PMT in Javascript

Question

I am trying to code the equivalent of the EXCEL PMT function.

in Java Script, the formula looks like this:

function PMT (ir, np, pv, fv ) { /* ir - interest rate per month np - number of periods (months) pv - present value fv - future value (residual value) */ pmt = ( ir * ( pv * Math.pow ( (ir+1), np ) + fv ) ) / ( ( ir + 1 ) * ( Math.pow ( (ir+1), np) -1 ) ); return pmt; } 

This is great for PMT calculations, where Type = 1 (i.e. payments occur at the beginning of the period)

However, I am trying to code a script of type 0 (that is, payments occur at the end of the period)).

Any math wizards who can tell me how to change the formula?

thanks

Answer 1

@ dps123: When I recently had to work with some financial equations to convert functions from an Excel workbook, I came across an EGM Mathematical Finance class that tries to mimic Excel functions. It might be worth taking a look at this, just to see how the functions look / work like Excel.

Using an example:

 <?php /** * Case use of financial class. * * @version $Id: financial_example.php,v 1.0.5 2004-06-23 09:03:56-05 egarcia Exp $ * @author Enrique Garcia M. < [e-mail] > * @copyright (c) 2002-2004 EGM :: Ingenieria sin fronteras * @since Saturday, November 30, 2002 **/ /*************************************************************************** * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * ***************************************************************************/ include('financial_class.php'); echo '<pre>'; echo 'FV: ' . $f->FV(1.1, 1/360, 0, -100) . "\n"; echo 'PV: ' . $f->PV(1.1, 1/360, 0, -100.206306226) . "\n"; echo 'PMT: ' . $f->PMT(1.1, 1/360, -100) . "\n"; echo 'PMT: ' . $f->PMT(1.1, 1/360, 0, -100.206306226) . "\n"; echo 'NPER: ' . $f->NPER(1.1, 53428.7980679, -100) . "\n"; echo 'NPER: ' . $f->NPER(1.1, 0, -100, -100.206306226) . "\n"; echo 'FV: ' . $f->FV(0.1, 1/360, 0, -100) . "\n"; echo 'PV: ' . $f->PV(0.1, 1/360, 0, -100.026478555) . "\n"; echo 'PMT: ' . $f->PMT(0.1, 1/360, -100) . "\n"; echo 'PMT: ' . $f->PMT(0.1, 1/360, 0, -100.026478555) . "\n"; echo 'NPER: ' . $f->NPER(1.1, 37776.4114948, -100) . "\n"; echo 'NPER: ' . $f->NPER(1.1, 0, -100, -100.026478555) . "\n"; echo 'EFFECT: ' . $f->EFFECT(0.0525, 4) . "\n"; echo 'NOMINAL: ' . $f->NOMINAL(0.053543, 4) . "\n"; echo 'NPV: ' . $f->NPV(0.1, array(-10000,3000,4200,6800)) . "\n"; echo 'XNPV: ' . $f->XNPV(0.09, array(-10000,2750,4250,3250,2750), array( mktime(0,0,0,1,1,2008), mktime(0,0,0,3,1,2008), mktime(0,0,0,10,30,2008), mktime(0,0,0,2,15,2009), mktime(0,0,0,4,1,2009), )) . "\n"; echo 'XIRR: ' . $f->XIRR(array(-10000,2750,4250,3250,2750), array( mktime(0,0,0,1,1,2008), mktime(0,0,0,3,1,2008), mktime(0,0,0,10,30,2008), mktime(0,0,0,2,15,2009), mktime(0,0,0,4,1,2009), ), 0.1) . "\n"; echo 'IRR: ' . $f->IRR(array(-70000,12000,15000,18000,21000)) . "\n"; echo 'DISC: ' . $f->DISC( mktime(0,0,0,1,25,2007), mktime(0,0,0,6,15,2007), 97.975, 100, 0) . "\n"; echo 'DISC: ' . $f->DISC( mktime(0,0,0,1,25,2007), mktime(0,0,0,6,15,2009), 97.975, 100, 1) . "\n"; echo 'DISC: ' . $f->DISC( mktime(0,0,0,1,25,2007), mktime(0,0,0,6,15,2007), 97.975, 100, 2) . "\n"; echo 'DISC: ' . $f->DISC( mktime(0,0,0,1,25,2007), mktime(0,0,0,6,15,2007), 97.975, 100, 3) . "\n"; echo 'DISC: ' . $f->DISC( mktime(0,0,0,1,25,2007), mktime(0,0,0,6,15,2007), 97.975, 100, 4) . "\n"; echo 'INTRATE: ' . $f->INTRATE( mktime(0,0,0,2,15,2008), mktime(0,0,0,5,15,2008), 1000000, 1014420, 2) . "\n"; echo 'IPMT: ' . $f->IPMT(0.1/12, 3, 3, 8000) . "\n"; echo 'IPMT: ' . $f->IPMT(0.1, 3, 3, 8000) . "\n"; echo 'RECEIVED: ' . $f->RECEIVED( mktime(0,0,0,2,15,2008), mktime(0,0,0,5,15,2008), 1000000, 0.0575, 2) . "\n"; echo 'DOLLARDE: ' . $f->DOLLARDE(1.02, 16) . "\n"; echo 'DOLLARDE: ' . $f->DOLLARDE(1.1, 32) . "\n"; echo 'DOLLARFR: ' . $f->DOLLARFR(1.125, 16) . "\n"; echo 'DOLLARFR: ' . $f->DOLLARFR(1.125, 32) . "\n"; echo 'FVSCHEDULE: ' . $f->FVSCHEDULE(1, array(0.09,0.11,0.1)) . "\n"; echo 'PPMT: ' . $f->PPMT(0.1/12, 1, 2*12, 2000) . "\n"; echo 'PPMT: ' . $f->PPMT(0.08, 10, 10, 200000) . "\n"; echo 'RATE: ' . $f->RATE(4*12,-200, 8000) . "\n"; echo 'RATE: ' . $f->RATE(4*12,-200, 8000)*12 . "\n"; echo 'SYD: ' . $f->SYD(30000, 7500, 10, 10) . "\n"; echo 'SLN: ' . $f->SLN(30000, 7500, 10) . "\n"; echo 'DDB: ' . $f->DDB(1000000, 100000, 10, 4) . "\n"; echo 'DELTA: ' . $f->DELTA(5, 4) . "\n"; echo 'DELTA: ' . $f->DELTA(5, 5) . "\n"; echo 'PRICEDISC: ' . $f->PRICEDISC(mktime(0,0,0,2,16,2008), mktime(0,0,0,3,1,2008), 0.0525, 100, 2) . "\n"; echo 'YIELDDISC: ' . $f->YIELDDISC(mktime(0,0,0,2,16,2008), mktime(0,0,0,3,1,2008), 99.795, 100, 2) . "\n"; echo 'COUPNUM: ' . $f->COUPNUM(mktime(0,0,0,1,25,2007), mktime(0,0,0,11,15,2008), 2, 1) . "\n"; echo 'COUPDAYBS: ' . $f->COUPDAYBS(mktime(0,0,0,1,25,2007), mktime(0,0,0,11,17,2008), 1, 1) . "\n"; echo 'VDB: ' . $f->VDB(2400,300,10*365,0,1) . "\n"; echo 'VDB: ' . $f->VDB(2400,300,10*12,0,1) . "\n"; echo 'VDB: ' . $f->VDB(2400,300,10,0,1) . "\n"; echo 'VDB: ' . $f->VDB(2400,300,10*12,6,18) . "\n"; echo 'VDB: ' . $f->VDB(2400,300,10*12,6,18,1.5) . "\n"; echo 'VDB: ' . $f->VDB(2400,300,10,0,0.875,1.5) . "\n"; echo 'MIRR: ' . $f->MIRR(array(-120000,39000,30000,21000,37000,46000), 0.1, 0.12) . "\n"; echo 'MIRR: ' . $f->MIRR(array(-120000,39000,30000,21000), 0.1, 0.12) . "\n"; echo 'MIRR: ' . $f->MIRR(array(-120000,39000,30000,21000,37000,46000), 0.1, 0.14) . "\n"; echo '</pre>'; ?> 

