Yes, it's a boon that 0 * x is 0 for any x.  Some authors go even further:
Knuth propose to have 0 * x be 0 even when x is an expression which is in
error.  He wanted this to make "Iverson's convention" work in more
situations.  See https://arxiv.org/PS_cache/math/pdf/9205/9205211v1.pdf



On Fri, Apr 13, 2018 at 12:24 PM, Joe Bogner <joebog...@gmail.com> wrote:

> Well, I stumbled upon a fast way that takes advantage of simple
> multiplication
>
> _ * 0 = 0
> _ * 1 = _
> 5 * 1 = 5
>
>
>    _3 {. (-.@isblank * _&".) c1
> 1.01 _ 0
>    (6!:2) '(isblank * _&".) c1'
> 1.00593
>
> I'm still interested in alternatives that are similar speed if possible
>
>
>
>
>
> On Fri, Apr 13, 2018 at 3:19 PM, Joe Bogner <joebog...@gmail.com> wrote:
>
> > Thank you -- Yes, I didn't want to send along the full sample, although I
> > could have created some dummy data.
> >
> > Both Don and Raul's method are similar and are equally slow as the way I
> > had
> >
> > It's not terribly important but I have like 10 numeric columns that I
> > convert in a script and have to painfully wait 30 seconds each time I run
> > the script. If I could shave 20 seconds off something that I seem to be
> > running multiple times a day and learn something in the process, I
> figured
> > it was worthwhile to ask
> >
> >   (6!:2) '{.@(_&".)"1 c'
> > 3.44371
> >
> > Here's a reasonable approximation:
> >
> > c1 =. 1.5e6 #  (>' ';'bad';'10')
> >
> >
> >  c1=: (((5e6,4)$'1.01'),'bad'),' '
> >    $ c1
> > 5000002 4
> >    (6!:2) '{.@(_&".)"1 c1'
> > 3.29915
> >
> >
> > Compare to
> >
> >  (6!:2) '_ ". c1'
> > 1.01854
> >
> >    _3 {. _ ". c1 NB. bad values at the end... ideally the blank is 0
> though
> > 1.01 _ _
> >
> >
> > On Fri, Apr 13, 2018 at 2:13 PM, Raul Miller <rauldmil...@gmail.com>
> > wrote:
> >
> >> Your sample data does not match your problem description.
> >>
> >> That said,
> >>
> >>    {.@(_&".)"1 > ' ';'bad';'10'
> >> 0 _ 10
> >>
> >> Thanks,
> >>
> >> --
> >> Raul
> >>
> >>
> >> On Fri, Apr 13, 2018 at 2:03 PM, Joe Bogner <joebog...@gmail.com>
> wrote:
> >> > I have a large byte array that I want to convert to a number array. I
> >> want
> >> > to use _ to indicate a bad value, but blank should be treated as 0
> >> >
> >> >   $ c
> >> > 4862343 10
> >> >
> >> > Instead of this:
> >> >
> >> > asnumber =: _&".
> >> >
> >> >    asnumber  (>' ';'bad';'10')
> >> > _ _ 10
> >> >
> >> > I want this:
> >> >
> >> >
> >> >    asnumber  (>' ';'bad';'10')
> >> > 0 _ 10
> >> >
> >> > This works, but is much slower than I'd like -- nearly 3x slower than
> >> just
> >> > _ ".
> >> >
> >> >    asnumber =: _ ". '0' ,~^:([: */ ' '&=@])"1  ]
> >> >    asnumber  (>' ';'bad';'10')
> >> > 0 _ 10
> >> >
> >> >    (6!:2) 'asnumber c'
> >> > 3.32579
> >> >    (6!:2) '_&". c'
> >> > 1.35091
> >> >
> >> >
> >> > I have an isblank function that is fast
> >> >
> >> >    isblank =. ([: ({. -:"1 }.) ' '&,)
> >> >    (6!:2) 'isblank c'
> >> > 0.033164
> >> >
> >> > I can't seem to combine the two into something that performs well
> >> without
> >> > doing things at the atom/row level instead of the entire array. I
> >> suspect
> >> > I'm getting tripped up by rank
> >> > ------------------------------------------------------------
> ----------
> >> > For information about J forums see http://www.jsoftware.com/
> forums.htm
> >> ----------------------------------------------------------------------
> >> For information about J forums see http://www.jsoftware.com/forums.htm
> >>
> >
> >
> ----------------------------------------------------------------------
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