Log() not Returning what is Expected

[TX_trader]

Hello,

I am doing a simple log( c / ref(c,-1) ) calculation, but I am not getting the value that I expect. The calculation is ln( 90.2578 / 90.2353) which is returning 0.000249355 instead of 0.000249317. I checked in the quote editor to make sure there was rounding that I was missing, but I verified on a calculator and in Excel that the value is wrong. What is happening here?

[Cougar]

@TX_trader,

Tomasz discussed this earlier, please refer:

Double floating-point precision for all indicator/math calculations

Generally there are some misconceptions in your post: Misconception #1: you are mistaken about internal calculations in AmiBroker. Single precision is NOT used for calculation of indicator values. In 99% of cases AmiBroker uses much higher precision for internal calculations including all built-in indicators. What's more AmiBroker internal precision is 80 bit (16-bit more than double precision) in case of 32-bit version because all calculations are done on HARDWARE FPU, that has 80 bit regist…

And in manual:

About floating point arithmetic

Also this MS Excel doc:

Floating-point arithmetic may give inaccurate result in Excel - Office

Please feel free to share what you understood!

[empottasch]

i still have an ancient version of IDL (interactive data language). This is what I get:

IDL> print,alog( 90.2578 / 90.2353)
  0.000249355

[Tomasz]

Nothing is “hapenning” except the fact that you don’t understand how floating point works and that actually both numbers are correct. First is 32 bit the other one is 64 bit.

[TX_trader]

That's for sure. Thanks for the clarification.