# [racket] Decimal rounding problem

On Nov 29, 2012, at 9:44 AM, Greg Graham wrote:
>* I am trying to report GPA calculation results to 2 decimal places, so I thought real->decimal-string would do the trick. However, the following behavior surprised me:
*>*
*>>* (real->decimal-string 3.225 2)
*>* "3.23"
*>>* (real->decimal-string 4.225 2)
*>* "4.22"
*>*
*>* I would like the second answer to be "4.23", which is what a student would expect to see if they did the calculations themselves. The documentation for real->decimal-string says that it first converts the argument to an exact number. I suspect the problem has something to do with this:
*>*
*>>* (inexact->exact 4.225)
*>* 4 126663739519795/562949953421312
*>>* (/ 126663739519795.0 562949953421312.0)
*>* 0.22499999999999964
*>*
*>* Is there another rounding function that would just round the floating point without going through the conversion to exact?
*
Another rounding function won't help, because by the time the READER has finished with the character string "4.225", the internal form is already slightly less than 4225/1000. And if you're getting the inexact 4.225 from somewhere else, again it's not really 4225/1000 so it's already too late to round it the way you want.
You could work in a student language, where the reader reads decimals as exact by default :-)
Seriously, I'm sure there's a way to tell the reader in other languages to read decimals as exact; I just don't know what it is.
Stephen Bloch
sbloch at adelphi.edu