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Re^2: Trying to do multiple while loops on the same input file

by elef (Friar)
on Jun 01, 2011 at 14:17 UTC ( #907637=note: print w/ replies, xml ) Need Help??


in reply to Re: Trying to do multiple while loops on the same input file
in thread Trying to do multiple while loops on the same input file

It encourages "one-pass" processing of files. In other words it is more efficient to only read a file once (start to end). Structuring your code so that you don't need to make multiple passes through a file will make your program consume less resources and will most likely make your program more efficient.

Thanks. That makes some sense, but, honestly, I'd rather have perl treat coders as adults who can decide for themselves how many times they want to read their files. In this case, the file is about 50kB long. Even supposing that it's not left in memory between two reads, it can be read from my SSD again in a matter of milliseconds. Not a lot of time compared to the hours I spent looking to find out what's wrong, I think you'll agree.
On a more general note, I'd expect the TTMWTDI ethos to extend to allowing such "dumb" multiple reads on a filehandle, maybe throwing a warning if strictures are on. As it is, perl just fails to execute the second while loop without throwing any warning whatsoever, and whichever way you look at it, that's not very coder-friendly.


Comment on Re^2: Trying to do multiple while loops on the same input file
Re^3: Trying to do multiple while loops on the same input file
by zek152 (Pilgrim) on Jun 01, 2011 at 14:28 UTC

    It is much less a Perl way of doing things and more of an OS way of doing things. Perl gives you the seek function to do exactly what you want. Operating systems are made to handle 1kB files and also handle 3gB files. For your particular problem everything can be stored in ram (and possibly in a cache). For other problems the whole file cannot be stored in RAM.

    I am sorry that you spent hours looking for the issue. Trust me when I say that learning how file reading works will help you no matter what language you are using. I personally do not know of a language that provides the functionality that you desire in the basic read(file).I can tell you that C, C++, C#, Java, Perl and Python all read files in a start to finish manner.

    Update: fixed small typo.

      I can tell you that C, C++, C#, Java, Perl and Python all read files in a start to finish manner.

      I guess you meant "once only" instead of start to finish. That's fair enough, but if a command is going to fail, perl might as well give me some information about it. When you try to read a filehandle that never even existed, you get an "unopened filehandle" error message. I don't see why you shouldn't get an error message about an "already read" or "expired", "inactive" filehandle or something to that effect instead of the command failing silently, which makes debugging a nightmare.

        "I guess you meant 'once only' instead of start to finish."

        I did not mean that. Perhaps a better way would have been for me to say a beginning of file to end of file manner. All the aforementioned languages support some sort of seek which allows you to change the current position in the file.

        The reason that you should not get an error message is because the while(<FILE>){...} is behaving exactly as documented and in my opinion it behaves intuitively.

        As stated in perl documentation <FILE_HANDLE> will return undef when there is nothing left to read at the current location in the file (aka the end of the file). Undef evaluates as false and therefore the body of the while loop is never executed. It is important to note that the command did not fail, silently or otherwise. The command behaved exactly as documented. The filehandle is not "already read","expired" or "inactive". There is simply nothing left to be read at the current position in the file.

        I believe the primary issue is that your understanding of how file reading should work does not match how file reading actually works on modern operating systems. At the moment I do not have any advice on places to learn more about file reading other than the perldoc.perl.org. Perhaps another PerlMonk would have some additional reference material that would help you better understand files, file descriptors, and file reading.

Re^3: Trying to do multiple while loops on the same input file
by armstd (Friar) on Jun 01, 2011 at 15:14 UTC

    Well, I think I have to agree with Perl on this one. Its behavior is consistent. Once you reach the end of the file, "$line = <INPUTFILE>" is false. False ending the first loop, and still false at the beginning of any additional loops. It doesn't fail anything, it just doesn't change anything magically between loops either.

    It sounds like you would like it to be false, and then next loop reset back to the beginning of the file? That does not seem consistent to me. You'll note that you can do anything you like with INPUTFILE inside your while loop, and what would you expect the while() to do then?

    There is no explicit or implied relationship between your loop and what your loop does internal to Perl. Nor should there be imo. That's entirely for your code to establish.

    --Dave

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