Calculating interest rate in PHP with converted Excel RATE Function

Question

I'm having this issue as well - one example works and the next doesn't

Here's my code

define('FINANCIAL_MAX_ITERATIONS', 128); define('FINANCIAL_PRECISION', 1.0e-08);

function RATE($nper, $pmt, $pv, $fv = 0.0, $type = 0, $guess = 0.1) {

$rate = $guess; 
if (abs($rate) < FINANCIAL_PRECISION) { 
    $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv; 
} else { 
    $f = exp($nper * log(1 + $rate)); 
    $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv; 
} 
$y0 = $pv + $pmt * $nper + $fv; 
$y1 = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv; 

// find root by secant method 
$i  = $x0 = 0.0; 
$x1 = $rate; 
while ((abs($y0 - $y1) > FINANCIAL_PRECISION) && ($i < FINANCIAL_MAX_ITERATIONS)) { 
    $rate = ($y1 * $x0 - $y0 * $x1) / ($y1 - $y0); 
    $x0 = $x1; 
    $x1 = $rate;
    if (($nper * abs($pmt)) > ($pv - $fv))
        $x1 = abs($x1);

    if (abs($rate) < FINANCIAL_PRECISION) { 
        $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv; 
    } else { 
        $f = exp($nper * log(1 + $rate)); 
        $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv; 
    } 

    $y0 = $y1; 
    $y1 = $y; 
    ++$i; 
} 

$rate = abs(round($rate*100,5));

return $rate; 

} // function RATE()

And some examples RATE(60,-1338.88,274775,0,0,0.03200); // works - returns 3.40321

RATE(60,-2415.44,448925,0,0,0.04150); // doesn't work - returns 3.16288 when it should be 4.32

Any ideas? I've tried 4-5 pieces of code that all look similar and this one seems to give me the best results, even though they aren't consistent. I feel the code works, because how could it give me one correct result and the rest aren't??

p.s: I'm a newb at posting here, I've been reading this site for about 5 years though - hope I was thorough enough ;)

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Another good question, but still doesn't do what I need... Calculating interest rate in PHP

Calculating interest rate in PHP with converted Excel RATE Function

0 Answers0