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Re^2: How to replace Tab with spaces not altering postion

by Aristotle (Chancellor)
on Oct 11, 2002 at 21:45 UTC ( #204681=note: print w/replies, xml ) Need Help??

in reply to Re: How to replace Tab with spaces not altering postion
in thread How to replace Tab with spaces not altering postion

The problem is it's using $` which incurs a huge performance penalty.

Here's another OWTDI:

$_ = join "", map { $_ . " " x (8 - length() % 8) } split /\t/, $_, -1;
Update: blakem points out that this code is broken. It will pad all parts of the string to a multiple of 8, including the last one. Getting it not to is disappointingly awkward..
$_ = join "", do { my @bits = split /\t/, $_, -1; (map { $_ . " " x (8 - length() % 8) } @bits[0..$#bits-1]), $bits[ +-1] };
(Update is untested.)

Makeshifts last the longest.

Replies are listed 'Best First'.
Re: Re^2: How to replace Tab with spaces not altering postion
by petral (Curate) on Oct 14, 2002 at 15:27 UTC
    join "", map{ $_ . (/\n/ ? "" : " " x (8 - length() & 7)) } split /\t/

      $_ = "You\tare\tmaking\ttoo\tmany\tassumptions...";

      Makeshifts last the longest.

        Right, I was hurrying to put it up and leave.   Forgot to point out it only works for normal input files where each line ends in a newline.

        Actually, your algorithm solves the problem without abandoning regexen.   Unless we're missing something, Larry was thinking along the lines of bart's logic above, that you have to know the new length of the line up to this point.   But you are just depending on the text in front of the present tab(s) to be aligned either with the beginning of the line or the previous tab stop, so there is no need to consider the entire preceding line segment:
        $ perl -we'$_="\t\tYou\tare\tmaking\t\ttoo\tmany\tassumptions...\t\t"; s/(^|[^\t]+)(\t+)/$1." " x (length($2) * 8 - (length($1) & 7))/ge; print' You are making too many assump +tions... $ ( s/(.*?)(\t+)/...
        is simpler but presumably less efficient (which was your initial concern).   However, note that it will work correctly on a multiline string:
        $ perl -we'$_="\t\tYou\tare\tmaking\t\ttoo\n\tmany\tassumptions...\t"; s/(.*?)(\t+)/$1." "x(length($2)*8-(length($1)&7))/ge; print' You are making too many assumptions... $


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