ucar.ma2.Array.factory(DataType type, int[] shape) creates an array of unexpected size when shape is empty

[elzbietaa]

Summary

When creating an Array given an empty shape, the value returned by Array.getSize() is equal to 1 e.g.:

Array array = Array.factory(DataType.INT, new int[0]);

whereas we'd expect it to be zero.

Note 1: whether the DataType version of the method is used, or an overloaded one taking a Class does not matter. Neither does the type - the size is always == 1 for empty shapes.

Note 2: We're using edu.ucar:cmd-4.6.7.

If this behaviour is expected, can you please point out to documentation highliting the reasoning for that (if available).

[lesserwhirls]

Greetings!

The getSize() method returns the total number of elements in the array, so even though the dimensionality is zero (scalar), there is still one value to be found. However, if you use the getRank() method, you will get 0, as that is the rank of the array.

As an example:

Array scalar = Array.factory(DataType.INT, new int[0]);
Array multidim = Array.factory(DataType.INT, new int[] {3, 4});

System.out.printf("scalar Rank: %d\n", scalar.getRank());
System.out.printf("scalar Number of elements: %d\n", scalar.getSize());
System.out.println();
System.out.printf("multidim Rank: %d\n", multidim.getRank());
System.out.printf("multidim Number of elements: %d\n", multidim.getSize());

would give:

scalar Rank: 0
scalar Number of elements: 1

multidim Rank: 2
multidim Number of elements: 12

[elzbietaa]

Greetings, thank you for the prompt reply!

So just to clarify: the only way to create an Array of size == 0 is to use a non-empty shape array where the elements' product is equal to 0, e.g.:

Array.factory(DataType.INT, new int[]{0}); ,
Array.factory(DataType.INT, new int[]{0, 0, 0}); , or
Array.factory(DataType.INT, new int[]{2, 0, 0}); ...
?

Side question: do I understand correctly then, that the reported behaviour is not considered a bug but a feature?

Best regards

[lesserwhirls]

Hmmm, that's a good question. I'm not sure if it even makes sense for netCDF-Java to be able to create a zero sized Array object (an Array with no values), or if that in and of itself is a bug. I know we treat unlimited Dimensions as having a length of 0, and a vlen Dimensions as having a length = -1. Here we are are not talking about Dimension objects, but rather the number of elements in an Array object. @ethanrd - do you have any insight to offer?

[DennisHeimbigner]

It is possible to create a variable with a zero-size dimension using UNLIMITED.
When initially created, the size of the UNLIMITED dimension should be zero. So
the size of the variable should be zero.
e.g.

netcdf foo {
dimensions:
   D = UNLIMITED;
variables:
	double x(D) ;
}

[lesserwhirls]

A Variable, yes, but an Array?

[DennisHeimbigner]

May be relevant: #1212