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

I have the following in my script - 
$coord_{$key1}{$key2}[[2]] = $sth->fetchrow_array;

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where fetchrow_array belongs to DBI

Sometimes though fetchrow_array will not find an entry, so what I want to do is store missing entries in my database as 0 in the corresponding variable. So lets say the corresponding entry for $coord_{"AB"}{5}[[2]] has no entry in the database, then I want $coord_{"AB"}{5}[[2]] = 0 instead of $coord_{"AB"}{5}[[2]] = "uninitialized value";

So I could use something like - 

if($coord_{$key1}{$key2}[2] eq "uninitialized value")
{
  $coord_{$key1}{$key2}[2] = 0;
}

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but how does perl represent this "uninitialized value" that I'm using above? I hope that makes sense.

Thanks
Barry.

Replies are listed 'Best First'.
Re: use of uninitialized values
by nothingmuch (Priest) on Apr 06, 2003 at 14:13 UTC
    Perl has a function named undef, which either undefines a value (as in undef @bar), or returns an undefined value ($foo = undef). To check for definedness, you use the function defined: 
    if(not defined($coord_{$key1}{$key2}[2]))
 {
   $coord_{$key1}{$key2}[2] = 0;
 }

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    or more simply 
    $coord_{$key1}{$key2}[2] = 0 unless defined $coord_{$key1}{$key2}[2];

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    or even better 
    $coord_{$key1}{$key2}[2] ||= 0;

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    Update:I forgot to say that undef is the 'uninitialized value' - an undefined (not only false but with no value).

    -nuffin
    zz zZ Z Z #!perl
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[d/l]
[select]
Re: use of uninitialized values
by Anonymous Monk on Apr 06, 2003 at 14:13 UTC
    $coord_{$key1}{$key2}[2] ||= 0;
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Re: use of uninitialized values
by graff (Chancellor) on Apr 07, 2003 at 01:08 UTC
    Assuming that you're using DBI to do SQL queries, you could have the SQL statement return something suitable in place of a null (empty) field value -- e.g.: 
    select NVL(some_column,0) from some_table where some_condition...

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    The "NVL( column, value )" expression in SQL (in oracle, at least) will yield "value" in the case where "column" is null.
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      PostgreSQL and SQL Server use COALESCE. You can give COALESCE a list of values. It will return the first one that is not NULL.

      90% of every Perl application is already written. ⇒
      dragonchild
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