GN=Gary Ng, RM=Raul Miller
GN> What would be the lookup result of _1 { f ?
RM> _1 { f
RM> gives you the last item in f.
GN> thanks then why would I get
GN> 0 0 when the lookup index is _1 _1
For the same reason that when you write:
0 1 2 { f
you'd get the first 3 items of f . In short: the dyad { maps indices
(x) to items (y) . Its inputs are a list of indices and a universe, and
its output is the items of the universe which exist at those indices.
Things like _1 _1 work because repeated indices are allowed. At first
blush, this may seem useless, but in practice it's a useful feature (plus
there's no good reason to restrict the definition of { to prohibit repeats).
If you're wondering why _1 _1 doesn't give the bottom-right corner of the
array, like f[0][0] (or maybe f[0;0]) would give the top-left corner of a 2D
array in a C-like language, the answer is: in J, compound
(multi-dimensional) indices are addressed with boxes.
There are many ways to present the rationale for this decision, but an easy
one is: conceptually, a compound index is a single, atomic thing, yet has
structure (this seems contradictory until you work through some examples,
e.g. "the bottom-right corner" of a 2D array). In J, atoms with (arbitrary)
structure are represented with boxes. Voila.
GN> if I just type -1 { f, I get something else
GN> [compared to _1 { f].
In J, _1 and -1 are very different things. The rule of thumb is, where you
would write -1 in another language, you would write _1 in J, and where you
would write negate(numeric_argument) in another language, you'd write
-numeric_argument in J.
That is, - is a function, applied to a (numeric) argument, whereas _1 is how
you spell the name of the first negative integer (i.e. the _ is an
inseparable part of the name of the constant).
In more formal, and perhaps illuminating, terms: _1 is one word, and -1 is
two. Compare ;:'_1' with ;:'-1' . For a longer treatment of this
topic, see [1] .
-Dan
[1] Forum thread "why is _5+_4 not the same as -5 + -4 ?":
http://www.jsoftware.com/pipermail/general/2007-April/029806.html
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