Check even numbers?

[Anonymous Monk]

Hello there monks!
I have a counter inside a Perl script, which counts the number of occurences of a certain pattern in a file.
The lines in the file are something like:
>protein1
sequence1
>protein2
sequence2
>protein3
sequence3
etc
So, if, in each sequence, the program finds, for instance, the letter 'A', it adds 1 to the counter. What I want to print out is those proteins and their sequences, that have even number of occurences of the letter 'A' (2,4,6,8 etc).
Is there a function or something that checks if a number is odd or even?
Thank you!

[jeffa]

You can use the modulus operator for that.

my $is_even = $number % 2 == 0;
my $is_odd  = $number % 2 == 1;

[GrandFather]

Check the least significant bit. It is set for odd numbers:

print "$_ is " . ($_ & 1 ? "odd" : "even") . "\n" for 0..3;

Prints:

0 is even
1 is odd
2 is even
3 is odd

[liverpole]

A very simple method for checking whether a number $N is odd or even is to ask yourself "is there any remainder when I divide $N by 2?".

So, for example:

if (0 == $N % 2) {
    print "The number $N is even\n";    # There is no remainder
} else {
    print "The number $N is odd\n";     # There is a remainder (of 1)
}

[cees]

If you want to count the number of occurences of a single character in a string, then you should look at using 'tr'.

$str = 'ABABAABBA';
$count = $str =~ tr/A//;  # $a = 5;

If you omit the second arg to tr, it knows you are not changing the string, but still counts up the occurences for you, which makes this very efficient.

After that you just have to see if the number is positive ($count > 0) and use Grandfather's test above to see if it is even ( !($count & 1) ).

if ($count && !($count & 1)) {
  # Do something with this protein sequence
}

[jwkrahn]

Perhaps you could use a regular expression:

print $sequence if $sequence =~ /^[^A]*(?:A[^A]*A)+[^A]*$/;

[Corion]

Dominus wrote a treatise that gives you the two functions odd and even which tell you whether a number is even or odd in his list of Infrequently Asked Questions about Perl.

[BrowserUk]

I assume you are aware, (and it probably should be mentioned), that the referenced method is (I assume), a joke along the same lines as the included method of finding tomorrow's date:

sub tomorrow_date {
    sleep 86_400; 
    return localtime();
}

Without all the compile-time reductions, lazy evaluation etc. that most properly functional languages benefit from, this method of determining the eveness of a number is ludicrously inefficient--6000+ times slower than the normal method:

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];
no warnings 'recursion';

sub odd { my $number = shift; return !even ($number); }
sub even { my $number = abs shift; return 1 if $number == 0; return od
+d ($number - 1); }

cmpthese -5, {
    normal     => q[ my $n;     $_ & 1 and $n++ for 1 .. 1000; ],
    functional => q[ my $n; even( $_ ) and $n++ for 1 .. 1000; ],
};

__END__
c:\test>junk
              s/iter functional     normal
functional      1.50         --      -100%
normal     2.41e-004    625042%         --

[lima1]

EMBOSS pepstats is probably what you want.

Bioperl supports starting of EMBOSS programs and parsing output files.