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POEvenly select N elems from array
 

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    <p>I need to <strong>evenly</strong> select <em>n</em> elements from an array. I guess the best way to explain is by example.</p> <p>say I have:</p> <p>array [0,1,2,3,4] and I need to select 3 numbers.. 0,2,4.</p> <p>of course, if the array length &lt;= <em>n</em>, I just need to return the whole array.</p> <p>I'm pretty sure there's a defined algorithm for this, been trying to search, and I took a look at <em>Introduction to algorithms</em> but couldn't find anything that met my needs (probably overlooked it)</p> <p>The problem I'm having is that I can't figure out a way to scale this up to any array [ p..q ], selecting N evenly elements.</p> <p>note: I can't just select the even elements from the example above..</p> <p>A couple other examples;</p> <p>array[0,1,2,3,4,5,6], 3 elements ; I need to get 0,3,6<br> array[0,1,2,3,4,5], 3 elements ; I need to get 0, either 2 or 3, and 5</p> <p>EDIT:</p> <p>more examples:<br> array [0,1,2], 2 elems : 0,2<br> array [0,1,2,3,4,5,6,7], 5 elems : 0,2, either 3 or 4, 5,7</p> <p>and yes, I'd like to include first and last elements always.</p> <p>EDIT 2:</p> <p>what I was thinking was something like .. first + last element, then work my way up using the median value. Though I got stuck/confused when trying to do so.</p> <p>I'll take a look at the algo you're posting. thanks!</p> <p>EDIT 3:</p> <p>Here's a souped up version of <em>incrediman</em> solution with PHP. Works with associative arrays as well, while retaining the keys.</p> <pre><code>&lt;?php /** * Selects $x elements (evenly distributed across $set) from $set * * @param $set array : array set to select from * @param $x int : number of elements to select. positive integer * * @return array|bool : selected set, bool false on failure */ ///FIXME when $x = 1 .. return median .. right now throws a warning, division by zero function select ($set, $x) { //check params if (!is_array($set) || !is_int($x) || $x &lt; 1) return false; $n = count($set); if ($n &lt;= $x) return $set; $selected = array (); $step = ($n - 1) / ($x - 1); $keys = array_keys ($set); $values = array_values($set); for ($i=0; $i&lt;$x; $i++) { $selected[$keys[round($step*$i)]] = $values[round($step*$i)]; } return $selected; } ?&gt; </code></pre> <p>You can probably implement an <a href="http://www.php.net/manual/en/spl.iterators.php" rel="noreferrer">Iterator</a> but I don't need to take it that far.</p>
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    1. USJean Paul Galea
    1. USJean Paul Galea
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    1. COWhat numbers do you need to select? Be more specific about your pattern.
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      1. USKevin Crowell
    2. COI think you need some more examples, cause I still don't understand what you're trying to do. What about longer arrays and a different number of elements to select?
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      1. USDean Harding
    3. COIf I'm reading this correctly, the OP wants to selects a number of array elements whose indices follow some regular pattern. I think Rex Kerr's answer might better explain what is being asked here.
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      1. USbta
 

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