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   9 <title>section 7.8.7: Random Number Generation</title>
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  14 <H2>section 7.8.7: Random Number Generation</H2>
  15
  16 <p>There is a typo in some printings;
  17 the code for returning a floating-point random number in the
  18 interval [0,1) should be
  19 <pre>   #define frand() ((double) rand() / (RAND_MAX+1.0))
  20 </pre></p><p>If you want to get random integers from M to N,
  21 you can use something like
  22 <pre>   M + (int)(frand() * (N-M+1))
  23 </pre><pre></pre></p><p>``[Setting] the seed for <TT>rand</TT>''
  24 refers to the fact that,
  25 by default,
  26 the sequence of pseudo-random numbers
  27 returned by <TT>rand</TT>
  28 is the same each time your program runs.
  29 To randomize it,
  30 you can call <TT>srand</TT> at the beginning of the program,
  31 handing it some truly random number,
  32 such as a value having to do with the time of day.
  33 (One way is with code like
  34 <pre>   #include &lt;stdlib.h&gt;
  35         #include &lt;time.h&gt;
  36 <br>
  37 <br>
  38         srand((unsigned int)time((time_t *)NULL));
  39 </pre>which uses the <TT>time</TT> function mentioned on page 256 in
  40 appendix B10.)
  41 </p><p>One other caveat about <TT>rand</TT>:
  42 don't try to generate random 0/1 values
  43 (to simulate a coin flip, perhaps)
  44 with code like
  45 <pre>   rand() % 2
  46 </pre>This looks like it ought to work,
  47 but it turns out that
  48 on some systems <TT>rand</TT> isn't always perfectly random,
  49 and returns values which consistently alternate
  50 even, odd, even, odd, etc.
  51 (In fact, for similar reasons,
  52 you shouldn't usually use
  53 <TT>rand() % N</TT>
  54 for any value of <TT>N</TT>.)
  55 A good way to get random 0/1 values would be
  56 <pre>   (int)(frand() * 2)
  57 </pre>based on the other <TT>frand()</TT> examples above.
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  66 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
  67 // <a href="copyright.html">Copyright</a> 1995, 1996
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