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    <p>All the answers so far are mathematically wrong. Returning <code>rand() % N</code> does not uniformly give a number in the range <code>[0, N)</code> unless <code>N</code> divides the length of the interval into which <code>rand()</code> returns (i.e. is a power of 2). Furthermore, one has no idea whether the moduli of <code>rand()</code> are independent: it's possible that they go <code>0, 1, 2, ...</code>, which is uniform but not very random. The only assumption it seems reasonable to make is that <code>rand()</code> puts out a Poisson distribution: any two nonoverlapping subintervals of the same size are equally likely and independent. For a finite set of values, this implies a uniform distribution and also ensures that the values of <code>rand()</code> are nicely scattered.</p> <p>This means that the only correct way of changing the range of <code>rand()</code> is to divide it into boxes; for example, if <code>RAND_MAX == 11</code> and you want a range of <code>1..6</code>, you should assign <code>{0,1}</code> to 1, <code>{2,3}</code> to 2, and so on. These are disjoint, equally-sized intervals and thus are uniformly and independently distributed.</p> <p>The suggestion to use floating-point division is mathematically plausible but suffers from rounding issues in principle. Perhaps <code>double</code> is high-enough precision to make it work; perhaps not. I don't know and I don't want to have to figure it out; in any case, the answer is system-dependent.</p> <p>The correct way is to use integer arithmetic. That is, you want something like the following:</p> <pre><code>#include &lt;stdlib.h&gt; // For random(), RAND_MAX // Assumes 0 &lt;= max &lt;= RAND_MAX // Returns in the closed interval [0, max] long random_at_most(long max) { unsigned long // max &lt;= RAND_MAX &lt; ULONG_MAX, so this is okay. num_bins = (unsigned long) max + 1, num_rand = (unsigned long) RAND_MAX + 1, bin_size = num_rand / num_bins, defect = num_rand % num_bins; long x; do { x = random(); } // This is carefully written not to overflow while (num_rand - defect &lt;= (unsigned long)x); // Truncated division is intentional return x/bin_size; } </code></pre> <p>The loop is necessary to get a perfectly uniform distribution. For example, if you are given random numbers from 0 to 2 and you want only ones from 0 to 1, you just keep pulling until you don't get a 2; it's not hard to check that this gives 0 or 1 with equal probability. This method is also described in the link that nos gave in their answer, though coded differently. I'm using <code>random()</code> rather than <code>rand()</code> as it has a better distribution (as noted by the man page for <code>rand()</code>).</p> <p>If you want to get random values outside the default range <code>[0, RAND_MAX]</code>, then you have to do something tricky. Perhaps the most expedient is to define a function <code>random_extended()</code> that pulls <code>n</code> bits (using <code>random_at_most()</code>) and returns in <code>[0, 2**n)</code>, and then apply <code>random_at_most()</code> with <code>random_extended()</code> in place of <code>random()</code> (and <code>2**n - 1</code> in place of <code>RAND_MAX</code>) to pull a random value less than <code>2**n</code>, assuming you have a numerical type that can hold such a value. Finally, of course, you can get values in <code>[min, max]</code> using <code>min + random_at_most(max - min)</code>, including negative values.</p>
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    1. POHow to generate a random integer number from within a range
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    1. USRyan Reich
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    1. COtried this in xcode 4.5.1 .. am getting this error -> Undefined symbols for architecture i386: "_random_in_range",
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    2. CO@raw3d How is the method called in your program? This is a recursive function that calls *itself*.
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    3. CO@Adam Rosenfield,@Ryan Reich : In a related question where Adam had answered:http://stackoverflow.com/questions/137783/expand-a-random-range-from-15-to-17..In the most upvoted answer : The usage of 'modulus' would then be incorrect, no? To generate 1..7 from 1..21,the procedure what Ryan has described should be used.Please correct me if I am wrong.
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