mortgage calculator

[peterjaz]

I'm trying to create a mortagage calculator where visitors can enter a loan amount, interest rate(whole num converted to decimal), and loan length in years. i'm screwing up with exponents and i always get zero as an asnwer. can someone help?
thanks a bunch.

i have a test page posted here:
http://www.preferredhomeloans.com/test.php

here's the formula i'm trying to code:
here's the formula for calculating the payment.
M = P ( J / (1 - (1 + J) -N))
Variables
M = monthly payment
P = principal, the initial amount of the loan
I = the annual interest rate (from 1 to 100 percent)
L = length, the length (in years) of the loan, or at least the
length over which the loan is amortized.
J = monthly interest in decimal form = I / (12 x 100)
N = number of months over which loan is amortized = L x 12
M = P ( J / (1 - (1 + J) -N))

I've attached my code in a text file.

[laserlight]

From what I see, this could be what you need:

<?php
/*
M = monthly payment 
P = principal, the initial amount of the loan 
I = the annual interest rate (from 1 to 100 percent) 
L = length, the length (in years) of the loan, or at least the 
length over which the loan is amortized. 
J = monthly interest in decimal form = I / (12 x 100) 
N = number of months over which loan is amortized = L x 12 
M = P * ( J / (1 - (1 + J) ** -N))

User gives variables P, I, L
*/

function calcMonthlyPayment($init_amt, $int_rate, $duration) {
	//calc monthly interest rate, store in $int_rate
	$int_rate /= 1200.0; //0.0 < int_rate < 1.0
	//calc # of months over which loan is amortized
	$duration /= -12; //negative since -N is used

//now let's sub in the values
$payment = $int_rate + 1;
$payment = pow($payment, $duration);
$payment = 1 - $payment;
$payment = $int_rate / $payment;
$payment *= $init_amt;

//job done, mission accomplished (etc)
return $payment;
}
?>

[sijis]

I've played around with the code and i think i know why you are getting a zero.

first of the statement:
$expon = pow($base, $exp);
outputs this: "INF", which i believe means INFINITE.

the reason that occurs is because the values

$base and $exp are ALWAYS negative.
I've also noticed that $exp always seems to product -360

note this:

if $exp is negative output ok
if $base is negative output ok
if $exp AND $base are negative out ok
this is true if i used whole numbers...

if $exp is a negative decimal with a negative $base i get an error.
otherwise is $exp is a negative decimal with a positive $base, no error.

i think your problem lies within the calculations... why the negatives? then it translates into the pow() issue.
I don't know mortgage, but there must be another way to calulate this.

hth.

[laserlight]

the base is negative?

that shouldnt be the case
By the formula, we must consider:

(1 + J)**(-N)

Now,
J = I / 1200
since 0 < I < 100
J must be postive
As such, (1+J) must be positive.

[laserlight]

Conversely, we could avoid using power() with any negative arguments by using:

function calcMonthlyPayment($init_amt, $int_rate, $duration) {
    //calc monthly interest rate, store in $int_rate
    $int_rate /= 1200; //0.0 < int_rate < 1.0
    //calc # of months over which loan is amortized
    $duration /= 12;

//now let's sub in the values
$payment = $int_rate + 1;
$payment = pow($payment, $duration);
$payment = ($payment - 1) / $payment;
$payment = $int_rate / $payment;
$payment *= $init_amt;

//job done, mission accomplished (etc)
return $payment;
}
?>

mathematically,
M = P ( J / (1 - (1 + J) -N))
M = J / (((J + 1)N - 1) / (J + 1)**N) P

which looks more complex but could actually be simpler since we can place (J + 1)**N into 1 single variable and re-use it.

[sijis]

i always seem to get a negative...

i must admit though, i have no idea what "" means in (1 + J)(-N) but i guessing its exponents

but based on the code in the attachment, it looks different to your explanation.

$base = 1 - ( 1+ $j);

this person is substracting 1 from (1+j)
so its

(1 - (1 + J))**(-N)

in your terms.

[laserlight]

Well, normally a**b denotes "a to the power of b", i.e. exponent.

Exponents take precedence over subtraction where precedence is not clarified by parentheses.

EDIT:
incidentally, it makes sense here, since then one would obviously simply the formula to:
M = P * ( J / (J**-N))

which can be further simplified to:
M = P * ( J**(N + 1) )

[peterjaz]

Thanks to you both! You helped me discover my errors and it is now working. Your kindness and generocity will be rewarded and good fortune will come your way. Peter🙂

[trex005]

laserlight, This function you provided doesn't seem to do the trick.
calcMonthlyPayment(98000, 6.00, 30);
gives me :
Monthly Payments are 39543.4276804

any ideas?

[trex005]

I thought it might want intrest rate in dec format.. so I tried :
calcMonthlyPayment(98000.00, (6.00/100), 30);
and got :
Monthly Payments are 39203.430042769