Change variable multiple times within a single string?

[JTomb]

Hi. If I have the following:

$string="A111B111A111B111";
$yes_or_no="no";

Is there a way to read that string and set/reset $yes_or_no to "yes" each time it finds an A and back "no" when it finds the subsequent B?

Thanks!

Joe

[toolic]

split

use warnings;
use strict;

my $string="A111B111A111B111";
my $yes_or_no="no";
for (split //, $string) {
    if ($_ eq 'A') {
        $yes_or_no = 'yes';
        print "$_ yes_or_no=$yes_or_no\n";
    }
    if ($_ eq 'B') {
        $yes_or_no = 'no';
        print "$_ yes_or_no=$yes_or_no\n";
    }
}

__END__

A yes_or_no=yes
B yes_or_no=no
A yes_or_no=yes
B yes_or_no=no

[hdb]

Use a translation table (hash):

use strict;
use warnings;

my $string="A111B111A111B111";
my $yes_or_no="no";
my %decision = ( A => 'yes', B=> 'no' );

for (split //, $string ) {
  $yes_or_no = $decision{$_} if exists $decision{$_};
  print "$_ ($yes_or_no)\n";
}

[2teez]

Hi hdb,
Your solution would not only either set/reset and print the value 'yes' and 'no' for 'A' and 'B', but would also do for subsequent '1' also. And such that either 'yes' or 'no', were printed when the value of '1' is seen.
Though, the OP wanted 'yes' or 'no', for 'A' and 'B'.
So, I added this next if /1/; like so:

for (split //, $string ) {
  next if /1/;
  $yes_or_no = $decision{$_} if exists $decision{$_};
  print "$_ ($yes_or_no)\n";
}

Though the OP didn't give any information on what he expect when '1' is seen.

[hdb]

That is not correct! The value of the variable $yes_or_no will only be changed on the occurrence of 'A' or 'B' in the string. That is what  if exists $decision{$_}; is for, there is no entry of for '1' in the hash.

Printing the variable in each line is only showing the current value. The OP had not really said what he wanted to do while running through the string, so I made something up.

In a way, the print statement is completely superfluous, as the script would set the variable according to the wishes of the OP, but it would not be observable...

[davido]

my $state  = 'no';
my $string = 'A111B111A111B111';
while(
  $string =~ m/
    (?<chunk>
      (?<condition>[AB])
      (?<value>\d+)
    )
  /gx
) {
    $state = $+{condition} eq 'A' ? 'yes' : 'no';
    print "($+{chunk}) => State: ($state) => Value ($+{value})\n";
}

[toolic]

That prints all yeses for me and a bunch of warnings (change == to eq, perhaps?).

[davido]

:) Thanks. Fixed.

(== should have been eq)

[BillKSmith]

If you only need the final state. This code returns 'yes' only if there is an 'A' in the string and there is no 'B' after it.

use warnings;
use strict;
my $string="A111B111A111B111";
my $yes_or_no= ($string =~ m/A[^B]*$/) ? 'yes' : 'no';
print "yes_or_no=$yes_or_no\n";

Update: The original description is not quite accurate. It should read "This code returns 'yes' only if there is at least one 'A' in the string and there is no 'B' after the final 'A'."

[AnomalousMonk]

Another possibility:

