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

Dear Monks,

I've the following problem:
my @a = &requestDB(..........) ;
if ( defined @a) {

} else { .... }

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So, for example, if test requests something from a database. The request can return nothing (no match found inside the database), or something went wrong.....
How do I tell the differnce from this array ?
Should I use references ?

Thanks in advance
Luca

2006-01-21 Retitled by planetscape, as per Monastery guidelines
Original title: 'sub retruns array or empty array'

Replies are listed 'Best First'.
Re: sub returns array or empty array
by holli (Abbot) on Jan 20, 2006 at 14:27 UTC
    You should simply test for 
    if ( @a )

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    That will catch all cases:
    • the sub returns undef -> false
    • the sub returns an emtpy array ->false
    • the sub returns a filled array ->true
    The problem with your code is, that even an empty array is defined, so your comparison always returns true.


    holli, /regexed monk/
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Re: sub returns array or empty array
by Fletch (Bishop) on Jan 20, 2006 at 14:11 UTC

    If "nothing went wrong but there were no results" is a valid case you need to distinguish then you'd probably be best off returning undef on error and an array reference if successful. Then you could use defined $results to check for error, and check for @{ $results } == 0 to see if there were no matches.

    Update: Gah, chopped out the "... is a ... distinguish ..." phrase somehow while editing. Makes a bit more sense now.

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      sounds good!
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Re: sub returns array or empty array
by radiantmatrix (Parson) on Jan 20, 2006 at 15:23 UTC

    Somewhat related to the comment by Fletch above, you could use an ARRAYref on success (which would be empty for no results), and a scalar containing an error message on failure. 

    sub requestDB {
    ...
    if ($some_error) {
        return "This error occurred: $DBI::errstr";
    }

    return \@results; # reference to results array
}

my $a = requestDB(...);
if ( ! ref $a ) {
    # if $a is not a reference, it's a scalar that holds the error mes
+sage
    die $a;
}
elsif (@$a) {
    # if @$a is nonzero, there were results
    ...
}
else {
    # there weren't results
    print "No results found, sorry!"
}

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    <-radiant.matrix->
    A collection of thoughts and links from the minds of geeks
    The Code that can be seen is not the true Code
    I haven't found a problem yet that can't be solved by a well-placed trebuchet
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Re: sub returns array or empty array
by ptum (Priest) on Jan 20, 2006 at 14:02 UTC

    If something goes wrong in my database connection or statement, and I'm using the DBI, I can check $DBI::errstr. Both $DBI::err and $DBI::errstr should be false in the case where there was no match, but the statement was OK.


    No good deed goes unpunished. -- (attributed to) Oscar Wilde

    2006-01-21 Retitled by planetscape, as per Monastery guidelines
    Original title: 'Re: sub retruns array empty array'

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Re: sub returns array or empty array
by Sioln (Sexton) on Jan 20, 2006 at 15:03 UTC 
    my @a = &requestDB(..........) ;
if ( defined @a) {

} else { .... }

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    my @a = &requestDB(..........) ;
if ( scalar(@a)) {

} else { .... }

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Re: sub returns array or empty array
by jeanluca (Deacon) on Jan 20, 2006 at 14:14 UTC
    That makes sense, but what if you don't know what is happening inside this sub. This sub could read from file...!
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