multidimensional hash modified by test

[Mike Solomon]

I ran the following script and got what I consider to be a strange result


my %data;

$data{a}{a} = 1;

my $total = keys %data;

print "$total\n";

#prints 1

print "NOT IN HASH\n" unless $data{b}{b};

#prints NOT IN HASH
$total = keys %data;

print "$total\n";

#prints 2

Why does the total keys increase to 2 when I test for non existent data?

Thanks

 

[A. Sinan Unur]

(e-mail address removed) (Mike Solomon) wrote in

I ran the following script and got what I consider to be a strange result


my %data;

$data{a}{a} = 1;

my $total = keys %data;

print "$total\n";

#prints 1

print "NOT IN HASH\n" unless $data{b}{b};

This autovivifies $data{b}.

....

Why does the total keys increase to 2 when I test for non existent data?

You should use exists if you want to test for existence:

perldoc -f exists

Sinan

 

[xhoster]

I ran the following script and got what I consider to be a strange result

my %data;

$data{a}{a} = 1;

my $total = keys %data;

print "$total\n";

#prints 1

print "NOT IN HASH\n" unless $data{b}{b};

#prints NOT IN HASH
$total = keys %data;

print "$total\n";

#prints 2

Why does the total keys increase to 2 when I test for non existent data?

It is called autovivification. You can read about it in perldoc perlref.

Xho

 

[Tad McClellan]

print "NOT IN HASH\n" unless $data{b}{b};

$total = keys %data;
print "$total\n";
#prints 2

Why does the total keys increase to 2 when I test for non existent data?

Autovivification.

See:

perldoc perlreftut
perldoc perlref


And Randal has written a column on that (of course):

http://www.stonehenge.com/merlyn/UnixReview/col44.html

 

[LEH]

(e-mail address removed) (Mike Solomon) wrote in


This autovivifies $data{b}.

...


You should use exists if you want to test for existence:

perldoc -f exists

Sinan

using exists,
ie. print "NOT IN HASH\n" unless exists $data{b}{b};
also seems to autovivify $data{b}

according to perldoc -f exists

if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into
existence just because its existence was tested, any intervening
ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will
spring into existence due to the existence test for the $key
element above.

Larry

 

[LEH]

using exists,
ie. print "NOT IN HASH\n" unless exists $data{b}{b};
also seems to autovivify $data{b}

according to perldoc -f exists

if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into
existence just because its existence was tested, any intervening
ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will
spring into existence due to the existence test for the $key
element above.

Larry

but this works
print "NOT IN HASH\n" unless exists $data{b} && $data{b}{b};
although perhaps there is a better way.
Larry

 

[A. Sinan Unur]

....

according to perldoc -f exists

if (exists &{$ref->{A}{B}{$key}}) { } ....
ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will
spring into existence due to the existence test for the $key
element above.

Aaaah ... I don't know how I forgot that but thanks for catching the
error.

Sinan.

 

[Mike Solomon]

(e-mail address removed) (Mike Solomon) wrote in


This autovivifies $data{b}.

...


You should use exists if you want to test for existence:

perldoc -f exists

Sinan

Thanks everyone

By the way it behaves exactly the same way if you use exists

 

[Anno Siegel]

[...]

using exists,
ie. print "NOT IN HASH\n" unless exists $data{b}{b};
also seems to autovivify $data{b}

according to perldoc -f exists

if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into
existence just because its existence was tested, any intervening
ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will
spring into existence due to the existence test for the $key
element above.

Larry

but this works
print "NOT IN HASH\n" unless exists $data{b} && $data{b}{b};
although perhaps there is a better way.

You need "exists" in front of "$data{b}{b}" too. As written, it will
still autovivify $data{ b}->{ b} if only $data{ b} exists. Also, it
will fail (claim nonexistence) if $data{ a}->{ a} is defined but false.

Anno

 

[xhoster]

You need "exists" in front of "$data{b}{b}" too.

True, but....

As written, it will
still autovivify $data{ b}->{ b} if only $data{ b} exists.

$data{ b}->{ b} is not used in a hashref-like context, so there is nothing
to autovivify, whether the "exists" is added or not.

Also, it
will fail (claim nonexistence) if $data{ a}->{ a} is defined but false.

Yes, that is true.

Xho