use global price inside the function and before the for loop as one of the answers below, it works

Sheldore Over a year ago

@nosklo: I know, it is to replace the OP's for loop. But unless I define global price inside the function, I am getting this UnboundLocalError.

Sheldore · Accepted Answer · 2018-09-21 20:49:38Z

0

If you want to stick to using a function as in your code, following is one way:

opnDays = ["mon", "tue", "wed", "thr", "fri"]

price = 10

def discount(array):
    disPrice = price # assign initial price to discount
    for day in array:
        disPrice = disPrice*0.9 # Reduce the discount by 10 percent every time
        print(day, disPrice)

discount(opnDays)

Output

mon 9.0
tue 8.1
wed 7.29
thr 6.561
fri 5.9049000000000005

answered Sep 21, 2018 at 20:49

Sheldore

39.2k9 gold badges63 silver badges76 bronze badges

Comments

PM 2Ring · Accepted Answer · 2018-09-21 21:00:06Z

Your code is pretty close, but on each day you need to modify the price of the previous day.

I would pass the starting price into the function, just like you pass the list of day names. I've also used .format, so we get the prices printed as dollars & cents. This only affects how the price is printed, it doesn't affect the actual value, so we don't lose any accuracy.

I've also changed your names slightly so that they conform with the usual Python PEP-0008 style.

def discount(array, price):
    for day in array:
        price = price - (price * .1)
        print("{}: {:.2f}".format(day, price))

open_days = ["mon", "tue", "wed", "thr", "fri"]
start_price = 10.00
discount(open_days, start_price)

output

mon: 9.00
tue: 8.10
wed: 7.29
thr: 6.56
fri: 5.90

mad_ · Accepted Answer · 2018-09-21 21:07:37Z

0

You can use global and keep your logic as it is

opnDays = ["mon", "tue", "wed", "thr", "fri"]

price = 10
def discount(array):

    global price

    for day in array:
        price = price - (price * .1)
        print(day, price)
discount(opnDays)

In order to avoid global pass the price as an argument and the preferred approach

opnDays = ["mon", "tue", "wed", "thr", "fri"]

price = 10
def discount(array,price):

    for day in array:
        price = price - (price * .1)
        print(day, round(price,2))
discount(opnDays,price)

In case you need the final price after successive iterations which I believe might be a requirement for you

opnDays = ["mon", "tue", "wed", "thr", "fri"]

price = 10
def discount(array,price):

    for day in array:
        price = price - (price * .1)
        print(day, round(price,2))
    return price
final_price = discount(opnDays,price)
print(final_price)

edited Sep 21, 2018 at 21:07

answered Sep 21, 2018 at 20:54

mad_

8,2832 gold badges32 silver badges46 bronze badges

5 Comments

PM 2Ring Over a year ago

You could use global for this, but it shouldn't be encouraged. Modifiable globals break program modularity. It doesn't matter much for a tiny program like this, but it's a bad habit to get into.

mad_ Over a year ago

I am usually not a fan of using global but in this case, I believe the final price should be the result of iterations in the sense that it will reduce 7 times as original price and should be able to consume later.

Sheldore Over a year ago

@mad_: If I try the code without global price, I get UnboundLocalError: local variable 'price' referenced before assignment

mad_ Over a year ago

@Bazingaa obviously price has not been declared inside the funtion

Sheldore Over a year ago

Yeah, so the solution where you just use price = price - (price * .1) without passing the price to the function would not work. That's what I am saying

Collectives™ on Stack Overflow

Python 'For' Loop Iterate Array

5 Answers 5

1 Comment

3 Comments

Comments

Comments

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

1 Comment

3 Comments

Comments

Comments

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related