How to interrogate array cell?

[Richard Heintze]

I have an array of hashes. What function should I be
using to interrogate each array cell when I want to
know if it is occupied?

"exists" seemed to do the job nicely. What about
"defined"?

Now I am curious: how would I implement a switch
statement (er, I mean, set of if-elsif statements) for
a hetrogeneous array where some array cells contain
arrays, others integers, other hashes?

I tried saying
my @x;
if(@x && $x[$ii] && %{$x[$ii]} && exists
$x[$ii]{"xyz"}){
my $z = $x[$ii]{"xyz"};
...}

But that did not work. There must be some function
that will tell me "this array cell contains a hash,
this other array cell contains another array...".

Thanks,
Sieg

__________________________________
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[Jan Eden]

Hi,

Richard Heintze wrote:

>I have an array of hashes. What function should I be
>using to interrogate each array cell when I want to
>know if it is occupied?
>
> "exists" seemed to do the job nicely. What about
>"defined"?
>

exists only works with hashes. Please check the recent conversation about "Array containment", where Rob Dixon advised me to turn my array into a hashto use the exists function.

- Jan
--
There are two major products that come out of Berkeley: LSD and UNIX. We don't believe this to be a coincidence. - Jeremy S. Anderson

[Wiggins D Anconia]

> Hi,

>
> Richard Heintze wrote:
>

> >I have an array of hashes. What function should I be
> >using to interrogate each array cell when I want to
> >know if it is occupied?
> >
> > "exists" seemed to do the job nicely. What about
> >"defined"?
> >

> exists only works with hashes. Please check the recent conversation

about "Array containment", where Rob Dixon advised me to turn my array
into a hash to use the exists function.

>

Caveat version... depends on the version of Perl. Newer 'exists' can
check arrays... from perldoc -f exists:

"Given an expression that specifies a hash element or array element,
returns true if the specified element in the hash or array has ever been
initialized...."

Works at least as of 5.6.1.

As to your other question, check out:

perldoc -f ref

You may also find,

perldoc UNIVERSAL

particularly 'isa' helpful...

[url]http://danconia.org[/url]

[James Edward Gray II]

On Feb 10, 2004, at 8:50 AM, Richard Heintze wrote:

> I have an array of hashes. What function should I be
> using to interrogate each array cell when I want to
> know if it is occupied?
>
> "exists" seemed to do the job nicely. What about
> "defined"?

exists() tests if the slot was ever assigned to. defined() tests if
the slot contains a defined value (read: not undef). It's a subtle
difference and it gets a little fuzzier inside a loop, where skipped
slots will come up, as long as a later slot has been assigned to. (See
code below.) For that reason, I generally want defined() when talking
about arrays.

> Now I am curious: how would I implement a switch
> statement (er, I mean, set of if-elsif statements) for
> a hetrogeneous array where some array cells contain
> arrays, others integers, other hashes?

See if this give you some ideas:

#!/usr/bin/perl

use strict;
use warnings;

my @complex = ( [ 1, 2, 3 ], 405, { dogs => 200, cats => 16 } );
$complex[4] = qr/a regex/;

print "Skipped slot.\n\n" unless exists $complex[3];

foreach (@complex) {
unless (defined $_) {
print "Undefined slot.\n";
next;
}
my $type = ref($_) || "INTEGER";

if ($type eq "ARRAY") { print "Array found: ", join(", ", @$_), "\n";
}
elsif ($type eq "HASH") {
my %hash = %$_;
print "Hash Found: ",
join(", ", map { "$_ => $hash{$_}" } keys %hash), "\n"; }
elsif ($type eq "INTEGER") { print "Integer found: $_\n"; }
else { print "Unknown type: $type\n"; }
}

__END__

James

[Rob Dixon]

Jan Eden wrote:

>
> Richard Heintze wrote:
>

> >I have an array of hashes. What function should I be
> >using to interrogate each array cell when I want to
> >know if it is occupied?
> >
> > "exists" seemed to do the job nicely. What about
> > "defined"?
> >

>
> exists only works with hashes. Please check the recent conversation about "Array
> containment", where Rob Dixon advised me to turn my array into a hash to use the
> exists function.

Sorry if I misled you Jan, but 'exists' works on both hash and array elements. Look:

use strict;
use warnings;

my @array;

@array[2, 4] = (1, 1);

for (0 .. 6) {
print "Element $_ Exists\n" if exists $array[$_];
}

**OUTPUT

Element 2 Exists
Element 4 Exists


The only reason I suggested that you generate a hash from your array was that,
if the array was invariant and you needed to do several lookups, it would be
faster doing hash accesses than linear searches through the array.

Look at

perldoc -f exists

for this topic, and make sure you read

perldoc -q "array contains"

about your previous question.

Rob