Unexpected error when calling getColIntegrality while all columns are continuous

[rschwarz]

Observation: In a purely continuous model, a call to getColIntegrality results in an error message: ERROR: Model integrality does not exist for index 0 as well as HighsStatus::kError.

Expectation: Even though no integrality was ever set, I'd simply expect a result of HighsVarType::kContinuous and no error.

Motivation: In some situations, a MIP model is built, but then some variables are relaxed. This way, we can end up with a problem where all variables are continuous. This need not result in an error for otherwise reasonable queries.

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Some explanation: When the integrality_ vector is empty, any call to getColIntegrality (with valid column index) will fail. See:

HiGHS/src/lp_data/Highs.cpp

Lines 2980 to 2996 in fd86653

	HighsStatus Highs::getColIntegrality(const HighsInt col,
	HighsVarType& integrality) const {
	const HighsInt num_col = this->model_.lp_.num_col_;
	if (col < 0 || col >= num_col) {
	highsLogUser(options_.log_options, HighsLogType::kError,
	"Index %d for column integrality is outside the range [0, "
	"num_col = %d)\n",
	int(col), int(num_col));
	return HighsStatus::kError;
	}
	if (static_cast<size_t>(col) < this->model_.lp_.integrality_.size()) {
	integrality = this->model_.lp_.integrality_[col];
	return HighsStatus::kOk;
	} else {
	highsLogUser(options_.log_options, HighsLogType::kError,
	"Model integrality does not exist for index %d\n", int(col));
	return HighsStatus::kError;

The integrality_ vector is initially empty, and only filled when the user calls changeColIntegrality the first time. Further, the integrality_ vector may also be cleared (to save memory?) after all variables are changed back to continuous, see:

HiGHS/src/lp_data/HighsLpUtils.cpp

Lines 1622 to 1624 in fd86653

	// If integrality_ contains only HighsVarType::kContinuous then
	// clear it
	if (!lp.isMip()) lp.integrality_.clear();

[rschwarz]

I've prepared a test that demonstrates the behaviour (based on the C API, but it should be the same for C++), as well as a potential fix, in branch https://github.com/rschwarz/HiGHS/tree/rs/getColInt

[jajhall]

I guess I felt that the error return was a flag to the user of an futile query for discrete status in a continuous problem

Better to return HighsVarType::kContinuous, now you point it out

[rschwarz]

According to this test, this behaviour is apparently not unexpected:

HiGHS/check/TestHighsModel.cpp

Line 106 in fd86653

REQUIRE(highs.getColIntegrality(0, integrality) == HighsStatus::kError);

With my proposed change, this will fail.

I guess for users of C++, there is also the option to call isMip() and then query getColIntegrality only when needed. But isMip() is not exposed in the C API, so this option is not available.

[rschwarz]

Either way can be defended, I guess ;-)

[jajhall]

Closed by #2262