How do I get this formula to work and put the return function into the print statement so that it just outputs a money value?

Question

def compound_interest(P, r, n, Y):
    '''
    Computes the future value of investment after Y years

    P: initial investment principal
    r: the annual interest rate
    n: the number of times per year interest will be compounded
    Y: the number of years over which to invest

    P = float(input("Enter your starting principal ($): "))
    r = float(input("Enter the annual interest rate (value between 0 and 1): "))
    n = float(input("Enter the number of times per year to compound interest: "))
    Y = float(input("Enter the number of years over which to invest: "))

    returns: the future value of investment
    '''
    compound_interest = ((P)((1+(r/n))**(ny)))
    print("After 10 year (s), you will have", "$" + str(compound_interest))
    return compound_interest

Answer 1

Here is a solution to your problem:

import math

def compound_interest():

    P = float(input("Enter your starting principal ($): "))
    r = float(input("Enter the annual interest rate (value between 0 and 1): "))
    n = float(input("Enter the number of times per year to compound interest: "))
    Y = float(input("Enter the number of years over which to invest: "))
    cpd_interest = P * math.pow(r+1, n * Y)
    print("After {} year (s), you will have {} $".format(Y, cpd_interest))
    return cpd_interest

compound_interest()

I've removed the parameters you give in your function, because you don't need them if you ask for them as input() from the user.

I also improved your calculation: When you want to calculate the interest it should be the starting principal * (the interest percentage +1 to the power (number of years times number of times per year)). I used the math.pow() function for this, and you can see here how it works exactly.

I renamed the variable name from compound_interest to cpd_interest, as it's a bad idea to have variable names the same name as your function.

I also rewrote your print statement, and used a replacement field to correctly format the invested years & interest. You cannot return inside a print statement, returning is always the last thing the function does (unless it returns nothing).