open and read text file
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From: Ben Morrow on 12 Mar 2010 18:00 Quoth jt(a)toerring.de (Jens Thoms Toerring): > cerr <ron.eggler(a)gmail.com> wrote: > > I'm just trying to open a text file and print line by line. > > My Code: > > my $HANDLE = $filename; > > open(HANDLE) or die("Could not open GPS source file."); > > Because your use of open() is completely broken. No kind of > open() function I have ever seen (as far as I remember) works > like that. It's a Perl 4ism that is never used nowadays, but still documented and supported. A bare open HANDLE; takes the filename from the global $HANDLE in the current package. In the OP's case it's failing because he sets a lexical $HANDLE instead of a global. > All take a file name and return a handle that in > subsequent calls of functions to read from or write to the > file etc. is used. open doesn't return a filehandle (though some things would be more convenient if it did). It opens an existing filehandle, auto-vivifying it if necessary. (It is perfectly OK to pass an already-open filehandle to open, and it will close it first.) Ben
From: Jens Thoms Toerring on 12 Mar 2010 18:23 Ben Morrow <ben(a)morrow.me.uk> wrote: > Quoth jt(a)toerring.de (Jens Thoms Toerring): > > cerr <ron.eggler(a)gmail.com> wrote: > > > I'm just trying to open a text file and print line by line. > > > My Code: > > > my $HANDLE = $filename; > > > open(HANDLE) or die("Could not open GPS source file."); > > > > Because your use of open() is completely broken. No kind of > > open() function I have ever seen (as far as I remember) works > > like that. > It's a Perl 4ism that is never used nowadays, but still documented and > supported. A bare > open HANDLE; Uuups, that's from before my time with Perl. And do I feel lucky;-) > takes the filename from the global $HANDLE in the current package. In > the OP's case it's failing because he sets a lexical $HANDLE instead of > a global. > > All take a file name and return a handle that in > > subsequent calls of functions to read from or write to the > > file etc. is used. > open doesn't return a filehandle (though some things would be more > convenient if it did). It opens an existing filehandle, auto-vivifying > it if necessary. (It is perfectly OK to pass an already-open filehandle > to open, and it will close it first.) Mmmm, what means auto-vivifying in this context? And what "opens an existing file handle"? Obviously, not understanding enough I just see a variable, not an "existing file handle". Since I can't remember you being wrong on such things I guess there's some kind of magic going on I didn't grok yet, so a bit more of explanation would be great! And I also didn't know about a file getting closed automatically when the file handle is re-used. What happens when you do e.g. open my $h1, '<', 'filename1.txt'; my $h2 = $h1; open $h1, '<', 'filename2.txt'; Does that close the first file or does it stay open because of '$h2' still refering to it? Best regards, Jens -- \ Jens Thoms Toerring ___ jt(a)toerring.de \__________________________ http://toerring.de
From: John Bokma on 12 Mar 2010 18:35 jt(a)toerring.de (Jens Thoms Toerring) writes: > open my $handle, '<', $filename or die "Can't open file\n"; To me the acceptable minimum for the rhs of or would be: die "Can't open '$filename': $!"; this reports and the filename and why it couldn't be opened. Personally I prefer to add "for reading" since I consider it more clear compared to just "Can't open". -- John Bokma j3b Hacking & Hiking in Mexico - http://johnbokma.com/ http://castleamber.com/ - Perl & Python Development
From: Uri Guttman on 12 Mar 2010 19:02 >>>>> "JTT" == Jens Thoms Toerring <jt(a)toerring.de> writes: >> It's a Perl 4ism that is never used nowadays, but still documented and >> supported. A bare >> open HANDLE; JTT> Uuups, that's from before my time with Perl. And do I feel lucky;-) and it still is supported. nasty old concept. it is used in golfing sometimes. >> open doesn't return a filehandle (though some things would be more >> convenient if it did). It opens an existing filehandle, auto-vivifying >> it if necessary. (It is perfectly OK to pass an already-open filehandle >> to open, and it will close it first.) JTT> Mmmm, what means auto-vivifying in this context? And what "opens an JTT> existing file handle"? Obviously, not understanding enough I just see JTT> a variable, not an "existing file handle". Since I can't remember you JTT> being wrong on such things I guess there's some kind of magic going on JTT> I didn't grok yet, so a bit more of explanation would be great! open my $handle will autovivify a handle in the scalar if it was undef. same as $ref->{foo} will do if $ref is undef. it used to be you had to have a typeglob or ref to one in the variable so open would work. the better lazy way is for perl to do it for you. JTT> And I also didn't know about a file getting closed automatically when JTT> the file handle is re-used. What happens when you do e.g. JTT> open my $h1, '<', 'filename1.txt'; JTT> my $h2 = $h1; JTT> open $h1, '<', 'filename2.txt'; JTT> Does that close the first file or does it stay open because of JTT> '$h2' still refering to it? the handle isn't really in the scalar, but is refered to by the scalar (the handle is actually a typeglob entry). the file itself will be closed as will the handle itself. the extra copy now refers to a closed file and will spit out an error if used for i/o. uri -- Uri Guttman ------ uri(a)stemsystems.com -------- http://www.sysarch.com -- ----- Perl Code Review , Architecture, Development, Training, Support ------ --------- Gourmet Hot Cocoa Mix ---- http://bestfriendscocoa.com ---------
From: J�rgen Exner on 12 Mar 2010 19:07
cerr <ron.eggler(a)gmail.com> wrote: >I'm just trying to open a text file and print line by line. >My Code: > my $HANDLE = $filename; > open(HANDLE) or die("Could not open GPS source file."); This style is an anachronism from the dark ages, supported only for backward compatibility of ancient code. Today you should use the three-argument form of open() with a lexical file handle open ($Handle, '<', $filename) and in the error message I would at least include the filename and the reason why the error occured: or die ("Couldn't open GPS source file '$filename': $!";) jue |