[Jprogramming] Grammar question: 2: * i.

[Elijah Stone]

A single digit followed by a colon is a _constant verb_, which always 
produces the same result no matter its operand.  For example:

    f=. 2:
    f 5
2
    f 7 7
2
    3 f 'a'
2

So, '2: * i.' is a V V V fork, whereas '2 * i.' is an N V V fork.  I guess 
that the latter were not yet implemented when that entry of the phrasebook 
was written.

FWIW the functions are documented in NuVoc; the entry link is titled '_9: 
to 9:; Constant Functions', and can be read at 
https://code.jsoftware.com/wiki/Vocabulary/zeroco

There is also a dictionary entry: 
https://www.jsoftware.com/help/dictionary/dconsf.htm

  -E


On Tue, 19 Oct 2021, Jon Quant wrote:

> Hi all,
>
> The J phrases book has simple uses of colon which I can't find in the NuVoc
> grammar.
> Could someone help explain the grammar of the following?
>
> Verb trains:
>
> even=: 2: * i.
> odd=: 1: + 2: * i.
>
> These seem to work without the colons anyway. What is the grammar rule and
> purpose for an integer followed by a colon?
>
> Great fun so far with J!
>
> Thanks
>
> Jon
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