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How to add a SEED into your own function

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To add a SEED to your own function and make it work you need to follow three simple rules:

  1. Add keyword SEED into the function arguments.
  2. (rnd-seed seed) should be placed at the beginning of the function.
  3. Nested functions that use seed need to use a (seed) function and not a seed argument.

Example:

(defun rnd-number (n low high &key seed)
  (do-verbose ("rnd-number")
    (rnd-seed seed)
    (if (zerop n)
        nil
        (cons (round (+ low (rnd-round 0 (- high low) :seed (seed))))
              (rnd-number (decf n) low high :seed (seed))))))

 

Each time we evaluate the expression we get a different result.

(rnd-number 12 0 5)
=> (0 3 2 5 3 3 4 3 2 2 4 4)

 

Adding seed to the expression will produce always the same result.

(rnd-number 12 0 5 :seed 45)
=> (2 2 5 4 5 0 5 3 5 3 1 1)

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