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Rate() returning NAN

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Apr 6th, 2012
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  1. //The following consts are defined:
  2. //FINANCIAL_PRECISION=1.0e-08
  3. //FINANCIAL_MAX_ITERATIONS=128
  4.  
  5.  
  6. //Function by PHPExcel project;
  7. protected function RATE($nper, $pmt, $pv, $fv = 0.0, $type = 0, $guess = 0.1) {
  8.         $rate = $guess;
  9.         if (abs($rate) < self::FINANCIAL_PRECISION) {
  10.             $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv;
  11.         } else {
  12.             $f = exp($nper * log(1 + $rate));
  13.             $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;
  14.         }
  15.         $y0 = $pv + $pmt * $nper + $fv;
  16.         $y1 = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;
  17.  
  18.         // find root by secant method
  19.         $i  = $x0 = 0.0;
  20.         $x1 = $rate;
  21.         while ((abs($y0 - $y1) > self::FINANCIAL_PRECISION) && ($i < self::FINANCIAL_MAX_ITERATIONS)) {
  22.             $rate = ($y1 * $x0 - $y0 * $x1) / ($y1 - $y0);
  23.             $x0 = $x1;
  24.             $x1 = $rate;
  25.  
  26.             if (abs($rate) < self::FINANCIAL_PRECISION) {
  27.                 $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv;
  28.             } else {
  29.                 $f = exp($nper * log(1 + $rate));
  30.                 $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;
  31.             }
  32.  
  33.             $y0 = $y1;
  34.             $y1 = $y;
  35.             ++$i;
  36.         }
  37.         return $rate;
  38.     }
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