How to do a backward calculation for each record in a dataset

Some (quite a few!) years ago I wrote myself a wee function called
compInt() ("compound interest") to do --- I think --- just what you require.
I have attached the code for this function and a help file for it.

If anyone else wants this code, and if the attachments don't get through the list,
let me know and I can send the stuff to you directly.

   cheers,

       Rolf Turner

On 02/18/2013 10:34 PM, Prakasit Singkateera wrote:

Hi all,

Firstly, it is not a homework. I am working for a hotel booking company in
Thailand but I don't want to explain a complex equation and concept here so
I keep it simple and closely related to what I am trying to solve.I apology
if my question is not clear enough.

I am new to R and previously this problem can be solved easily in Excel
using the "Goal Seek" tool. An example related to my question is when we
use the PMT formula (in Excel) to find the loan payment amount for the
given values of parameters i.e. interest rate, total number of payments,
and principal amount of the loan.

loan_payment_amt_of_each_period =
PMT(interest_rate,total_number_of_payments,principal_amt)

The question is when you know exactly on a monthly basis that you can
afford only X amount of money to pay and you want to know how many months
you have to do the payment given your monthly affordable money, the fixed
interest rate, and the principal amount of loan. Using Goal Seek tool in
Excel, it is like a backward solving for X given Y by not having to
transform anything from the original equation. Simply put the
loan_payment_amt_of_each_period you want and let the software calculate the
total_number_of_payments for you.

Thanks arun. But that was you solved the original equation and put it as a
new formula to R to calculate the result which is easy as long as the
original equation is not complex.


Thanks you,
Prakasit Singkateera


On Mon, Feb 18, 2013 at 1:18 AM, Bert Gunter <gunter.berton at gene.com> wrote:

Homework? We don't do homework here.

-- Bert

On Sun, Feb 17, 2013 at 5:10 AM, Prakasit Singkateera
<asltjoey.rsoft at gmail.com> wrote:

Hi Experts,

I have a dataset of 3 columns:

customer.name     product     cost
John     Toothpaste     30
Mike     Toothpaste     45
Peter     Toothpaste     40

And I have a function of cost whereby

cost = 3.40 + (1.20 * no.of.orders^2)

I want to do a backward calculation for each records (each customer) to
find his no.of.orders and create a new column named "no.of.orders" in

that

dataset but I don't know how to do.

Please help me.

Thank you everyone,
Prakasit

--

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
Website:

http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm

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