Regarding getting the size of array in a hash of array structure

Question

I wrote the following Perl function

  sub Outputing
  { 
     my $featureMatrix = shift;
     my $indexRow = shift;
     my $fileName = "/projectworkspace/input.dat";

     open(DATA, "> $fileName");
     printf DATA "%d", $#$indexRow;
     print DATA "\n";

     my $numDataPoints = $#{$featureMatrix{$indexRow->[1]}};
     printf DATA "%d", $numDataPoints;
     print DATA "\n";

     close DATA;    
    }

I calling Outputing as follows:

Outputing($matrix, $Rows);e

$matrix is a hash of array, whose structure is like this

   my $matrix 
     = { 200 => [ 0.023, 0.035, 0.026 ], 
          110 => [ 0.012, 0.020, 0,033], 
        };

Rows is an array storing the sorted key of matrix, it is obtained as follows

my @Rows = sort keys %matrix;

both matrix and Rows are used as parameters passed to Outputing.

The printed out $numDataPoints is -1, which is not correct? I do not know which might be the reason that causes this problem? If we use the above example, and assume $indexRow->[1]=110, then $numDataPoints should be 2. I am not sure whether the $#{$featureMatrix{$indexRow->[1]}}; is the correct way to get the size of this array.

ruakh · Accepted Answer · 2012-02-16 03:24:15Z

3

Assuming that you've included all the relevant code, this:

my @indexRow = sort keys %featureMatrix;

should be this:

my @indexRow = sort keys %$featureMatrix;

and this:

     my $numDataPoints = $#{$featureMatrix{$indexRow->[1]}};

should be this:

     my $numDataPoints = $#{$featureMatrix->{$indexRow->[1]}};

That is, the problem is that in some places, you're using a hash named %featureMatrix, and in others, you're using a hashref named $featureMatrix that refers to an anonymous hash.

You should be using use warnings and use strict to prevent such mistakes: those would have prevented you from using %featureMatrix when you've only declared $featureMatrix. (Actually, use warnings might not help in this case — it could detect if you used %featureMatrix exactly once, but in your case, you use it a few times — but use strict would almost certainly have helped.)

edited Feb 16, 2012 at 3:24

answered Feb 16, 2012 at 3:19

ruakh

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5 Comments

user785099 Over a year ago

Thanks, but I still not very clear about this. First, my @indexRow = sort keys %featureMatrix; was used outside the function of "Outputing" rather than inside the "Outputing". The variable name I used may cause some confusion. I will edit my original post.

user785099 Over a year ago

I just edited my original post. Does that affect your answer? In accordance with your answer, I should have written my @Rows = sort keys %$matrix;

ruakh Over a year ago

@user785099: Are you familiar with the difference between my $featureMatrix = { ... } and my %featureMatrix = ( ... )? Your question says you're using the former, but various parts of your code would only work if you were using the latter.

user785099 Over a year ago

can you elaborate more on the difference between my $featureMatrix = { ... } and my %featureMatrix = ( ... ).

ruakh Over a year ago

@user785099: my %featureMatrix = ( ... ) creates a hash named %featureMatrix. Its elements can be referred to with the notation $featureMatrix{'...'}. my $featureMatrix = { ... } creates an anonymous hash, and a reference to that hash. The reference is named $featureMatrix. The hash can be referred to with the notation @$featureMatrix, and its elements can be referred to with the notation $featureMatrix->{'...'}.

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