[SOLVED] perl check array

qrange · 11-21-2012, 01:29 PM

@markush
how do you see if array is subset of another?
my program takes input from user in form : '2,3,10...'
it then uses csv to extract elements and store it in array. I need to check if user didn't accidentally type in wrong number. hash has all valid values.

markush · 11-21-2012, 01:56 PM

Well, if one array has about 10 elements and the other about 3 elements I can see if the second is a subset of the first.

I'm not sure if I want to see the rest of your program....

Perl is an interesting language and worth learning it. What I want to say is: you should really try to understand the concepts. Hashes and regular expressions are the most fundamental concepts of Perl and one should (at least) these try to understand before writing programs.

Here is how I would solve your problem

Code:

# check if a is subset of b
sub subset { 
        my ( $a, $b ) = @_ ;
        my %b ;

        foreach ( @$b ) {
                $b{$_} = 1 ;
        }
        foreach ( @$a ) {
                exists $b{$_} || return 0 ;
        }
        return 1 ;
}

here the arrays are passed as references to the function subset, one would call it this way

Code:

my @array1 = qw(2 3 10 7 11  15 16 19 20 21 22 25 26) ;
my @array2 = qw(2 4 7) ;
print "yes\n" if subset(\@array2, \@array1) ;

In the function subset the first foreach loop populates the hash %b and the second loop checks if the elements of the other array are stored as key in the hash. I'd recommend to understand this code. This code will also work when the arrays have thousands of elements.

Markus

qrange · 11-22-2012, 03:45 AM

thanks, but your program doesn't print invalid entry.
I don't understand:

Code:

exists $b{$_} || return 0

markush · 11-22-2012, 05:55 AM

Quote:

Originally Posted by qrange

thanks, but your program doesn't print invalid entry.

you're kidding, right? why don't you write yourself a print-statement as you need it, for example:

Code:

exit "Invalid input!" if !subset(\@array1, \@array2);

Quote:

I don't understand:

Code:

exists $b{$_} || return 0

it's a shortcut, || means "or". If the statement left from the || is true, the complete statement is true and the next entry of the loop is checked. But if the first statement is wrong which means $_ is not in the hash, then the statement right from || is executed, the function returns 0 and exits. This means that the function returns 0 immediately when it found a number which isn't in the hash.

You can uses this also for the above printstatement:

Code:

subset(\@array1, \@array2) || exit "invalid input!";

one doesn't need paranthesis in Perl:

Code:

subset \@array1, \@array2 or exit "invalid input!";

Which can be understood even if one doesn't know Perl.

Markus

qrange · 11-22-2012, 09:48 AM

thanks.
I expected it to print bad number, (if some are ok), not a generic "Invalid input!". but, nevermind, I'll modify it.
yeah, I knew || is 'or'; nice shortcut.

Sergei Steshenko · 11-22-2012, 11:12 AM

Quote:

Originally Posted by qrange

thanks.
I expected it to print bad number, (if some are ok), not a generic "Invalid input!". but, nevermind, I'll modify it.
yeah, I knew || is 'or'; nice shortcut.

qrange · 11-22-2012, 03:27 PM

well I didn't know about 'short-circuit evaluation'. don't recall seeing it before in other languages. I've used BASIC and a bit of asm on ZX, then some PASCAL, C and some other in college. but all that was many years ago.
again, sorry for being ignorant.