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POPython - Splitting an array into two using an optimized for loop
 

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  1. POPython - Splitting an array into two using an optimized for loop
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    <p>This is a followup question to a question I posted <a href="https://stackoverflow.com/questions/20223962/python-operations-on-all-elements-of-an-array-without-a-loop">here</a>, but it's a very different question, so I thought I would post it separately.</p> <p>I have a Python script which reads an very large array, and I needed to optimize an operation on each element (see referenced SO question). I now need to split the output array into two separate arrays.</p> <p>I have the code:</p> <pre><code>output = [True if (len(element_in_array) % 2) else False for element_in_array in master_list] </code></pre> <p>which outputs an array of length <code>len(master_list)</code> consisting of True or False, depending on if the length of <code>element_in_array</code> is odd or even. My problem is that I need to split <code>master_list</code> into two arrays: one array containing the <code>element_in_array</code>'s that correspond to the <code>True</code> elements in <code>output</code> and another containing the <code>element_in_array</code>'s corresponding to the <code>False</code> elements in <code>output</code>.</p> <p>This can clearly be done with traditional array operators such as <code>append</code>, but I need this to be as optimized and as fast as possible. I have many millions of elements in my <code>master_list</code>, so is there a way to accomplish this without directly looping through <code>master_list</code> and using <code>append</code> to create two new arrays.</p> <p>Any advice would be greatly appreciated. Thanks!</p>
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    1. COSo you're appending all the trues, aka even, to the first list. It should be a single for loop, which means O(n), you really can not go faster than a linear time loop here.
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    2. COIf you have a very large array, can you use a [NumPy](http://www.numpy.org) array instead of a pure Python list? If so, you can probably do it in simpler code, which takes about 1/10th as long to run, and uses about 1/4th the storage.
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    3. COAs a side note, `True if foo else False` is simpler (and often faster) as `bool(foo)`.
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