I think you mean "finds elements of a list which would be representable exactly as either integers or booleans". Typically, your list will be all floating point numbers. But, also, I do not think you want to exclude 1 nor 0.
This boils down to a test for fractionality with a range test. In many languages, this sort of testing is made more convenient with constants representing the largest and smallest possible integers. J currently does not have that. But we can define our own: MAXINT=: #.1#~31+32*IF64 MININT=: <:-MAXINT Now all we have to do is enhance your fractionality test with a range test. isinteger=: (=<.) * <:&MAXINT * >:&MININT Which gives us: isinteger 1 2.5 __ 3 4.5 6 1 0 0 1 0 1 That said, I would want to talk about complex numbers and rational and extended precision numbers before I tried to implement an 'isfloating'. This fractionality test throws a domain error for complex values, and technically all extended precision values are integers, even after a floor operation (though you can defeat this by appending an infinity to the list). Thanks, -- Raul On Sun, Aug 2, 2020 at 8:24 AM Skip Cave <s...@caveconsulting.com> wrote: > > What I'm really looking for, is a verb that finds integers in a list: > > datatype 2.5 > > floating > > datatype 3 > > integer > > datatype __ > > floating > > > So J considers __ as "floating" > > > So I want a verb "isinteger" that marks the integers in a vector, where __ > is in the list, and is considered floating: > > isinteger 1 2.5 __ 3 4.5 6 > 1 0 0 1 0 1 > > And maybe the inverse also: > > isfloating 1 2.5 __ 3 4.5 6 > > 0 1 1 0 1 0 > > > My (=<,) doesn't do it: > > (=<.)1 2.5 __ 3 4.5 6 > > 1 0 1 1 0 1 > > > So what would "isinteger" look like? > > > Skip > > > Skip Cave > Cave Consulting LLC > > > On Sun, Aug 2, 2020 at 1:44 AM Skip Cave <s...@caveconsulting.com> wrote: > > > I use the (=<.) verb to find integers in a list: > > > > > > * (=<.)1 2.5 2.7 3 4.5 6* > > > > *1 0 0 1 0 1* > > > > * (#~(=<.))1 2.5 2.7 3 4.5 6* > > > > *1 3 6* > > > > I ran across an interesting result when infinity is in the list: > > > > * (=<.)1 2.5 __ 3 4.5 6* > > > > *1 0 1 1 0 1* > > > > * (#~(=<.))1 2.5 __ 3 4.5 6* > > > > *1 __ 3 6* > > > > > > So J is saying that the floor of infinity is infinity (and the ceiling of > > infinity is also infinity). Since infinity is not a number, it would seem > > that an error should be generated when taking the floor of infinity, or > > perhaps NAN, or a zero? In any case, this messes up my nice integer-finding > > verb. Is the\re a mathematical justification for defining the floor of > > infinity to be infinity? > > https://math.stackexchange.com/questions/981708/limit-of-floor-function-when-x-goes-infinity > > > > > > Skip > > > > > > Skip Cave > > Cave Consulting LLC > > > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
