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Postings from September 2000
At 12:00 AM 9/12/00 +1100, Jeremy Howard wrote: >c.soeller@auckland.ac.nz wrote: > > Reading through the examples left me wondering about some > > technicalities: > > > > > @t[|i;|j] = @a[|j;|i]; # transpose 2-d @a > > > > Written like this it would require that @a is exact 2-dim, i.e. it would > > not just swap the first two dims of any n-dim array? I suppose if I'd > > want that I'd write > > > > @t[|i;|j;] = @a[|j;|i;]; # trailing ';' implies there might be > > trailing dims > > >Not necessary. Since arrays support all the syntax of a plain old list of >lists, and the |i syntax just creates an implicit loop, the example quoted >from the RFC will work with other sized arrays. In fact, if it was only 2d, >it would more properly be: > > $t[|i;|j] = $a[|j;|i]; # transpose 2-d @a I think it's fair to say that I goofed, and was probably inconsistent with my use of @ and $ in RFC207. It was the hardest (conceptually) of the RFCs I wrote, and it was written late at night -- my thinking probably wasn't as clear as it could have been. I know I delayed in writing it because it posed interesting challenges to syntax and semantics. I think what I was thinking was that |i would act as a list, therefore @t[|i] is syntactically an array slice, and therefore uses @, not $. I'm not so certain that that makes a tremendous amount of sense. The problem is that the isolated expression ~a[|i;|j] (using ~ as a placeholder for $ or @) evaluates to a 2D array, so should use @, but the |i;|j syntax is used to refer to a single element, so it's sort of a scaler, and should use $. While something like "$t[|i;|j] = $a[|j;|i] >With three dimensions, each implicit loop in > > @t[|i;|j] = @a[|j;|i]; # transpose 2-d @a > >is assigning the _list_ (or 1d array) at @a[|j;|i] to the appropriate index >in @t. Ditto for >3 dimensions, except that it is a >1d array (or LOL) that >is being assigned at each index through the implicit loop.Thread Previous