Thanks, I haven't used pre and post conditions before but this seems like a
good place to start.
On Thursday, May 8, 2014 8:17:53 PM UTC-4, James Reeves wrote:
>
> If you're representing a matrix with vectors, then any public function
> that operates on a matrix should return vectors.
>
> You may want to consider a function like:
>
> (defn matrix? [m]
> (and (vector? m)
> (every? vector m)
> (apply = (count m) (map count m)))
>
> Then add it as a pre and post condition:
>
> (defn transpose [m]
> {:pre [(matrix? m)] :post [(matrix? %)]}
> (apply mapv vector m))
>
> That'll provide some type guarantees.
>
> - James
>
>
> On 9 May 2014 00:28, Joseph Rollins <[email protected] <javascript:>>wrote:
>
>> I am confused about the best way to assure that I am dealing with a
>> vector when dealing with core library functions that often return seq where
>> I then lose my ability to access it using associative accesses (get-in
>> specifically).
>>
>> My particular use case is as follows:
>>
>> I am modeling a matrix as nested vectors
>>
>> [[0 0 0] [0 1 1] [0 1 1]]
>>
>> represents
>>
>> 0 0 0
>> 0 1 1
>> 0 1 1
>>
>> I know matrix libraries exist, but for the sake of learning am rolling my
>> own functionality. I have several functions that operate on this:
>>
>> (defn transpose [piece]
>> (apply mapv vector piece))
>>
>> (defn translate-block
>> ([mm] (let [one? #(if (= 1 %) true false)]
>> (translate-block mm one?)))
>> ([mm f] (map #(map f %) mm)))
>>
>> (defn rotate-left [b]
>> (->> b
>> transpose
>> reverse))
>>
>> The problem I am running into is that these do not output nested vectors,
>> but nested seqs (due to all of the map operations). I am using these
>> matrices to program Tetris, and I am writing a function that takes a Tetris
>> block, a Tetris board (both represented as matrices) and a position of the
>> block inside the board. The function is supposed to determine if the
>> placement of the block is valid (not overlapping already placed blocks:
>>
>> (defn is-valid?
>> "Determine if a game state is valid (is current piece place-able)."
>> [{board :board {:keys [piece pos]} :piece}]
>> (let [[x y] pos
>> coords (for [rows (range (count piece))
>> cols (range (count (first piece)))]
>> [[rows cols] [(+ y rows) (+ x cols)]])]
>> (map (fn [[c cc]]
>> (and (get-in board cc) (get-in piece c)))
>> coords)))
>>
>> The matrices hold true and false values for if occupied. When testing
>> this function I realized get-in was just returning nil because it wouldnt
>> get in a seq (non-associative). What is the best way to assure that when I
>> need associative guarantees that they haven't been thrown away through
>> previous transformations of the board or piece matrices?
>>
>> The two choices I see are making sure every function that operates on the
>> matrices explicitly outputs nested vectors again, or transforming the input
>> to functions that need the guarantees to nested vectors explicitly. Am I
>> missing something here, or, if not, what is the idiomatic way to solve this
>> problem?
>>
>> Thanks
>>
>> -Joseph
>>
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