You must not have executed something:
nPr =: [:*/-i.
always fails with a domain error.
([:*/-i.)
is
[: (*/ - i.)
is
y [: (x */ y) - (x i. y)
which is not useful.
*/@(-i.)
is
*/ x (-i.) y
is
*/ x - i. y
which is permutation.
For a discussion of rank, I like chapter 6 of JfC, which you can get to
from the help page -- but I would, because I wrote it.
Henry Rich
Alex Gian wrote:
> Although, funnily enough if we define
> nPr =: [:*/-i.
> then it works
> 14 nPr 1.9
> 1 14 182 2184 24024 240240 2162160 17297280 121080960
>
> I'm beginning to understand the issue with the rank, although it seems
> to me that "*/@(-i.)" and "[:*/-i." are not a million miles from each
> other, so I can't quite see _which_ is the verb that allows the one to
> work, but not the other.
>
> Can anyone point me to the best introduction to read about verb rank?
>
>
> (BTW, I do understand that if we want the whole sequence
> */\ 14 - i. 8
> is the better way, since it will avoid redundant recomputation.)
>
>
>
> On Fri, 2010-02-19 at 13:12 +0100, Zsbán Ambrus wrote:
>
>> The way we defined that verb, it won't automatically work for an array
>> of numbers. J builtins that do numerical computations on single
>> numbers are defined to be extended to arrays. For example, ^!._1 is
>> such a builtin, so we get
>>
>> 14 ^!._1 i. 9
>> 1 14 182 2184 24024 240240 2162160 17297280 121080960
>>
>> Other builtins and user functions do use array arguments as a whole,
>> so J won't automatically apply our defined verbs to each element,
>> which is why the above doesn't work. You can use the rank conjuction
>> to apply them to each element and assemble the results.
>>
>> 14 */@(-i.)"0 i. 9
>> 1 14 182 2184 24024 240240 2162160 17297280 121080960
>>
>> Although if you need the whole sequence, it might be better to compute
>> it like this,
>>
>> 1, */\ 14 - i. 8
>> 1 14 182 2184 24024 240240 2162160 17297280 121080960
>>
>> Ambrus
>
>
>
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