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POList Comprehensions in Python : efficient selection in a list
 

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    <p>Let's suppose that I have a list of elements, and I want to select only some of them, according to a certain function (for example a <em>distance</em> to an other element).</p> <p>I want to have as a result a list of tuple, with the distance and the element. So, I wrote the following code</p> <pre><code>result = [ ( myFunction(C), C) for C in originalList if myFunction(C) &lt; limit ] </code></pre> <p>But <code>myFunction</code> is a very time-consuming function, and the <code>originalList</code> quite big. So doing like that, <code>myFunction</code> will be call twice for every selected element.</p> <p>So, is there a way to avoid this ??</p> <p>I have two other possibilities, but they are not so good:</p> <ol> <li><p>the first one, is to create the unfiltered list</p> <pre><code>unfiltered = [ (myFunction(C),C) for C in originalList ] </code></pre> <p>and then sort it</p> <pre><code>result = [ (dist,C) for dist,C in unfiltered if dist &lt; limit ] </code></pre> <p>but in that case, I duplicate my <code>originalList</code> and waste some memory (the list could be quite big - more than 10,000 elements)</p></li> <li><p>the second one is tricky and not very pythonic, but efficient (the best we can do, since the function should be evaluated once per element). <code>myFunction</code> stores it last<br> result in a global variable (<code>lastResult</code> for example), and this value is re-used in the List comprehension </p> <pre><code>result = [ (lastResult,C) for C in originalList if myFunction(C) &lt; limit ] </code></pre></li> </ol> <p>Do you have any better idea to achieve that, in an efficient and pythonic way ??</p> <p>Thanks for your answers.</p>
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