fix metric alias

[guolinke]

refer to #2209 (comment)

[StrikerRUS]

@guolinke Is it possible to share std::string GetMetricType(const std::string& type) function for objective too? It seems to be that the difference is in l2/l2_root and multiclass/multiclassova only.

[StrikerRUS]

@guolinke I updated docs accordingly.

[guolinke]

Sorry, didn't get it. Could explain with more details?

[StrikerRUS]

@guolinke I mean, we have practically the same code for objective function here
https://github.com/microsoft/LightGBM/blob/master/src/objective/objective_function.cpp as for metric function which you've updated in this PR. So I wonder can we use std::string GetMetricType(const std::string& type) for replacing aliases with main names for both metrics and objectives.

UPD: After that all || in if conditions for objective functions will be removed.

[StrikerRUS]

@guolinke In other words, replace

LightGBM/src/io/config.cpp

Line 70 in 7189743

*objective = value;

with

objective = GetMetricType(value);

This will allow to work only with objectives names in further code, without considering aliases.

[guolinke]

@StrikerRUS Thanks, good point! will update it accordingly.

[StrikerRUS]

@guolinke Don't you want to merge GetMetricType and ParseObjective functions into one function (e.g. ParseFunctionAliases)? And that function can take one additional enum argument function_type with two possible values metric and objective to help with l2/l2_root and multiclass/multiclassova cases, which are different in two current functions.

[guolinke]

@StrikerRUS I tried it, and think it is more easy-to-read to use two functions.
Actually, the duplicated aliases are no many, just two or three.
but introducing the function_type, with caring about the duplicate or not, will increase the complexity of this function.

[StrikerRUS]

@guolinke OK, got it!

[StrikerRUS]

Left some comments, please check.

[StrikerRUS]

I updated docs according to new objective names.

[StrikerRUS]

I think that the following code can be simplified by removing checks for aliases, since we have already done it earlier:

LightGBM/src/io/config.cpp

Lines 198 to 234 in 7189743

	bool CheckMultiClassObjective(const std::string& objective) {
	return (objective == std::string("multiclass")
	|| objective == std::string("multiclassova")
	|| objective == std::string("softmax")
	|| objective == std::string("multiclass_ova")
	|| objective == std::string("ova")
	|| objective == std::string("ovr"));
	}
	void Config::CheckParamConflict() {
	// check if objective, metric, and num_class match
	int num_class_check = num_class;
	bool objective_custom = objective == std::string("none") || objective == std::string("null")
	|| objective == std::string("custom") || objective == std::string("na");
	bool objective_type_multiclass = CheckMultiClassObjective(objective) || (objective_custom && num_class_check > 1);
	if (objective_type_multiclass) {
	if (num_class_check <= 1) {
	Log::Fatal("Number of classes should be specified and greater than 1 for multiclass training");
	}
	} else {
	if (task == TaskType::kTrain && num_class_check != 1) {
	Log::Fatal("Number of classes must be 1 for non-multiclass training");
	}
	}
	for (std::string metric_type : metric) {
	bool metric_custom_or_none = metric_type == std::string("none") || metric_type == std::string("null")
	|| metric_type == std::string("custom") || metric_type == std::string("na");
	bool metric_type_multiclass = (CheckMultiClassObjective(metric_type)
	|| metric_type == std::string("multi_logloss")
	|| metric_type == std::string("multi_error")
	|| (metric_custom_or_none && num_class_check > 1));
	if ((objective_type_multiclass && !metric_type_multiclass)
	|| (!objective_type_multiclass && metric_type_multiclass)) {
	Log::Fatal("Multiclass objective and metrics don't match");
	}
	}

[StrikerRUS]

I added new aliases for regression objectives in docs.

[StrikerRUS]

@guolinke Can we merge?

[guolinke]

Yes