More elegant, more obvious error for the enduser (#229)

R-package/R/lgb.Dataset.R

@@ -80,12 +80,12 @@ Dataset <- R6Class(
       if (!is.null(private$categorical_feature)) {
         if (typeof(private$categorical_feature) == "character") {
             cate_indices <- as.list(match(private$categorical_feature, private$colnames) - 1)
-            if (length(cate_indices) != length(private$categorical_feature)) {
-              stop("lgb.self.get.handle: cannot find feature name ", sQuote(private$categorical_feature[!private$categorical_feature %in% private$colnames]))
+            if (sum(is.na(cate_indices)) > 0) {
+              stop("lgb.self.get.handle: supplied an unknown feature in categorical_feature: ", sQuote(private$categorical_feature[is.na(cate_indices)]))
             }
           } else {
             if (max(private$categorical_feature) > length(private$colnames)) {
-              stop("lgb.self.get.handle: supplied a too large value in categorical_feature")
+              stop("lgb.self.get.handle: supplied a too large value in categorical_feature: ", max(private$categorical_feature), " but only ", length(private$colnames), " features")
             }
             cate_indices <- as.list(private$categorical_feature - 1)
           }