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2 points by jsgrahamus 618 days ago | link | parent

Here is a related post: http://arclanguage.org/item?id=4370


2 points by jsgrahamus 618 days ago | link

Here is a version from the linked post altered to take any size n. Can you see the cause of the error?

  (def valid? (stack)
    (let i 0
      (if (or (pos stack.0 cdr.stack)
              (some (fn(x)(++ i)(is i (abs:- x stack.0))) cdr.stack))
        nil
        t)))

  (def queens2 (max size stack)
    (if (is size max)
      (and (prall rev.stack "[ ") (prn " ]"))
      (for rank 1 max
        (push rank stack)
        (if (valid? stack)
            (queens2 max (+ size 1) stack))
        (pop stack))))

  (def queens (max)
     (with (size 0 stack nil)
        (queens2 max size stack)))

  arc> (queens 4)
  Error: "_R: undefined;\n cannot reference undefined identifier"
  arc>

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3 points by rocketnia 618 days ago | link

Whenever you see an error message complain about an undefined identifier named "_R", that's because of a long-standing issue with the Racket reader in Windows terminals. If you paste multiple lines into the terminal, what you paste needs to have a blank line at the end already or else Racket will think it sees an R somewhere in the middle of your code.

It's not your fault. It's a bummer to have to work around this.

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2 points by akkartik 618 days ago | link

BTW, you can always simplify (if <expression> nil t) to just (no <expression>).

Also, what's that i variable doing in valid?? I don't understand how valid? works..

You _really_ don't want to be making pass-by-reference changes like incrementing i inside some, since there's no guarantee about the order in which it'll run. If you want imperative updates, just use an explicit loop like each.

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1 point by akkartik 618 days ago | link

Hmm, that works for me on both Anarki and Arc 3.1. Are you running on Windows or something like that?

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1 point by akkartik 618 days ago | link

Ah, thanks! Lots to learn there.

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