[Octave-bug-tracker] [bug #31287] Certain assignments of empty arrays give errors (Matlab incompatibility)

[Olaf Till]

Follow-up Comment #9, bug #31287 (project octave):

Rules deduced for Matlab 2009b

In the following "dimensions" means dimensions as returned by size().

1. Special case "null assignment" (= [], literally)

The rule for the left hand side is given by the error message "A null
assignment can have only one non-colon index."

The following, and only the following, qualify for a "colon index" in the
sense of this rule:

- a colon ":",
- any numeric array with at least one dimension empty,
- only if the lhs is at most 2-dimensional: a logical index of arbitrary
length with all elements false.

2. The other empty assignments

Definitions for assignments:
lhs(idx) = rhs;
ilhs means lhs(idx)

Matlab seems to treat 2 cases differently

2.1. First case: lhs <= 2-dimensional AND ilhs <= 2-dimensional AND rhs <=
2-dimensional

Empty assignment succeeds if and only if (any dimension in ilhs is 0 AND any
dimension in rhs is 0).

2.2. Second case: all other empty assignments

Empty assignment succeeds if and only if all dimensions != 1 (i.e. 0 or >= 2)
are in the same order in ilhs and rhs (which implies that there is the same
number of these non-singleton dimensions in ilhs and rhs).

I.e. dimensions of 1 can be in arbitrary numbers and positions in ilhs and
rhs.

End of rules.

I can make a test script, but not before January. I would like to have these
rules reviewed and tested before (though of corse I will also test them with
the script). Should I post them to a list for a wider audience of potential
testers?

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