[RESOLVED] For Loop Help

amreason

EDIT

I am making a modified compound interest calculator.
Here is my problem:
Say you put $100 (amount) a month in to a 8% (rate) interest rate a year account.
You add this $100 a month for 30 years (years). With the interest compounding every year. How much would you have at the end of the 30 years.

here it is on my server: www.gotomountainview.com/input.php

What is going wrong? The $count is not adding up right somewhere.

<?php
$amount = $POST['amount'];
$years = $POST['years'];
$rate = $_POST['rate'];
?>

<?php

for ($year = 1; $year < $years; $year++)
{

$count = 0 + $count;
$subtotal = 12 ($amount + $count);
$final = $subtotal $rate;
$count = $subtotal + $final;

}
print "$count"
?>

drew010

<?php

$amount = 100;
$period = 30;
$rate   = .08;

for ($i = 1; $i <= $period; ++$i) {
  $amount += $amount * $rate;
}

echo $amount;

?>

seemed to work for me, alternatively, avoid a loop and use the compound interest formula, unless you want to show an amortization table or something.

echo $amount * pow(1 + $rate, $period);

amreason

that does not seem correct. It only came up with 1006.27 dollars. Thats way too small for adding 100 a month for 30 years.

amreason

you forgot to figure in the 12 months a year. How would that be done using your example?

drew010

ah i read it wrong, i didnt see the add 100 a month thing. ill post an update in a minute

amreason

Thanks! My mind is just not getting this one! The answer is probably right under my nose.

drew010

<?php

$amount = 100;
$period = 30;
$rate   = .08;


$balance = 0;

for ($year = 1; $year <= $period; ++$year) {
  //add 100$ a month
  for($month = 1; $month <= 12; ++$month) {
    $balance += $amount;
    echo "After Month $month Year $year: $balance\n";
  }
  $balance += $balance * $rate;
  echo "Compound Interest Year $year: $balance\n";
}

?>

I get a final balance of 146815.04160299

drew010

of course you could do away with the 12 step month inner for loop and just do a multiplication by 12 like you have. just for illustration in my example.

amreason

Your Awesome! Thank you so much.

kburger

$count = 0 + $count;

I'm sure you've figured out by now that this line is pointless.

Weedpacket

Or do away with all the looping and just recognise a geometric progression when you see one.

$pmt = 1200; // $100/month = $1200/interest period
$i = 0.08; // interest per interest period (year)
$n = 30; // number of interest periods

$total  = $pmt * (1+$i) * (pow(1+$i, $n)-1)/$i;

Then you can have the fun of working out the effect that inflation will have over the same period 😃

drew010 wrote:
...[doesn't really matter. I just wanted to say 'Long Time No See']...

amreason

Good Good! Im learning a lot faster with your help! Thank You!