Beefy Boxes and Bandwidth Generously Provided by pair Networks
Your skill will accomplish
what the force of many cannot
 
PerlMonks  

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??
All,
There is a long story behind this that involves a Java programmer asking for some help with Perl. I won't get into the particulars other than to say the question asked was:

What's the easiest way to loop through a comma delimited file and append the line minus 1 column into a new file that is the same name as the excluded column?

The file in question was about 20 lines long. I gave my disclaimer about normally using a module to handle CSV, but the following code should work:

#!/usr/bin/perl use strict; use warnings; while ( <DATA> ) { my @field = split /,/; my $file = splice @field, 2, 1; open (OUTPUT, '>>', $file) or die $!; print OUTPUT join ',', @field; } __DATA__ 1,2,foo,3 4,5,bar,6 7,8,foo,9
I asked the Java programmer the next day how it worked and I was informed that it was too slow and that a Java program was being written instead. Scratching my head, I asked if the same file I was shown before was the one actually being used. It wasn't - multiple files millions of lines long each. Opening and closing file(s) that many times is bound to be slow. I offered the following modification* of the code provided the column being excluded was fairly repetetive in the file:
#!/usr/bin/perl use strict; use warnings; my %fh; while ( <DATA> ) { my @field = split /,/; my $file = splice @field, 2, 1; if ( ! $fh{$file} ) { open ($fh{$file}, '>>', $file) or die $!; } print { $fh{$file} } join ',', @field; } __DATA__ 1,2,foo,3 4,5,bar,6 7,8,foo,9
I explained that the reason for the disclaimer was that that the hash only bought performance if a file had more than 1 line getting appended to it. Additionally, if there are too many unique files, memory and/or open file descriptors may cause a problem. I was then told that the Java code was nearly done but thanks anyway. *shrug* - exit stage right.

I think I am missing how Java is going to be that much faster. I assume Java is still going to open and close the file each time through the loop unless there is a similar trick. Given that I don't really know Java I could be out in left field here.

Leaving Java aside, is there more run-time efficient way than my second suggestion in Perl? I haven't given it a lot of thought because the Java developer is just being silly. It is a run one and done script so it would already be finished if the first version (wrapped in a tiny shell script) had been allowed to run. On the other hand, this is the sort of thing that I like to be aware of in the future. (Prior Planning Prevents Poor Performance)**

Cheers - L~R

* The actual code used ARGV
** I learned this in the military, but there were a couple extra explicitive Ps

In reply to Performance Trap - Opening/Closing Files Inside a Loop by Limbic~Region

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • Outside of code tags, you may need to use entities for some characters:
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others scrutinizing the Monastery: (6)
    As of 2014-09-23 03:56 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      How do you remember the number of days in each month?











      Results (210 votes), past polls