i. returns the index of the first occurrence of a value within an array.
So, when implementing an algorithm which needs the index of the second
occurrence of the value within a (large) array, we need to do some
additional work.
let's say that our array is A, the value is V
F=: A i. V NB. the index of the first occurrence of V
What's the most efficient way of finding the second occurrence?
One possibility is
S=: (1+F) + ((1+F) }. A) i. V
Another possibility, assuming that V is numeric and not zero, would be
S=: (0 F} A) i. V
But A is large, so perhaps a faster approach would be:
S=: {{ while. V~:y{A do. y=. y+1 end. y }} F
(Which has me wishing that S=: A i.!.F V would do the job, though I'm
not sure that that's completely appropriate...)
But, we can probably eliminate the need to generate a copy of A with a
little extra work:
A=: 0 F} A
S=: A i. V
A=: V F} A
It seems to me that this is probably going to be the fastest approach.
Can anyone think of a faster approach (or something with comparable
speed which isn't quite so unwieldy?)
Thanks,
--
Raul
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