[R] strange floor rounding

[David Winsemius]

On Jul 9, 2010, at 10:27 AM, Trafim Vanishek wrote:

> Thanks everybody for referring me to FAQ 7.31 but I don't see how to  
> solve
> it.
> I am giving a concrete number and I need to get 58 not 57. Seems,  
> there is
> no way?
>
Building on an example on the help page for as.integer,  this seems to  
be working:

 > trnc2 <- function(x) trunc(x) + (trunc(x) < x)*sign(x)*(abs(x -  
trunc(x)) < .Machine$double.eps^0.5)

 > trnc2(0.01*100)
[1] 1
 > trnc2(seq(0, 1, by=0.01)*100)
   [1]   0   1   2   3   4   5   6   8   8   9  10  11  12  13  15   
15  16  17  18  19
  [21]  20  21  22  23  24  25  26  27  29  28  30  31  32  33  34   
35  36  37  38  39
  [41]  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54   
56  57  58  57  59
  [61]  60  61  62  63  64  65  66  67  68  69  70  71  72  73  74   
75  76  77  78  79
  [81]  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94   
95  96  97  98  99
[101] 100
 > trnc2(-seq(0, 1, by=0.01)*100)
   [1]    0   -1   -2   -3   -4   -5   -6   -7   -8   -9  -10  -11   
-12  -13  -14  -15
  [17]  -16  -17  -18  -19  -20  -21  -22  -23  -24  -25  -26  -27   
-28  -28  -30  -31
  [33]  -32  -33  -34  -35  -36  -37  -38  -39  -40  -41  -42  -43   
-44  -45  -46  -47
  [49]  -48  -49  -50  -51  -52  -53  -54  -55  -56  -57  -57  -59   
-60  -61  -62  -63
  [65]  -64  -65  -66  -67  -68  -69  -70  -71  -72  -73  -74  -75   
-76  -77  -78  -79
  [81]  -80  -81  -82  -83  -84  -85  -86  -87  -88  -89  -90  -91   
-92  -93  -94  -95
  [97]  -96  -97  -98  -99 -100

And after looking at that a bit I came up with a shorter alternate  
that seems to work as well:

trnc3 <- function(x) trunc(x+sign(x)* .Machine$double.eps^0.5)

See also  ?all.equal

-- 
David.
> On Fri, Jul 9, 2010 at 4:05 PM, David Winsemius <dwinsemius at comcast.net 
> >wrote:
>
>>
>> On Jul 9, 2010, at 9:46 AM, Trafim Vanishek wrote:
>>
>> Dear all,
>>>
>>> might seem and easy question but I cannot figure it out.
>>>
>>> floor(100*(.58))
>>> [1] 57
>>>
>>> where is the trick here?
>>>
>>
>> FAQ 7.31
>>
>>
>> And how can I end up with the right answer?
>>>
>>
>> Define right, please. (There have been several questions in the  
>> last week
>> for which FAQ 7.31 was the answer and some of the responses had  
>> useful
>> links.)
>>
>> --
>> David Winsemius, MD
>> West Hartford, CT
>>
>>
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
West Hartford, CT