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@ -3,6 +3,8 @@
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@aoc-title[1]
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@aoc-title[1]
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@defmodule[aoc-racket/day1]
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@link["http://adventofcode.com/day/1"]{The puzzle}. Our @link-rp["day1-input.txt"]{input} is a string of parentheses that controls an elevator. A left parenthesis @litchar{(} means go up one floor, and a right parenthesis @litchar{)} means go down.
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@link["http://adventofcode.com/day/1"]{The puzzle}. Our @link-rp["day1-input.txt"]{input} is a string of parentheses that controls an elevator. A left parenthesis @litchar{(} means go up one floor, and a right parenthesis @litchar{)} means go down.
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@chunk[<day1>
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@chunk[<day1>
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@ -22,6 +24,7 @@ The building has an indefinite number of floors in both directions. So the ultim
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@chunk[<day1-setup>
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@chunk[<day1-setup>
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(require racket rackunit)
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(require racket rackunit)
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(provide (all-defined-out))
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(define up-char #\()
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(define up-char #\()
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(define down-char #\))
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(define down-char #\))
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@ -35,11 +38,7 @@ The building has an indefinite number of floors in both directions. So the ultim
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@chunk[<day1-q1>
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@chunk[<day1-q1>
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(define (q1 str)
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(define (q1 str)
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(displayln (format "ups = ~a" (get-ups str)))
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(get-destination str))]
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(displayln (format "downs = ~a" (get-downs str)))
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(define destination (get-destination str))
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(displayln (format "destination = ~a" destination))
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destination)]
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@subsection{Alternate approach: numerical conversion}
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@subsection{Alternate approach: numerical conversion}
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@ -53,9 +52,7 @@ Rather than counting matches with @racket[regexp-match*], we could also convert
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-1)))
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-1)))
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(define (q1-alt str)
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(define (q1-alt str)
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(define destination (apply + (elevator-string->ints str)))
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(apply + (elevator-string->ints str)))]
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(displayln (format "destination = ~a" destination))
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destination)]
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@section[#:tag "q2"]{At what point does the elevator enter the basement?}
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@section[#:tag "q2"]{At what point does the elevator enter the basement?}
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@ -77,15 +74,13 @@ We could characterize this as a problem of tracking @italic{cumulative values} o
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#:break (in-basement? movements-so-far))
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#:break (in-basement? movements-so-far))
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(cons (get-destination (~a c)) movements-so-far)))
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(cons (get-destination (~a c)) movements-so-far)))
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(define basement-position (length relative-movements))
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(length relative-movements))]
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(displayln (format "basement entered at position = ~a" basement-position))
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basement-position)]
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@subsection{Alternate approaches: @tt{for/first} or @tt{for/or}}
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@subsection{Alternate approaches: @tt{for/first} or @tt{for/or}}
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When you need to stop a loop the first time a condition occurs, you can also consider @racket[for/first] or @racket[for/or]. The difference is that @racket[for/first] ends after the first evaluation of the body, but @racket[for/or] evaluates the body every time, and ends the first time the body is not @racket[#f].
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When you need to stop a loop the first time a condition occurs, you can also consider @racket[for/first] or @racket[for/or]. The difference is that @racket[for/first] ends after the first evaluation of the body, but @racket[for/or] evaluates the body every time, and ends the first time the body is not @racket[#f].
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The two are similar. The choice comes down to readability and efficiency — meaning, if each iteration of the loop is expensive, you probably will want to cache intermediate values, which means you might as well use @racket[for/fold].
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The two are similar. The choice comes down to readability and efficiency — meaning, if each iteration of the loop is expensive, you'll probably want to cache intermediate values, which means you might as well use @racket[for/fold].
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@chunk[<day1-q2-alt>
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@chunk[<day1-q2-alt>
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(define (q2-for/first str)
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(define (q2-for/first str)
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@ -94,7 +89,6 @@ The two are similar. The choice comes down to readability and efficiency — me
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(for/first ([idx (in-range (length ints))]
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(for/first ([idx (in-range (length ints))]
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#:when (negative? (apply + (take ints idx))))
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#:when (negative? (apply + (take ints idx))))
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idx)))
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idx)))
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(displayln (format "basement entered at position = ~a" basement-position))
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basement-position)
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basement-position)
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(define (q2-for/or str)
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(define (q2-for/or str)
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@ -102,7 +96,6 @@ The two are similar. The choice comes down to readability and efficiency — me
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(let ([ints (elevator-string->ints str)])
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(let ([ints (elevator-string->ints str)])
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(for/or ([idx (in-range (length ints))])
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(for/or ([idx (in-range (length ints))])
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(and (negative? (apply + (take ints idx))) idx))))
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(and (negative? (apply + (take ints idx))) idx))))
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(displayln (format "basement entered at position = ~a" basement-position))
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basement-position)]
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basement-position)]
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