财务费率函数在JavaScript不能正常工作
在简单的财务费率函数中给出的函数在javascript中并没有给我和excel函数一样的答案。 对于http://allfinancialmatters.com/2009/11/03/how-to-use-the-rate-function-in-excel/给出的问题，对于我的testing用例来说，它是完美的。 其结果不同于excel率。 这是奇怪的行为。 我无法解决这个问题。 我的testing用例（用excel输出）是
RATE(360,-665.3, 99000) = 0.0059 RATE(360,-958.63, 192000) =0.0036 RATE(180,-1302.96,192000) = 0.0023 RATE(360, -889.19, 192000) =0.00312 RATE(360, -1145.8, 240000) = 0.0033 我的code.js是
function rate(paymentsPerYear, paymentAmount, presentValue, futureValue, dueEndOrBeginning, interest) { //If interest, futureValue, dueEndorBeginning was not set, set now //if (interest == null) // not working here :D if (isNaN(interest)) interest = 0.1; //interest = 0.1; if (isNaN(futureValue)) futureValue = 0; if (isNaN(dueEndOrBeginning)) dueEndOrBeginning = 0; var FINANCIAL_MAX_ITERATIONS = 128;//Bet accuracy with 128 var FINANCIAL_PRECISION = 0.0000001;//1.0e-8 var y, y0, y1, x0, x1 = 0, f = 0, i = 0; var rate = interest; // initiallizing rate to our guess interest if (Math.abs(rate) < FINANCIAL_PRECISION) { y = presentValue * (1 + paymentsPerYear * rate) + paymentAmount * (1 + rate * dueEndOrBeginning) * paymentsPerYear + futureValue; } else { f = Math.exp(paymentsPerYear * Math.log(1 + rate)); y = presentValue * f + paymentAmount * (1 / rate + dueEndOrBeginning) * (f - 1) + futureValue; } y0 = presentValue + paymentAmount * paymentsPerYear + futureValue; y1 = presentValue * f + paymentAmount * (1 / rate + dueEndOrBeginning) * (f - 1) + futureValue; // find root by Newton secant method i = x0 = 0.0; x1 = rate; while ((Math.abs(y0 - y1) > FINANCIAL_PRECISION) && (i < FINANCIAL_MAX_ITERATIONS)) { rate = (y1 * x0 - y0 * x1) / (y1 - y0); x0 = x1; x1 = rate; if (Math.abs(rate) < FINANCIAL_PRECISION) { y = presentValue * (1 + paymentsPerYear * rate) + paymentAmount * (1 + rate * dueEndOrBeginning) * paymentsPerYear + futureValue; } else { f = Math.exp(paymentsPerYear * Math.log(1 + rate)); y = presentValue * f + paymentAmount * (1 / rate + dueEndOrBeginning) * (f - 1) + futureValue; } y0 = y1; y1 = y; ++i; } return rate; //return String(parseFloat(rate).toFixed(3)); // rounding it to 3 decimal places //return parseFloat(rate).toFixed(3); }
和我的HTML文件是
<head><title>JavaScript Loan Calculator</title> <script src="code.js"></script> </head> <body bgcolor="white"> <form name="loandata"> <table> <tr> <td>1)</td> <td>paymentsPerYear:</td> <td><input type="text" name="paymentsPerYear" size="12" onchange="calculate();"></td> </tr> <tr> <td>2)</td> <td>paymentAmount:</td> <td><input type="text" name="paymentAmount" size="12" onchange="calculate();"></td> </tr> <tr> <td>3)</td> <td>presentValue:</td> <td><input type="text" name="presentValue" size="12" onchange="calculate();"></td> </tr> <tr> <td>4)</td> <td>futureValue:</td> <td><input type="text" name="futureValue" size="12"></td> </tr> <tr> <td>5)</td> <td>dueEndOrBeginning:</td> <td><input type="text" name="dueEndOrBeginning" size="12"></td> </tr> <tr> <td>6)</td> <td>interest:</td> <td><input type="text" name="interest" size="12"></td> </tr> <tr><td colspan="3"> <input type="button" value="Compute" onClick="calculate();"> </td></tr> <tr> <td>APR:</td> <td><input type="text" name="APR" id="APR" size="12"></td> </tr> </table> </form> <script language="JavaScript"> function calculate() { var paymentsPerYear = document.loandata.paymentsPerYear.value; var paymentAmount = document.loandata.paymentAmount.value; var presentValue = document.loandata.presentValue.value; var futureValue = document.loandata.futureValue.value; var dueEndOrBeginning = document.loandata.dueEndOrBeginning.value ; var interest = document.loandata.interest.value ; var ans = rate(parseFloat(paymentsPerYear), parseFloat(paymentAmount), parseFloat(presentValue), parseFloat(futureValue), parseFloat(dueEndOrBeginning), parseFloat(interest)); document.loandata.APR.value=ans; //alert(futureValue); } </script> </body> </html>
如果任何人仍然在寻找一个Excel的Rate函数的javascript实现，这就是我想出的：
var rate = function(nper, pmt, pv, fv, type, guess) { // Sets default values for missing parameters fv = typeof fv !== 'undefined' ? fv : 0; type = typeof type !== 'undefined' ? type : 0; guess = typeof guess !== 'undefined' ? guess : 0.1; // Sets the limits for possible guesses to any // number between 0% and 100% var lowLimit = 0; var highLimit = 1; // Defines a tolerance of up to +/- 0.00005% of pmt, to accept // the solution as valid. var tolerance = Math.abs(0.00000005 * pmt); // Tries at most 40 times to find a solution within the tolerance. for (var i = 0; i < 40; i++) { // Resets the balance to the original pv. var balance = pv; // Calculates the balance at the end of the loan, based // on loan conditions. for (var j = 0; j < nper; j++ ) { if (type == 0) { // Interests applied before payment balance = balance * (1 + guess) + pmt; } else { // Payments applied before insterests balance = (balance + pmt) * (1 + guess); } } // Returns the guess if balance is within tolerance. If not, adjusts // the limits and starts with a new guess. if (Math.abs(balance + fv) < tolerance) { return guess; } else if (balance + fv > 0) { // Sets a new highLimit knowing that // the current guess was too big. highLimit = guess; } else { // Sets a new lowLimit knowing that // the current guess was too small. lowLimit = guess; } // Calculates the new guess. guess = (highLimit + lowLimit) / 2; } // Returns null if no acceptable result was found after 40 tries. return null; };
用Abdul的testing用例testing这个函数给出如下结果：
rate(360,-665.3, 99000); 0.005916521358085446 rate(360,-958.63, 192000); 0.0036458502960158515 rate(180,-1302.96,192000); 0.0022917255526408564 rate(360, -889.19, 192000); 0.0031250616819306744 rate(360, -1145.8, 240000); 0.003333353153720964