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copied!<p>Standard Scheme does not provide a random number generator, and although most Scheme implementations provide one, they tend to differ in their details. If you want to write a portable Scheme program, it's easy to build your own random number generator; here's a method due to Knuth:</p> <pre><code>(define random (let ((a 69069) (c 1) (m (expt 2 32)) (seed 19380110)) (lambda new-seed (if (pair? new-seed) (set! seed (car new-seed)) (set! seed (modulo (+ (* seed a) c) m))) (/ seed m)))) </code></pre> <p>Calling <code>(random)</code> returns a random fraction between 0 (inclusive) and 1 (exclusive). The random fractions cycle with period <em>m</em>. Calling <code>(random seed)</code> resets the seed of the random number generator, so that two random sequences starting from the same seed will be identical; dates in the form YYYYMMDD make good seeds (that's Knuth's birthday above). If you want to flip a coin, say: <code>(if (< (random) 1/2) 'heads 'tails)</code>.</p> <p>Sometimes you want a random integer over a range. The <code>randint</code> function shown below returns a random integer on the range <em>lo</em> (inclusive) to <em>hi</em> (exclusive); <em>lo</em> defaults to 0:</p> <pre><code>(define (randint . args) (cond ((= (length args) 1) (floor (* (random) (car args)))) ((= (length args) 2) (+ (car args) (floor (* (random) (- (cadr args) (car args)))))) (else (error 'randint "usage: (randint [lo] hi)")))) </code></pre> <p>Random numbers such as these are good enough for simple simulations, but beware they are not suitable for cryptographic applications. If you are interested, I have several random number generators, including some suitable for cryptographic applications, at <a href="http://programmingpraxis.com/contents/themes/#Random%20Number%20Generators" rel="nofollow">my blog</a>.</p>

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