May i ask a question about reading file?
while(0){ print 'hi' }
Will never print hi.
cat 1.txt:
0
open FH, '1.txt' or die;
while(<FH>) { print 'hi' }
This will print hi.
Since $_ == 0 here, why while become true?
I am confused about this.
Regards
November 20, 2022, 8:57 PM, "Kang-min Liu" <gu...@gugod.org> wrote:
>
> linux...@gmx.net writes:
>
> >
> > I have a large file which has millions of lines.
> > They are text only lines.
> > If I have to change one line in the file, what's the efficient way?
> > I don't want to slurp the whole file into memory, change that line and
> > write the full content back to disk again.
> >
>
> It seems like the editing is line-based, I could recommend checking out
> 'perl -i' for doing in-place editing, if you are OK with regexp-based
> search & replace -- it's basically the same as doing it in vim,
>
> perl -p -i.orig -e "s/hello/wow/" input.txt
>
> This finds the first line that matces regexp ^foo$ , then replaces the
> entire line with "foobar", shift the remainder of the file correctly and
> wrote everything back to input.txt -- while keep an original copy of
> input.txt at
> input.txt.orig
>
> However that may match on multiple lines, if you know the line number in
> advance, you could check the line number variable $.
>
> perl -p -i -e "s/^.+$/wow/ if $. == 2" input.txt
>
> Note that doing this would still scan the entire input.txt line by
> line. And adding `exit()` or `next` in the body of `-e` would make the
> program finish early but would also truncate input.txt -- which is
> probblay not what we want.
>
> ----
>
> Alternatively, if you are looking for doing this with some code but not
> with "perl" command, read on...
>
> How efficient it could be depends a little bit on how the target line is
> identified and the kind of editing that's required.
>
> For sure you could avoid slupring by doing doing line-based changes
> like:
>
> while (defined(my $line = <$fh>)) {
> ...
> }
>
> If the editing is replacing $line with something that's equal in length,
> then it can be pretty efficient -- just print the thing and the file is
> modified in place.
>
> If, say, we want to just just the 42nd line in the file, here's how I
> would do:
>
> # Open as read-write mode.
> open my $fh, "+<", "input.txt";
>
> # Seek to the beginning of 42nd line
> my $lineno = 1;
> while (defined(my $line = <$fh>)) {
> $lineno += 1;
> last if $lineno == 42;
> }
>
> # Print the new content at the begging of 42nd line.
> print $fh $newcontent;
>
> This is the most efficient scenario because the program can end here
> without reading the remainder of input.txt.
>
> However, if $newcontent is longer than the 42nd line, the program would
> still finish and when we inspect the file, we'll see that the text in
> $newcontext bleed over to the 43rd line and maybe further lines.
>
> Similarly, if the $newcontent is shorter, the original conten in the
> 42nd line will only be partially replaced.
>
> Most likely that's not the kind of editing we want to be doing.
>
> Meaninng, if $newcontent is longer or shorter, the remainder of the file
> should be shifted a few characters forward or backword and we want to
> re-print those lines back to $fh -- which also requires a lot of bookkeeping
> code just to get everything corner case right.
>
> If the editing we want is rather generic I'd say we probably want to put
> the output to a different file instead of doing in-place editing.
>
> And we will still end up slurping the entire file, but only keeping one
> line at a time in memory.
>
> --
> Cheers,
> Kang-min Liu
>
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