Subj : Re: Compiler and an interpreter
To   : comp.programming
From : Arthur J. O'Dwyer
Date : Sun Aug 07 2005 05:33 pm


On Sat, 6 Aug 2005, Gerry Quinn wrote:
> usenet@jdh30.plus.com says...
>>
>> nth(i, 0) = {i}
>> nth(i, 1) = neighbours(i)
>> nth(i, n) = union(j in nth(i,n-1), nth(i, 1)) - nth(i, n-1) - nth(i, n-2)
>>
>> where neighbours(i) gives the set of neighbours of "i".
>
> I don't understand the last line, so I may have solved a different
> problem!

   I don't know for sure, but I strongly suspect
     nth(i, n) = union(j in nth(i,n-1), nth(i, 1)) - ...
was supposed to read
     nth(i, n) = union(j in nth(i,n-1), nth(j, 1)) - ...

Otherwise, what's the point of mentioning 'j'?

   Once that change is made, I think it's easy to read the expression as 
calculating, as you said,
> nth( i, n ) = all neighbours of nth( i, n-1 ), that don't appear in
> nth( i, n-1 ) or nth( i, n-2 )

-Arthur

.