bimleshsharma has asked for the wisdom of the Perl Monks concerning the following question:

I have csv file and i want to modify some field's value. I tried by creating new csv with modified value and then renamed with original one but i want to open that file in read/write mode and do simultaneously(no rename). Here is what i tried but not successful. Thanks in advance.

sub modifying_csv
{
    my $file=shift;
    open FH,"+<$file";
    while(<FH>)
    {
        my $line= $_;
        if($line =~ /43950635/)
        {
            $line =~ s/(.*)(43950635)(.*)/"$1$2BIM$3"/;
            $line =~ s/\"//g;
            #print  "$line";
            print FH "$line";
        }
    }
    close FH;

[download]

Replies are listed 'Best First'.
Re: Modify csv file.
by Tux (Canon) on Nov 05, 2012 at 12:10 UTC

    Have a look at DBD::CSV.

    There are lots and lots of questions asked like these, so a search might help you too.


    Enjoy, Have FUN! H.Merijn
[reply]
Re: Modify csv file.
by BillKSmith (Monsignor) on Nov 05, 2012 at 13:27 UTC

    It is impossible to modify any type of file "in-place" unless the replacement fields each have exactly the same number of characters as the fields they replace. Even in that case it is usually easier to copy and rename the file as you suggested. The details often be handled automatically. (Refer to runtime options -i and -p in perldoc perlrun)

    Bill
[reply]

      Actually it's not impossible to modify any type of file "in place". Just for Fun ... to update a file "in place".

      ...
    open(my $READER,'+<',$Filename_S)
        or die "Can't open '$Filename_S'! $!";
    open(my $WRITER,'+<',$Filename_S)                      # Don't use
+ '+>'
        or die "Can't open '$Filename_S'! $!";
    # The overflow buffer:
    my @Buffer_a;
    while (<$READER>) {
        # Read (past tense) a line - buffer its replacement ...

        # Modify the line here!
        my $Update_s=...
         
        push(@Buffer_a,$Update_s);
        # Write from the overflow buffer if we can ...
        while (@Buffer_a && length($Buffer_a[0]) < tell($READER)-tell(
+$WRITER)) {
            # Enough room to write $Buffer_a[0] so write it ...
            print $WRITER shift(@Buffer_a);
             };
         };
    # Nothing more to read ...
    close($READER)
        or die "Can't close '$Filename_S'! $!";
    # If there's anything in the buffer write it ...
    while (@Buffer_a) {
        print $WRITER shift(@Buffer_a);
         };
    # Truncate the file, in case, what we're writing is shorter than w
+hat we read
    truncate($WRITER,tell($WRITER));
    #  ... and close
    close($WRITER)
        or die "Can't close '$Filename_S'! $!";
...

      [download]
      Be aware that if something should happen during the update one may be "up the creek without a paddle".
[reply]
[d/l]
Re: Modify csv file.
by Anonymous Monk on Nov 06, 2012 at 01:14 UTC
[reply]
Re: Modify csv file.
by marinersk (Priest) on Nov 06, 2012 at 07:16 UTC
    bimleshsharma,

    Excellent replies in this thread but I fear none communicate what appears to be the primary faulty assumption you seem to making.

    Modifying a file in place is usually left to the realm of binary file I/O for a list of reasons which could fill a chapter in CompSci 110.

    CSV files are notoriously left to the realm of sequential text file processing, for a list of reasons that could at least fill a Wiki page somewhere.

    You are, essentially, trying to adjust a sequential text file using a binary file approach. This is fraught with difficulties (and as one clever poster demonstrates, fun ones to solve) and for the sake of maintainable code, should probably be avoided.

    One approach which might suit you better (and it might not, it is an engineering decision you will have to make) is to use the slurp-digest-spew model. Read the whole CSV into an array. Adjust the array in memory. Write the whole array out to the original file. And pray there is not a power outage in the middle of the write loop or you risk corrupting your data file.

    I sincerely hope this, added to the immensely useful information already provided, may shed some light on why the thing you are attempting is giving you such headaches.

    You are trying to fit a square peg into a round hole, which upon occasion may require you to grind away at the corners of the peg. This may not constitute good software engineering for your project when all is said and done.

    As always, however, that is your call.

[reply]

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