CODEWITHSUNDEEP
perlhash
eve
eve

Reputation: 3049

Perl "Not an ARRAY reference" error

I'll be glad if someone can enlighten me as to my mistake:

my %mymap; 
@mymap{"balloon"} = {1,2,3};

print $mymap{"balloon"}[0] . "\n";

Upvotes: 5

Views: 17079

Answers (4)

TLP
TLP

Reputation: 67900

Well, first off, always use strict; use warnings;. If you had, it might have told you about what is wrong here.

Here's what you do in your program:

my %mymap;  # declare hash %mymap
@mymap{"balloon"} = {1,2,3};  # attempt to use a hash key on an undeclared 
                              # array slice and assign an anonymous hash to it

print $mymap{"balloon"}[0] . "\n";  # print the first element of a scalar hash value

For it to do what you expect, do:

my %mymap = ( 'balloon' => [ 1,2,3 ] );
print $mymap{'balloon'}[0];

Upvotes: 3

Axeman
Axeman

Reputation: 29854

$mymap{'balloon'} is a hash not an array. The expression {1,2,3} creates a hash:

 {
   '1' => 2,
   '3' => undef
 }

You assigned it to a slice of %mymap corresponding to the list of keys: ('balloon'). Since the key list was 1 item and the value list was one item, you did the same thing as 

$mymap{'balloon'} = { 1 => 2, 3 => undef };

If you had used strict and warnings it would have clued you in to your error. I got:

Scalar value @mymap{"balloon"} better written as $mymap{"balloon"} at - line 3. Odd number of elements in anonymous hash at - line 3.

Upvotes: 6

user554546
user554546

Reputation:

Okay, a few things...

%mymap is a hash. $mymap{"balloon"} is a scalar--namely, the value of the hash %mymap corresponding to the key "balloon". @mymap{"balloon"} is an attempt at what's called a hash slice--basically, you can use these to assign a bunch of values to a bunch of keys at once: @hash{@keys}=@values.

So, if you want to assign an array reference to $mymap{"balloon"}, you'd need something like:

$mymap{"balloon"}=[1,2,3].

To access the elements, you can use -> like so:

$mymap{"balloon"}->[0] #equals 1
$mymap{"balloon"}->[1] #equals 2
$mymap{"balloon"}->[2] #equals 3

Or, you can omit the arrows: $mymap{"balloon"}[0], etc.

Upvotes: 1

MichielB
MichielB

Reputation: 4285

If you had used 'use strict; use warnings;' on the top of your code you probably have had better error messages.

What you're doing is creating a hash called mymap. A hash stores data as key => value pairs. You're then assigning an array reference to the key balloon. Your small code snipped had two issues: 1. you did not addressed the mymap hash, 2. if you want to pass a list, you should use square brackets:

my %mymap;
$mymap{"balloon"} = [1,2,3];
print $mymap{"balloon"}[0] . "\n";

this prints '1'.

You can also just use an array:

my @balloon = (1,2,3);
print $balloon[0] . "\n";

Upvotes: 5

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