Not use EXPRESS ARRAYs

[pipauwel]

There are very few ARRAYs defined in IFC. SETs and LISTs are much more common. Is it an option to replace all ARRAYs with LISTs?

This seems an easy fix, and it makes translation to other languages easier.

In particular:

TYPE IfcComplexNumber = ARRAY [1:2] OF REAL;
END_TYPE;

[hlg]

No, it is not possible to simply replace ARRAYs with LISTs, because LISTs in EXPRESS can not have unset values, while ARRAYs can. Instead, IfcComplexNumber might be modelled like this:

ENTITY IfcComplexNumber;
    RealPart: OPTIONAL REAL;
    ImaginaryPart: OPTIONAL REAL;
END_ENTITY;

A similar construction would work for IfcMaterialLayerWithOffsets.OffsetValues with optional lower and upper offset values.

Alternatively, if there is a sensible default, like 0 for the REALS in IfcComplexNumber, then you could use a fixed length or length-constraint LIST. But for IfcMaterialLayerWithOffsets.OffsetValues that would not be as simple.

Longer and variable length arrays such as BSpline control points need a completely different consideration.

[aothms]

To my understanding ARRAYS are not by default with optional elements.

The spec says (emphasis mine)

An array data type definition may optionally specify that an array value cannot contain duplicate elements. It may also specify that an array value need not contain an element at every index position.

The formal def is:

165 array_type = ARRAY bound_spec OF [ OPTIONAL ] [ UNIQUE ] base_type .

So in my understanding only ARRAY [1:2] OF OPTIONAL REAL; would result in optional elements.

[hlg]

Upon rereading the spec, yes, you are totally right. I also misinterpreted the bounds to denote minimum and maximum cardinality instead of indizes. Then yes, for non-optional arrays, a fixed size list could be a possible replacement such as LIST[2] OF REAL in this case.

It still remains to check whether there are potential issues with indexes in any rules or functions, since ARRAY[1:2] would assume indizes 1 or 2.

[pipauwel]

I am aware that they are all four slightly different, bags, sets, lists, and sets. Fact is that many other languages don´t go in that diversity in expressiveness. So, in practice, they are anyway mapped to different constructs, often to lists. That seems to be also the common case in the IFC spec in general.

So, in an effort to remove the more exotic choices in the spec, and reduce parsing complexity, I would argue for trying to follow your suggestion:

a fixed size list could be a possible replacement such as LIST[2] OF REAL in this case.

There are a number of arrays used in rules and DERIVE constructs, I think -not many-. I would argue for an effort (project, one day activity, branch in github, or anything) to try to get these arrays out of the spec.

[TLiebich]

lets be specific here - the three diffences of arrays, compared to lists, are

1. the bounds are not min/max (in terms of members), but min/max in terms of indices
2. there are sparse arrays when OPTIONAL is used
3. there are arrays prohibiting duplicates when UNIQUE is used

using LIST instead does cover 1) [minor task, check if functions need to be changed] and 3) LIST can also be UNIQUE. This leaves only 2) open, as lists cannot be "sparse".

Now lets check IFC (latest official IFC4.2). We have:

TYPE IfcComplexNumber = ARRAY [1:2] OF REAL;
END_TYPE;

ENTITY IfcMaterialLayerWithOffsets
 SUBTYPE OF (IfcMaterialLayer);
	OffsetDirection : IfcLayerSetDirectionEnum;
	OffsetValues : ARRAY [1:2] OF IfcLengthMeasure;
END_ENTITY;

ENTITY IfcMaterialProfileWithOffsets
 SUBTYPE OF (IfcMaterialProfile);
	OffsetValues : ARRAY [1:2] OF IfcLengthMeasure;
END_ENTITY;

it could easily changed to (with only minor backward compatibility problems) into:

TYPE IfcComplexNumber = LIST [2:2] OF REAL;
END_TYPE;

ENTITY IfcMaterialLayerWithOffsets
 SUBTYPE OF (IfcMaterialLayer);
	OffsetDirection : IfcLayerSetDirectionEnum;
	LowerOffsetValue : OPTIONAL IfcLengthMeasure;
	UpperOffsetValue : OPTIONAL IfcLengthMeasure;
 WHERE
        MinOneOffset: EXISTS(LowerOffsetValue) OR EXISTS(UpperOffsetValue);
END_ENTITY;

ENTITY IfcMaterialProfileWithOffsets
 SUBTYPE OF (IfcMaterialProfile);
	LowerOffsetValue : OPTIONAL IfcLengthMeasure;
	UpperOffsetValue : OPTIONAL IfcLengthMeasure;
 WHERE
        MinOneOffset: EXISTS(LowerOffsetValue) OR EXISTS(UpperOffsetValue);
END_ENTITY;

alternative is to only change ARRAY to LIST also here (minor impact, less explicit)

ENTITY IfcMaterialLayerWithOffsets
 SUBTYPE OF (IfcMaterialLayer);
	OffsetDirection : IfcLayerSetDirectionEnum;
	OffsetValues : LIST [2:2] OF IfcLengthMeasure;
END_ENTITY;

ENTITY IfcMaterialProfileWithOffsets
 SUBTYPE OF (IfcMaterialProfile);
	OffsetValues : LIST [2:2] OF IfcLengthMeasure;
END_ENTITY;

plus some usage in derived (not directly exchanged) attributes and functions for b-spline curves and surfaces. This might need to be checked in detail, but would not be a show stopper.

[TLiebich]

check list

what do we win

• interoperability with other encoding systems

what do we loose

• nothing

schema impact

• minor
• replace ARRAY by LIST, make attributes of IfcMaterial[Layer/Profile]WithOffsets either lists or explicit, check required changes in derived and functions for bspline curves and surfaces

instance model impact

• none (if IfcMaterial[Layer/Profile]WithOffsets using LIST) or minor

backwards compatible

• none (if IfcMaterial[Layer/Profile]WithOffsets using LIST) or minor

automatic migration possible

• yes

[TLiebich]

I created a new label "validated proposal" meaning, that this issue has been thoughougly checked and is seen as a valid candidate to be included into upcoming version of IFC (sort of "pre-authorized" pull request).