Personally, I avoid paying attention to internal data types whenever possible as this leads us down an implementation-dependent rabbit hole with little upside.
My own rounding utility "roundNums" - found here: http://code.jsoftware.com/wiki/User:Devon_McCormick/Utilities - allows options for different kinds of rounding - up, down, and banker's - for the cases where we're on an exact halfway point. Banker's rounding - also referenced here http://code.jsoftware.com/wiki/Addons/general/misc - attempts to avoid systematic bias by rounding either up or down. This would be important, e.g., in the case of a series resulting from the average of two series of whole numbers where rounding either up or down would introduce directional bias. Also, I like my way of specifying the rounding precision with a minimum increment, i.e. "1" for whole numbers or "0.05" for nickels, as this is intuitive and flexible. On Thu, Jan 14, 2016 at 7:01 PM, Marshall Lochbaum <[email protected]> wrote: > Actually, you're off by one on the low end--a quirk of two's complement > notation is that the minimum possible value is actually one less than > minus the maximum. But the bigger problem is that doing things with rank > zero is very slow. I was working with audio data where there are > typically almost a hundred thousand values per second (and, admittedly, > where any more than 24 bits in the output is overkill), so this wouldn't > be fast enough. > > Marshall > > On Thu, Jan 14, 2016 at 09:39:20PM +0000, 'Pascal Jasmin' via Programming > wrote: > > anything wrong with this? > > > > (0.5 <.@:+ ])`(9223372036854775807 * *)@.(9223372036854775807 < |)("0) _ > 234234.3 __ > > > > works as an adverb where you provide your rounding function. > > > > > > roundA =: `(9223372036854775807 * *)(@.(9223372036854775807 < |))("0) > > > > (0.5 <.@:+ ]) roundA _ 234234.3 __ > > >. roundA _ 234234.3 __ > > > > ----- Original Message ----- > > From: Marshall Lochbaum <[email protected]> > > To: [email protected] > > Sent: Thursday, January 14, 2016 2:56 PM > > Subject: [Jprogramming] Round to nearest integer: harder than it seems > > > > Here's something I spent far too long on, and consequently thought was > > worth sharing. I can turn it into an essay on the J wiki if people want > > that. > > > > Recently I ran into the problem of rounding a J floating point number to > > an integer, and forcing the result to have integer type. This seems like > > a simple task: using a standard rounding function, we have > > 0.5 <.@:+ 2.3 5.1 7.6 3.9 > > 2 5 8 4 > > But with numbers that are too large, the result still contains > > floating-point numbers, and has type 8 (floating point) rather than 4 > > (integer). > > 0.5 <.@:+ _1e50 2e30 _ > > _1e50 2e30 _ > > 3!:0 ]0.5 <.@:+ _1e50 2e30 _ > > 8 > > When applied to a float, (<.) applies the C floor function, which yields > > another float, and than casts the results to integers if all of them are > > exactly representable as integers. They're not here, so they are left as > > floating-point numbers. > > > > To give a more accurate problem statement, I want the 64-bit signed > > integer which is closest to the function input. Thus numbers above the > > maximum representable integer should round to that integer, and likewise > > for numbers below the minimum representable integer. We define these two > > bounds now. > > MAX =: ->: MIN =: _2 <.@^ 63 > > Note that since MAX is one less than 2^63, trying to take (2<.@^63) > > would give us a float, and subtracting one would still leave us with a > > floating point number, which is not actually equal to MAX since (>:MAX) > > is representable as a float, while nearby integers are not. MIN on the > > other hand is safely computed as an exponent. Note the negative base, > > which works because 63 is odd. > > > > With these bounds, our problem should be easy: clamp to the integer > > range, then use (<.). > > ([: <. MIN>.MAX<.]) 0.5 + __ _1e30 _1e10 _100.3 > > _9223372036854775808 _9223372036854775808 _10000000000 _100 > > So far, so good... > > ([: <. MIN>.MAX<.]) 0.5 + 50.4 2e10 1e30 _ > > 50 2e10 9.22337e18 9.22337e18 > > Oops. What happened? > > MAX <. _ > > 9.22337e18 > > Since one of the arguments is a float, (<.) casts both to floats, and > > takes the minimum. But the closest floating-point number to MAX is > > (MAX+1), and that number's floor (MAX+1) isn't representable as an > > integer--it's one too big. We didn't have this problem with MIN, since > > it is exactly a negative power of two. > > <. MAX+1 > > 9.22337e18 > > > > We'll make a test case that contains numbers close to both bounds. I've > > included the addition of 0.5 in t so I can focus on the floor function > > from now on. The type of our result is a float, so we failed. > > t =. 0.5 + __,_,~ (MIN,0,MAX) +/(,@:) i:1e5 > > 3!:0 ([: <. MIN>.MAX<.]) t > > 8 > > > > We can fix the problem by using exact integers, but it's extremely slow. > > However, it serves as a good answer key. The ("0) is there for a > > reason--otherwise the big array of exact integers tends to flood RAM. > > fl_e =: (MIN>.MAX<.<.)&.:x:"0 NB. exact floor > > 3!:0 key =. fl_e t > > 4 > > 10 (6!:2) 'fl_e t' > > 3.62018 > > > > If we use a number small enough that its floating-point representation > > is equal to a 64-bit integer, then we can force our answer to be a > > float, but it's not correct since the results are sometimes too small. > > If that doesn't matter and speed is critical, this is the right method > > to use. > > MAX1 =. MAX - 512 > > fl_f =: [: <. MIN>.MAX1<.] NB. fast floor > > 3!:0 fl_f t > > 4 > > key -: fl_f t > > 0 > > 10 (6!:2) 'fl_f t' > > 0.0179205 > > > > Finally, my solution. It's not particularly elegant, but it is correct > > and has good performance. We reduce all the values larger than MAX to > > zero, then clamp on the minimum side and take the floor. For the values > > that we removed, we add MAX back in. The comparison (<:&MAX) is only > > computed once to save a little time. > > fl =: ((MAX*-.@]) + [: <. MIN>.*) <:&MAX > > key -: fl t > > 1 > > 10 (6!:2) 'fl t' > > 0.0426181 > > It's critical to use (<:) rather than (<) to test whether numbers are > > acceptable even though it fails MAX, which wouldn't break (<.). That's > > because comparisons cast their arguments to floats before comparing, so > > MAX < MAX+1 > > 0 > > > > Maybe there's a quicker solution to be found. The following rounds > > towards zero quickly by negating all the positive numbers, and restoring > > their signs later. However, adding in the cases to make it equal to (<.) > > on small numbers removes its advantage. > > fl_o =: (] * MIN <.@:>. *) -@:* NB. floor towards zero > > fl_o _4.6 _3 _2.8 _1.2 3.4 5.8 9 > > _5 _3 _3 _2 4 6 9 > > 10 (6!:2) 'fl_o t' > > 0.0324293 > > > > Any takers? > > > > Marshall > > ---------------------------------------------------------------------- > > For information about J forums see http://www.jsoftware.com/forums.htm > > ---------------------------------------------------------------------- > > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > -- Devon McCormick, CFA Quantitative Consultant ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
