finding the last occurence of substring in string

gri6507 has asked for the wisdom of the Perl Monks concerning the following question:

Fellow monks

I have a scalar which contains a stream of ASCII representation of binary data (one nibble = one ASCII char). If there are any bytes that are not defined, the ASCII representation of those bytes within the stream is '??'. There can be gaps within this stream and in general, I do not know the size of this stream up front. I need to get the byte address of the last defined byte within this stream. In the following example, that would be 4. While I can do something C-like, it just seems to me that there should be a much more Perl-ish way to do the same. Any ideas?

use warnings;
use strict;

# remember that a byte consists of 2 nibbles and each nibble can be re
+presented as an ASCII character within the stream.
my $stream = '0123??6789??????';
my $lastValidByteIndex;
for my $byteIndex (0 .. length($stream)/2-1) {
    my $byte = substr($stream, $byteIndex*2, 2);
    $lastValidByteIndex = $byteIndex unless $byte eq '??';
}
if (defined($lastValidByteIndex)) {
    print "last valid byte is at index $lastValidByteIndex\n";
} else {
    print "There is no valid data in the stream\n";
}
[download]

Comment on finding the last occurence of substring in string Download Code

Replies are listed 'Best First'.
Re: finding the last occurence of substring in string by BrowserUk (Patriarch) on Jul 26, 2012 at 16:03 UTC
The position of the (first of) last 2 non-? bytes that are followed exclusively by ?s to the end of the string: `$s =~ m[([^?]{2})[?]+$] and print $-[0];; 8` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l]
Re^2: finding the last occurence of substring in string by gri6507 (Deacon) on Jul 26, 2012 at 16:28 UTC
Thanks, the match operator gets me what I was looking for. But your post brings up another question. What is the meaning of `$-[0]` ?	[reply] [d/l]
Re^3: finding the last occurence of substring in string by BrowserUk (Patriarch) on Jul 26, 2012 at 16:40 UTC
What is the meaning of $-[0] ? See the definition of `@LAST_MATCH_START` in perlvar. `$-[0]` is the value of the first element of `@-`. Which, after a successful match, contains the position in the string at which the capturing parens matched. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l] [select]
Re: finding the last occurence of substring in string by johngg (Canon) on Jul 26, 2012 at 16:15 UTC
I'm not sure I'm understanding correctly what you want to do but this gives the answer you seem to want. `$ perl -Mstrict -Mwarnings -E ' > my $last; > my $pos; > my $stream = q{0123??6789??????}; > map { $last = $pos unless m{\?\?}; $pos ++ } > unpack q{(a2)*}, $stream; > say $last;' 4 $` [download] I hope this is helpful but perhaps you could clarify your requirement if I have misunderstood. Cheers, JohnGG	[reply] [d/l]
Re: finding the last occurence of substring in string by Ratazong (Monsignor) on Jul 26, 2012 at 16:37 UTC
Just for the sake of TIMTOWTDI (or fun, or for following Matthew 20:16), you may apply the following algorithm: get the index of the reversed substring in the reversed string calculate the position (lengthOfString minus index minus lengthOfSubstring)(with a possible off-by-one-correction) Rata	[reply]
Re^2: finding the last occurence of substring in string by RichardK (Parson) on Jul 27, 2012 at 10:20 UTC
if you want to get the reverse index why not just use rindex ? :)	[reply]
Re: finding the last occurence of substring in string by daxim (Curate) on Jul 26, 2012 at 16:15 UTC
edit: this was wrong	[reply]


We don't bite newbies here... much
	PerlMonks