master-blaster
Matthew Butterick 9 years ago
parent 8827154ace
commit bc06eb4f54

@ -47,7 +47,7 @@ Once the whole cell path is computed, the answer is found by removing duplicate
@subsection{Alternate approach: complex numbers} @subsection{Alternate approach: complex numbers}
Rather than use Cartesian coordinates, we could rely on Racket's built-in support for complex numbers to trace the path in the complex plane. Complex numbers have a real and an imaginary part — e.g, @racket[3+4i] — and thus, represent points in a plane just as well as Cartesian coordinates. The advantage is that complex numbers are atomic values, not lists. We can add them normally, without resort to @racket[map]. (It's not essential for this problem, but math jocks might remember that complex numbers can be rotated 90 degrees by multiplying by @racket[+i].) Rather than use Cartesian coordinates, we could rely on Racket's built-in support for complex numbers to trace the path in the complex plane. Complex numbers have a real and an imaginary part — e.g, @racket[3+4i] — and thus, represent points in a plane just as well as Cartesian coordinates. The advantage is that complex numbers are atomic values, not lists. We can add them normally, without resort to @racket[map]. (It's not essential for this problem, but math jocks might remember that complex numbers can be rotated 90 degrees counterclockwise by multiplying by @tt{+i}.)
Again, the problem has nothing to do with complex numbers inherently. Like pairs and lists, they're just another option for encoding dual-valued data. Again, the problem has nothing to do with complex numbers inherently. Like pairs and lists, they're just another option for encoding dual-valued data.
@ -56,11 +56,11 @@ Again, the problem has nothing to do with complex numbers inherently. Like pairs
(define start 0) (define start 0)
(define east 1) (define east 1)
(define moves (for/list ([s (in-list (regexp-match* #rx"." str))]) (define moves (for/list ([s (in-list (regexp-match* #rx"." str))])
(case s (* east (expt +i (case s
[(">") east] [(">") 0]
[("^") (* east +i)] [("^") 1]
[("<") (* east +i +i)] [("<") 2]
[("v") (* east +i +i +i)]))) [("v") 3])))))
(for/fold ([cells-so-far (list start)]) (for/fold ([cells-so-far (list start)])
([next-move (in-list moves)]) ([next-move (in-list moves)])
(define current-cell (car cells-so-far)) (define current-cell (car cells-so-far))

@ -12,4 +12,6 @@
@include-section[(submod "day1.scrbl" doc)] @include-section[(submod "day1.scrbl" doc)]
@include-section[(submod "day2.scrbl" doc)] @include-section[(submod "day2.scrbl" doc)]
@include-section[(submod "day3.scrbl" doc)] @include-section[(submod "day3.scrbl" doc)]
@include-section[(submod "day4.scrbl" doc)] @include-section[(submod "day4.scrbl" doc)]
@include-section[(submod "day5.scrbl" doc)]
@include-section[(submod "day6.scrbl" doc)]