Unlike in the K forum, the J-wires have been virtually silent on this
year's Advent of Code...
Anyway, today's problem, number 18, might interest J & APLers as part
1 is ALMOST
plug-it-in to J/APL; there's just one twist, so to speak, namely we
need to reverse an
arithmetic expression, eg
1 + (2 * 3) + (4 * (5 + 6))
needs to be evaluated as
((6 + 5) * 4) + (3 * 2) + 1
But, even though all numbers are single-digit, just reversing the
string doesn't work, fairly obviously:
|.'1 + (2 * 3) + (4 * (5 + 6))'
))6 + 5( * 4( + )3 * 2( + 1
So the brackets need swapping. The data is supplied as strings, so I
expected to use stringreplace,
as found in strings.ijs
But:
|. ( ')[' , '()' , '[(' ) stringreplace '1 + (2 * 3) + (4 * (5
+ 6))'
[[6 + 5) * 4) + [3 * 2) + 1
Why don't the intermediate '[' get changed to the final ')' ?
The second action in stringreplace is on the left hand argument:
_2 [\ ,(')[' , '()' , '[(' )
)[
()
[(
The first column should be items of old text and the second column
their new replacements. But the third pair seems to be ignored.
I did of course get round this little difficulty, but would have preferred
not to reinvent the wheel.
NB. Part 2 is a little more challenging, as + is defined there as taking
precedence over * .
Thanks,
Mike
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