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    <p>This probably isn't the best way to solve your problem, but if you want to know how to make it work:</p> <p>The problem here is that your inner <code>for line in f:</code> loop consumes the whole rest of the file—so when you get back to the outer loop, there's nothing left to read. (There's a second problem: When I run your code on your data, <code>len(aline)</code> is always <em>12</em>, not <em>11</em>. But that's a trivial fix.)</p> <p>This isn't specific to files; it's how all iterators work in Python. There are two general ways to deal with this for any iterator, plus one file-specific solution.</p> <p>First, there's <a href="http://docs.python.org/2/library/itertools.html#itertools.tee" rel="nofollow"><code>itertools.tee</code></a>. This takes an iterator, and returns <em>two</em> iterators, each of which can be advanced independently. Under the covers, it obviously has to use some storage to handle things if they get out of sync, which is why the documentation says this:</p> <blockquote> <p>In general, if one iterator uses most or all of the data before another iterator starts, it is faster to use list() instead of tee().</p> </blockquote> <p>And that's the other option: Read the whole iterator into a <code>list</code>, so you can loop over slices.</p> <p>This is clearly one of those cases where one iterator uses most of the data while the other one's sitting around waiting. For example, the first time through the inner loop, you're reading lines 1-20000 before the outer loop reads line 1. So, a <code>list</code> is a better option here. So:</p> <pre><code>f = open('t.log','r') contents = list(f) f.close() for idx, line in enumerate(contents): aline = line.replace(',','').split() if len(aline)==11: for line in contents[idx+1:]: bline = line.replace(',','').split() if len(bline)==11 and aline[2]==bline[2]: print 'a: ', aline print 'b: ', bline </code></pre> <p>Finally, if you have an fancy iterator that can be checkpointed and resumed in some way, you can checkpoint it right before the inner loop, then resume it right after. And fortunately, files happen to have such a thing: <a href="http://docs.python.org/2/library/stdtypes.html#file.tell" rel="nofollow"><code>tell</code></a> returns the current file position, and <a href="http://docs.python.org/2/library/stdtypes.html#file.seek" rel="nofollow"><code>seek</code></a> jumps to a specified position. (There's a big warning saying that "If the file is opened in text mode (without <code>'b'</code>), only offsets returned by <code>tell()</code> are legal." But that's fine; you're only using offsets returned by <code>tell</code> here.)</p> <p>So:</p> <pre><code>f = open('t.log','r') for line in f: aline = line.replace(',','').split() if len(aline)==11: pos = f.tell() for line in f: bline = line.replace(',','').split() if len(bline)==11 and aline[2]==bline[2]: print 'a: ', aline print 'b: ', bline f.seek(pos) </code></pre>
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    1. COabarnert, what does this do?: for line in contents[idx+1:]:
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    2. CObtw, your code seemed to get the job done for me, but I am still trying to understand why it works
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    3. COso far I think I can tell that contents[idx+1:] returns a list starting at the next 'line'(item) in the list and ending at the end of the list. So, in the outer part of the loop, we look for alines with len==11. When we find one of these, we know the index of it because of the enumerate function. Then we proceed to inner loop through 'contents', where we use the index+1 as the starting point (this allows us to scan for new potential matches ("blines") without scanning over already-scanned items). Finally I see that we check if a line and b line are actually a match, then print if true
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