posted 09-27-2000 01:36 AM                  

"A good way to accumulate money is to save regularly. Suppose you deposit D dollars into a bank account on January 1st, and the first of the month thereafter deposit an additional S dollars each month. Suppose further that the bank pays I percent interest annually, calculated once per month, sometime other than the first. Assuming rigorous self-control on your part, in that you don't touch the money until December 31st some Y years later, you will have accumulated quite a pile, depending of course on the values of D, S, I, Y. Write a procedure SAVINGS-ESTIMATOR that, given these four input values, calculates the amount of that you would accumulate under this plan. You should assume that Y is an integer, and the rest are floating point numbers.

Here is an example of how your procedure will be called: If someone were to start with $1000, with additional monthly deposits of $100/month for 18 years, assuming 6% annual interest, the call to your procedure would look like this:

(savings-estimator 1000.0 100.0 6.0 18)"

I know that that procedure's particular answer is 41572.1, so if I want to test mine when I'm finished I can find out if its right. Any ideas WHERE to start with this thing? I'm no good at word problems.

posted 09-27-2000 01:46 AM                  

Mmmm factorial easiness=good

(define (factorial n) (factorial-helper n 1))
(define (factorial-helper n result)
(if (not(integer? n)) #f
(if (negative? n) #f
(if (= n 0) result
(factorial-helper (- n 1)
(* result n))))))

Edit: Formatting lost in post

posted 09-27-2000 02:06 AM                  

code:

---

(define (savings-estimator d s i y)
        (savings-estimator-month  d s (/ i 12) (* y 12)))

(define (savings-estimator-month d s i m)
        (if (= m 0)
        d
        (savings-estimator-month (+ d s (* d (/ i 100))) s i (- m 1))))

---

its tail-recursive.

first, turn it into something nice and easy.
instead of dealing with annual interest and years past and month contributions, change it into monthl-based stuff.

so i made a helper fxn called estimate-interest-month that works in months, and just called that from the regular estimate-interest. (divide the interest rate by 12, and multiply years by 12 to get number of months. pass everyting else the same.

then the interesting stuff
each month, you'll end up with the amount you start with, plus interest for that month, plus monthly contribution.

keep doing that, until you have no more months left (- m 1)

[This message has been edited by Lt.Hawkins (edited 09-27-2000).]

posted 09-27-2000 02:20 AM                  

edit: its almost workin perfectly, will tweak

Lt: If you ever need to abuse processing power or bandwidth (or would like a northwestern email account) I'll try to hook you up.

posted 09-27-2000 02:32 AM                  

(actually, many times, so long as you make one of the variables a float and its used in calculations with all the others, then it'll still come out correct.

good ngith. i'm zonked from all those women you sent over.