Answer 2

@dps

You will need to change the denominator, where the coefficient of interest is changed to (ir * type + 1)

When this means annuity, which means the beginning of the payment of the period, a value of 1 for Type will insure the percentage ratio (ir + 1) , and when this is a regular annuity means the end of the payment period, a value of 0 for Type will reduce the coefficient to 1 .

 pmt = ( ir * ( pv * Math.pow ( (ir+1), np ) + fv ) ) / ( ( ir * type + 1 ) * ( Math.pow ( (ir+1), np) -1 ) ); 

You actually imagine how MS Excel calculates 5 functions Time Value of money, namely FV , PV , PMT , NPER and RATE . The first three are easily calculated by rearranging the equations and solutions for FV , PV or PMT . Other methods are needed for NPER and RATE , some use a binary search algorithm to find RATE , but the best and most elegant solution for finding RATE should be done using the Newton Raphson method.

Answer 3

Below is the code in java:

 double pmt = ((pv - fv) * ir / (1 - Math.pow((1 + ir), -np))); 

Answer 4

I'm not a math wiz, but a simple google search turned this topic up:

http://www.excelforum.com/excel-general/370948-pmt-function-does-anyone-know-the-formula.html

Here it has the following formula for type = 0:

 pmt = ((pv - fv) * ir / (1 - (1 + ir) ^ -(np))); 

Maybe this will work for you :)