At Thu, 29 Nov 2012 14:44:38 +0000, 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?
There are a couple of issues here. First, the inexact number 4.225 is actually slightly smaller than the exact value 4.225: > (< 4.225 #e4.225) #t > (pretty-print-exact-as-decimal #t) > (inexact->exact 4.225) 4.2249999999999996447286321199499070644378662109375 So, that's one reason to argue that "4.22" is the right answer. Even if you use the exact number 4.225, though, you get a "2" as the last digit: > (real->decimal-string #e4.225 2) "4.22" That's because rounding in Racket is to "even" --- a different convention than the one taught to most students, but one that it often preferred in computing (because it often reduces accumulated error, as I understand it). ____________________ Racket Users list: http://lists.racket-lang.org/users
