How to iterate until a condition is met in python for loop

Question

I have been working on this simple interest calculator and I was trying to make the for loop iterate until the amount inputted by the user is reached. But I am stuck at the range part, if I assign a range value like range(1 ,11) it will iterate it correctly and print the year in in contrast to the amount but I want the program to iterate until the year in which principal is greater than the amount is reached. My current code is bellow and the final product I want to reach is also attached bellow the current code. I'm new to python so please bare with me if I'm of track. Thanks in advance.

Current code:

principal = float(input("How much money to start? :"))
apr = float(input("What is the apr? :"))
amount = float(input("What is the amount you want to get to? :"))

def interestCalculator():
    global principal
    year = 1
    for i in range(1, year + 1):
        if principal < amount:
            principal = principal + principal*apr
            print("After year " + str (i)+" the account is at " + str(principal))
            if principal > amount:
                print("It would take" + str(year) + " years to reach your goal!")
        else:
            print("Can't calculate interest. Error: Amount is less than principal")

interestCalculator();

Final expected result:

"want the program to iterate until the year in which principal is greater than the amount is reached" - principal is always less than amount reach. — astrochun
– astrochun, Commented Mar 7, 2021 at 22:54
Using while is good practice, otherwise you could also use break to stop the loop just after printing the result. — Malo
– Malo, Commented Mar 7, 2021 at 23:06

DevLounge · Accepted Answer · 2021-03-07 23:43:28Z

5

Instead, you can use a while loop. What I mean here is you can simply:

principal = float(input("How much money to start? :"))
apr = float(input("What is the apr? :"))
amount = float(input("What is the amount you want to get to? :"))


def interestCalculator():
    global principal
    i = 1

    if principal > amount:
        print("Can't calculate interest. Error: Amount is less than principal")

    while principal < amount:
        principal = principal + principal*apr
        print("After year " + str (i)+" the account is at " + str(principal))
        if principal > amount:
            print("It would take" + str(year) + " years to reach your goal!")
        i += 1


interestCalculator()

edited Mar 7, 2021 at 23:43

DevLounge

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answered Mar 7, 2021 at 22:53

Daniyar Aubekerov

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4 Comments

sarem dave Over a year ago

Yes this worked! Still confused why it didn't work with the for loop though?

Malo Over a year ago

You need to add 'break' to stop the for loop when goal is reached.

Daniyar Aubekerov Over a year ago

@AnthonyPerot Thank you!

baptx Over a year ago

So if I understand correctly, Python does not allow an end condition in for loops (unless we break the loop) unlike other programming languages, so we have to use a while loop.

Holger Bille · Accepted Answer · 2022-12-02 11:24:33Z

0

A suggestion for a more pythonic solution

PRINCIPAL = float(input("How much money to start? :"))
APR = float(input("What is the apr? :"))
AMOUNT = float(input("What is the amount you want to get to? :"))


def interestCalculator(principal, apr, amount):
    year = 0
    yield year, principal
    while principal < amount:
        year += 1
        principal += principal*apr
        yield year, principal


for year, amount in interestCalculator(PRINCIPAL, APR, AMOUNT):
    print(f"After year {year} the account is at {amount:.2f}")

if year == 0:
    print("Can't calculate interest. Error: Amount is less than principal")
print(f"It would take {year} years to reach your goal!")

answered Dec 2, 2022 at 11:24

Holger Bille

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