Exercise: Savings goal – Hands-on Computer Science

Download as a script python-exercise-savings.py with the task as comments, to run in VS Code or Positron

Task

Write a program that works out how long savings with interest take to reach a savings goal. It asks for the starting amount, the interest rate in per cent and the goal. For the interest rate it keeps asking until the rate is greater than 0, because at 0 per cent or less the amount would never grow, and the loop over the years would run endlessly. It then prints the amount at the end of every year until the goal is reached, and finally after how many years that was the case.

What it should look like

Starting amount in euros? 1000
Interest rate in per cent? 0
The interest rate must be greater than 0, otherwise the amount never grows.
Interest rate in per cent? 4
Savings goal in euros? 1500
Year 1: 1040.00 euros
Year 2: 1081.60 euros
Year 3: 1124.86 euros
Year 4: 1169.86 euros
Year 5: 1216.65 euros
Year 6: 1265.32 euros
Year 7: 1315.93 euros
Year 8: 1368.57 euros
Year 9: 1423.31 euros
Year 10: 1480.24 euros
Year 11: 1539.45 euros
After 11 years the goal of 1500.00 euros is reached.

Your program

def main():
    amount = float(input("Starting amount in euros? "))
    rate = ask_rate()
    goal = float(input("Savings goal in euros? "))
    years = 0

    # Credit the interest year by year until the goal is reached,
    # and print the amount at the end of each year on the way.

    print(f"After {years} years the goal of {goal:.2f} euros is reached.")


def ask_rate():
    # Keep asking until the interest rate is greater than 0.
    rate = float(input("Interest rate in per cent? "))
    return rate


main()

SHOW HINT 1

In ask_rate the question belongs in a while True loop. Right below it, an if checks whether the interest rate is greater than 0 and in that case leaves the loop with break; otherwise the program prints the note, and the loop asks again from the start. After the loop, return gives back the interest rate.

SHOW HINT 2

A while loop with the condition amount < goal counts the years. In the body the interest is added, amount += amount * rate / 100, then years goes up by one, and the output shows year and amount with :.2f.

SHOW THE SOLUTION

def main():
    amount = float(input("Starting amount in euros? "))
    rate = ask_rate()
    goal = float(input("Savings goal in euros? "))
    years = 0

    while amount < goal:
        amount += amount * rate / 100
        years += 1
        print(f"Year {years}: {amount:.2f} euros")

    print(f"After {years} years the goal of {goal:.2f} euros is reached.")


def ask_rate():
    while True:
        rate = float(input("Interest rate in per cent? "))
        if rate > 0:
            break
        print("The interest rate must be greater than 0, otherwise the amount never grows.")
    return rate


main()

The program has two loops, and the first makes sure that the second comes to an end: only with an interest rate above 0 does amount grow in every round and eventually reach the goal. Nobody knows beforehand how many years that takes, so the second is a while loop in which years counts along.

For the quick ones

Extend the program with a fixed deposit at the end of every year, which it asks for at the start, say 100 euros. Then think about whether the interest rate still has to be greater than 0 with a deposit, and in which case an endless loop still threatens.