>perl -wMstrict -le
"for my $s (qw(
   B111  B111B111  B111B111B111
   A111  A111A111  A111A111A111
   A111B111  A111B111A111  A111B111A111B111  A111B111A111B111A111
   B111A111  B111A111B111  B111A111B111A111  B111A111B111A111B111
   B  BB  BBB  A  AA  AAA  BA  BAB  AB  ABA
   xxx  xx  x), '',
   ) {
   local our $y_n;
   ()= $s =~ m{ ([AB]) (?{ $y_n = $^N eq 'A' ? 'yes' : 'no' }) }xmsg;
   if (defined $y_n) {
     printf qq{%3s: '%s' \n}, $y_n, $s;
     }
   else {
     warn qq{no A or B found in '$s'};
     }
   }
"
 no: 'B111'
 no: 'B111B111'
 no: 'B111B111B111'
yes: 'A111'
yes: 'A111A111'
yes: 'A111A111A111'
 no: 'A111B111'
yes: 'A111B111A111'
 no: 'A111B111A111B111'
yes: 'A111B111A111B111A111'
yes: 'B111A111'
 no: 'B111A111B111'
yes: 'B111A111B111A111'
 no: 'B111A111B111A111B111'
 no: 'B'
 no: 'BB'
 no: 'BBB'
yes: 'A'
yes: 'AA'
yes: 'AAA'
yes: 'BA'
 no: 'BAB'
 no: 'AB'
yes: 'ABA'
no A or B found in 'xxx' at -e line 1.
no A or B found in 'xx' at -e line 1.
no A or B found in 'x' at -e line 1.
no A or B found in '' at -e line 1.

Update: And allowing for the possibility of side-effects (e.g., the values in the  %xlat hash could be anonymous subroutine references) (note:  //p and  ${^MATCH} only available in Perl 5.10+):

>perl -wMstrict -le
"my %xlat = qw(A yes  B no);
 ;;
 my $set = join '', map quotemeta, keys %xlat;
    $set = qr{ [$set] }xms;
 ;;
 my $xlat_keys = join ' or ', keys %xlat;
 ;;
 for my $s (qw(
   B111  B111B111  B111B111B111
   A111  A111A111  A111A111A111
   A111B111  A111B111A111  A111B111A111B111  A111B111A111B111A111
   B111A111  B111A111B111  B111A111B111A111  B111A111B111A111B111
   B  BB  BBB  A  AA  AAA  BA  BAB  AB  ABA
   xxx  xx  x), '',
   ) {
   local our $y_n;
   use re 'eval';
   ()= $s =~ m{ $set (?{ $y_n = $xlat{${^MATCH}} }) }xmspg;
   if (defined $y_n) {
     printf qq{%3s: '%s' \n}, $y_n, $s;
     }
   else {
     warn qq{no $xlat_keys found in '$s'};
     }
   }
"
 no: 'B111'
 no: 'B111B111'
 no: 'B111B111B111'
yes: 'A111'
yes: 'A111A111'
yes: 'A111A111A111'
 no: 'A111B111'
yes: 'A111B111A111'
 no: 'A111B111A111B111'
yes: 'A111B111A111B111A111'
yes: 'B111A111'
 no: 'B111A111B111'
yes: 'B111A111B111A111'
 no: 'B111A111B111A111B111'
 no: 'B'
 no: 'BB'
 no: 'BBB'
yes: 'A'
yes: 'AA'
yes: 'AAA'
yes: 'BA'
 no: 'BAB'
 no: 'AB'
yes: 'ABA'
no A or B found in 'xxx' at -e line 1.
no A or B found in 'xx' at -e line 1.
no A or B found in 'x' at -e line 1.
no A or B found in '' at -e line 1.

[dd-b]

Another variant I haven't seen; processing through the list backwards lets you stop as soon as you find an 'A' or 'B'. (That does mean this won't work if setting that variable is just a proxy for a more complex process with side-effects that really does need to be executed for each letter in the string.)

#! /usr/bin/perl

$string="A111B111A111B111";
$yes_or_no="no";

$yes_or_no = '';
for (my $x=1; $x<=length($string); $x++) {
    my $c = substr($string, -1 * $x, 1);
    $yes_or_no = "yes" if ($c eq 'A');
    $yes_or_no = "no" if ($c eq 'B');
    last if $yes_or_no;
}
print "yes_or_no $yes_or_no\n";

[GotToBTru]

use warnings;
use strict;

my $string="A111B111A111B111";
my $yes_or_no="no";
for (reverse(split //, $string)) {
    if ($_ eq 'A') {
        $yes_or_no = 'yes';
        print "$_ yes_or_no=$yes_or_no\n";
        last;
    }
    if ($_ eq 'B') {
        $yes_or_no = 'no';
        print "$_ yes_or_no=$yes_or_no\n";
        last;
    }
}