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POGenerate two lists at once
 

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  1. POGenerate two lists at once
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    <p><strong>Background</strong></p> <p>The algorithm manipulates financial analytics. There are multiple lists of the same size and they are filtered into other lists for analysis. I am doing the same filtering on different by parallel lists. I could set it up so that a1,b1,c2 occur as a tuple in a list but then the analytics have to stripe the tuples the other way to do analysis (regression of one list against the other, beta, etc.).</p> <p><strong>What I want to do</strong></p> <p>I want to generate two different lists based on a third list:</p> <pre><code>&gt;&gt;&gt; a = list(range(10)) &gt;&gt;&gt; b = list(range(10,20)) &gt;&gt;&gt; c = list(i &amp; 1 for i in range(10)) &gt;&gt;&gt; &gt;&gt;&gt; aprime = [a1 for a1, c1 in zip(a,c) if c1 == 0] &gt;&gt;&gt; bprime = [b1 for b1, c1 in zip(b,c) if c1 == 0] &gt;&gt;&gt; aprime [0, 2, 4, 6, 8] &gt;&gt;&gt; bprime [10, 12, 14, 16, 18] </code></pre> <p>It seems there should be a pythonic/functional programming/itertools way to create the two lists and iterate over the three lists only once. Something like:</p> <pre><code>aprime, bprime = [a1, b1 for a1, b1, c1 in zip(a,b,c) if c1 == 0] </code></pre> <p>But of course this generates a syntax error.</p> <p><strong>The question</strong></p> <p>Is there a pythonic way?</p> <p><strong>Micro-optimization shootout</strong></p> <p>The ugly but pythonic-to-the-max one-liner edges out the "just use a for-loop" solution and my original code in the ever popular <em>timeit</em> cage match:</p> <pre><code>&gt;&gt;&gt; import timeit &gt;&gt;&gt; timeit.timeit("z2(a,b,c)", "n=100;a = list(range(n)); b = list(range(10,10+n)); c = list(i &amp; 1 for i in range(n));\ndef z2(a,b,c):\n\treturn zip(*[(a1,b1) for a1,b1,c1 in zip(a,b,c) if c1==0])\n") 26.977873025761482 &gt;&gt;&gt; timeit.timeit("z2(a,b,c)", "n=100;a = list(range(n)); b = list(range(10,10+n)); c = list(i &amp; 1 for i in range(n));\ndef z2(a,b,c):\n\taprime, bprime = [], [];\n\tfor a1, b1, c1 in zip(a, b, c):\n\t\tif c1 == 0:\n\t\t\taprime.append(a1); bprime.append(b1);\n\treturn aprime, bprime\n") 32.232914169258947 &gt;&gt;&gt; timeit.timeit("z2(a,b,c)", "n=100;a = list(range(n)); b = list(range(10,10+n)); c = list(i &amp; 1 for i in range(n));\ndef z2(a,b,c):\n\treturn [a1 for a1, c1 in zip(a,c) if c1 == 0], [b1 for b1, c1 in zip(b,c) if c1 == 0]\n") 32.37302275847901 </code></pre>
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    1. COWhy are you trying to iterate over lists in parallel? What's the overall problem you're tying to solve? It's probably simpler than trying to iterate over multiple parallel lists.
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    2. COThe algorithm manipulates financial analytics. There are multiple lists of the same size and they are filtered into other lists for analysis. It really is the case that I am doing the same filtering on different by parallel lists. I could set it up so that a1,b1,c2 occur as a tuple in a list but then the analytics have to stripe the tuples the other way to do analysis (regression of one list against the other, beta, etc.).
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    3. COI don't get what the problem is with the code you have for creating aprime and bprime. In what way is it incorrect or unsuitable?
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