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    <p>You don't just need a random number generator here, you need one in conjunction with a shuffling algorithm.</p> <p>Create an array of <code>N</code> items (we'll use seven for our example), each containing the integer relating to its position:</p> <pre><code>+---+---+---+---+---+---+---+ | 1 | 2 | 3 | 4 | 5 | 6 | 7 | +---+---+---+---+---+---+---+ &lt;pool(7) </code></pre> <p>and set the pool size to 7.</p> <p>Then generate your random number, based on the pool size (i.e., get a number from 1 to 7). Let's say your generator returns 3.</p> <p>Pull out the value at position 3 then replace that with the top value, then reduce the pool size:</p> <pre><code>+---+---+---+---+---+---+---+ | 1 | 2 | 7 | 4 | 5 | 6 | 7 | -&gt; 3 +---+---+---+---+---+---+---+ &lt;pool(6) </code></pre> <p>Then you just keep doing this until you've gotten the quantity of values required. If our lotto was 5 from 7:</p> <pre><code>+---+---+---+---+---+---+---+ | 1 | 2 | 7 | 4 | 5 | 6 | 7 | +---+---+---+---+---+---+---+ &lt;pool(7) rnd(7) returns 3 +---+---+---+---+---+---+---+ | 1 | 2 | 7 | 4 | 5 | 6 | 7 | -&gt; 3 +---+---+---+---+---+---+---+ &lt;pool(6) rnd(6) returns 1 +---+---+---+---+---+---+---+ | 6 | 2 | 7 | 4 | 5 | 6 | 7 | -&gt; 1 +---+---+---+---+---+---+---+ &lt;pool(5) rnd(5) returns 5 +---+---+---+---+---+---+---+ | 6 | 2 | 7 | 4 | 5 | 6 | 7 | -&gt; 5 +---+---+---+---+---+---+---+ &lt;pool(4) rnd(4) returns 2 +---+---+---+---+---+---+---+ | 6 | 4 | 7 | 4 | 5 | 6 | 7 | -&gt; 2 +---+---+---+---+---+---+---+ &lt;pool(3) rnd(3) returns 1 +---+---+---+---+---+---+---+ | 7 | 4 | 7 | 4 | 5 | 6 | 7 | -&gt; 6 +---+---+---+---+---+---+---+ &lt;pool(2) </code></pre> <p>and there you have it, 5-from-7 numbers (3,1,5,2,6) extracted with no possibilities of duplicates and an efficient O(n) method for getting them. Any solution that relies on just getting random numbers and checking if they've already been used will be less efficient.</p>
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    1. POVB.NET - Removing a number from a random number generator
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    1. COI don't think you need to shuffle the numbers if you plan to randomly select one to remove. Feels a little bit like a belt with suspenders, it'll work, but a little redundant.
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    2. COThe shuffling is necessary to ensure that further selection is an O(1) operation. If you don't shuffle and rnd(7) returns 6 twice, that's no different to having to keep a list of previously used numbers to check against. The shuffling guarantees that selection of the same number twice is impossible, while still only having to call rnd() once. It's the best way of taking used numbers out of the pool.
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    3. COYou may be suffering under the misapprehension that shuffling means a lot of swaps. Shuffling in this case is simply moving the top element into the one you've just extracted and reducing the pool size. It means you can implement it with a simple fast array instead of a collection type.
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