Added naive left-join implementation, and test.

src/tech/ml/dataset.clj

@@ -64,7 +64,8 @@
                         ->>dataset
                         from-prototype
                         dataset->string
-                        join-by-column)
+                        join-by-column
+                        left-join)
 
 
 (par-util/export-symbols tech.ml.dataset.impl.dataset

src/tech/ml/dataset/base.clj

@@ -11,7 +11,8 @@
             [tech.io :as io]
             [tech.parallel.require :as parallel-req]
             [tech.parallel.for :as parallel-for]
-            [tech.parallel.utils :as par-util])
+            [tech.parallel.utils :as par-util]
+            [clojure.set])
   (:import [java.io InputStream]
            [tech.v2.datatype ObjectReader]
            [java.util List]
@@ -599,3 +600,28 @@ the correct type."
                          (map #(ds-col/select % rhs-indexes)))]
        (from-prototype lhs "join-table" (concat lhs-cols rhs-cols)))
      :rhs-missing-indexes rhs-missing}))
+
+
+(defn left-join
+  "Like tech.ml.dataset.base/join-by-column, except we include the
+   entries from lhs that did not make the join in the final
+   result, filling them with column-appropriate missing values."
+  [colname lhs rhs]
+  (let [inner    (-> (join-by-column colname lhs rhs) :join-table)
+        unused   (clojure.set/difference
+                    (set (lhs colname))
+                    (set (inner colname)))
+        leftover (->> (lhs colname)
+                      (map-indexed (fn [idx v]
+                                     (when (unused v) idx)))
+                      (filter identity)
+                      (select lhs (column-names lhs)))
+        n         (ds-row-count leftover)
+        new-columns (->> (clojure.set/difference
+                          (set (column-names inner))
+                          (set (column-names lhs)))
+                         (map (fn [name]
+                                (ds-col/empty-column name
+                                   (dtype/get-datatype
+                                        (column inner name)) n ))))]
+    (ds-concat inner (reduce add-column leftover new-columns))))

src/tech/ml/dataset/column.clj

@@ -210,6 +210,13 @@ Implementations should check their metadata before doing calculations."
           scanned-missing :missing} (ensure-column-reader data)]
      (col-impl/new-column name coldata metadata (or missing scanned-missing)))))
 
+(defn empty-column
+  "Like tech.ml.dataset.column/new-column, except we populate
+   the column with empty values corresponding to the underlying
+   datatype specified by dtype."
+  [name dtype n]
+  ;;TODO change the missing set to a flyweight set or something...
+  (new-column name (col-impl/make-container dtype n) nil (set (range n))))
 
 (defn ensure-column
   "Convert an item to a column if at all possible.  Currently columns either implement

src/tech/ml/dataset/impl/column.clj

@@ -37,7 +37,8 @@
      :string (make-string-table n-elems "")
      :text (let [list-data (ArrayList.)]
              (dotimes [iter n-elems]
-               (.add list-data "")))
+               (.add list-data ""))
+             list-data)
      (dtype/make-container :list dtype n-elems)))
   ([dtype]
    (make-container dtype 0)))
@@ -87,6 +88,7 @@
           (.read rdr idx))))))
 
 
+
 (deftype Column
     [^Set missing
      data

test/tech/ml/dataset/join_test.clj

@@ -20,6 +20,59 @@
     (is (empty? (seq rhs-missing)))))
 
 
+;;sample from https://www.w3schools.com/sql/sql_join_left.asp
+(deftest left-join-test
+  (let [lhs (ds/->dataset [{"CustomerID" 1,
+                            "CustomerName" "Alfreds Futterkiste",
+                            "ContactName" "Maria Anders",
+                            "Address" "Obere Str. 57",
+                            "City" "Berlin",
+                            "PostalCode" 12209,
+                            "Country" "Germany"}
+                           {"CustomerID" 2,
+                            "CustomerName" "Ana Trujillo Emparedados y helados",
+                            "ContactName" "Ana Trujillo",
+                            "Address" "Avda. de la Constitución 2222",
+                            "City" "México D.F.",
+                            "PostalCode" 5021,
+                            "Country" "Mexico"}
+                           {"CustomerID" 3,
+                            "CustomerName" "Antonio Moreno Taquería",
+                            "ContactName" "Antonio Moreno",
+                            "Address" "Mataderos 2312",
+                            "City" "México D.F.",
+                            "PostalCode" 5023,
+                            "Country" "Mexico"}])
+
+        rhs (ds/->dataset [{"OrderID" 10308,
+                            "CustomerID" 2,
+                            "EmployeeID" 7,
+                            "OrderDate" "1996-09-18",
+                            "ShipperID" 3}
+                           {"OrderID" 10309,
+                            "CustomerID" 37,
+                            "EmployeeID" 3,
+                            "OrderDate" "1996-09-19",
+                            "ShipperID" 1}
+                           {"OrderID" 10310,
+                            "CustomerID" 77,
+                            "EmployeeID" 8,
+                            "OrderDate" "1996-09-20",
+                            "ShipperID" 2}])
+        joined (ds/left-join "CustomerID" lhs rhs)
+        recs   (ds/mapseq-reader joined)
+        empty-int?    #{-32768}
+        empty-string? #{""}
+        empty-val?    #(or (empty-int? %) (empty-string? %))
+        realized       (some #(when (= (get % "CustomerID") 2) %) recs)
+        unrealized     (filter #(not= % realized) recs)]
+    (is (every? (complement empty-val?) (vals realized))
+        "Ana's record should be fully realized.")
+    (is (every? identity
+                (for [{:strs [OrderID OrderDate ShipperID]}
+                      unrealized]
+                  (every? empty-val? [OrderID OrderDate ShipperID])))
+        "Everyone else should have missing entries from RHS.")))
 
 
 (comment