Re: change one line in a large fine

Hi,

readline will be your friend.

Just have a look in the example mentioned this link:

https://www.tutorialspoint.com/perl/perl_readline.htm

Regards,
Ruprecht


Am 20.11.22 um 15:41 schrieb [email protected]:
> 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 $_ == 0 here, why while become true?
>
> I am confused about this.
>
> Regards
>
> November 20, 2022, 8:57 PM, "Kang-min Liu" <[email protected]> wrote:
>
>
>> [email protected] writes:
>>
>>> I have a large file which has millions of lines.
>>>   They are text only lines.
>>>   If I have to change one line in the file, what's the efficient way?
>>>   I don't want to slurp the whole file into memory, change that line and
>>>   write the full content back to disk again.
>>>
>> It seems like the editing is line-based, I could recommend checking out
>> 'perl -i' for doing in-place editing, if you are OK with regexp-based
>> search & replace -- it's basically the same as doing it in vim,
>>
>>   perl -p -i.orig -e "s/hello/wow/" input.txt
>>
>> This finds the first line that matces regexp ^foo$ , 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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>>

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