[Jprogramming] symbol index origin

[Dan Bron]

Pascal wrote:
> 6 s:s:'`'  NB. Oh!  The fill symbol hides.
> 0


Yes, symbol 0 is reserved (unsurprisingly) for the empty symbol, the symbolic equivalent of ‘’, which is the literal equivalent of … 0 (and not just in J, but broadly recognized as such in information theory and the discipline of formal languages).  

And, just like 0 in numerical arrays and ‘ ‘ in literal arrays and a: in boxed arrays, s:’ ‘ (the empty symbol) is used for fills in symbolic arrays.

If you had wanted, you could have asked J this question directly:


        _6 s: 0         NB. Which symbol has index 0 ?
     `

Or, less directly, but more cutely:

        6 s:^:_1: 0     
     `

This relationship between (-N)&s: and N&s:^:_1 is intentional, introduced for its mnemonic virtues, and is shared by some other primitives [1].

-Dan

[1] J Chat thread “Conversion”, March 2010, discussing x: and later p: :
    http://www.jsoftware.com/pipermail/chat/2010-March/003487.html <http://www.jsoftware.com/pipermail/chat/2010-March/003487.html>