Improve error message for row.getColumnGroup and fix warnings

Author: koperagen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/DataRow.kt

@@ -2,6 +2,7 @@ package org.jetbrains.kotlinx.dataframe
 
 import org.jetbrains.kotlinx.dataframe.api.next
 import org.jetbrains.kotlinx.dataframe.api.prev
+import org.jetbrains.kotlinx.dataframe.columns.ColumnKind
 import org.jetbrains.kotlinx.dataframe.columns.ColumnPath
 import org.jetbrains.kotlinx.dataframe.columns.ColumnReference
 import org.jetbrains.kotlinx.dataframe.impl.columnName
@@ -31,7 +32,17 @@ public interface DataRow<out T> {
     public operator fun get(first: String, vararg other: String): DataRow<T> = owner.get(first, *other)[index]
     public operator fun get(path: ColumnPath): Any? = owner.get(path)[index]
     public operator fun get(name: String): Any?
-    public fun getColumnGroup(columnName: String): AnyRow = get(columnName) as AnyRow
+    public fun getColumnGroup(columnName: String): AnyRow {
+        val value = get(columnName)
+        if (value == null) {
+            val kind = df()[columnName].kind()
+            if (kind != ColumnKind.Group) {
+                error("Cannot cast null value of a $kind to a ${DataRow::class}")
+            }
+        }
+        return value as AnyRow
+    }
+
     public fun getOrNull(name: String): Any?
     public fun <R> getValueOrNull(column: ColumnReference<R>): R?
 

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/DataRowApi.kt

@@ -24,6 +24,7 @@ public fun AnyRow.isNotEmpty(): Boolean = !isEmpty()
 
 public inline fun <reified R> AnyRow.valuesOf(): List<R> = values().filterIsInstance<R>()
 
+// region DataSchema
 @DataSchema
 public data class NameValuePair<V>(val name: String, val value: V)
 
@@ -39,6 +40,8 @@ public val DataRow<NameValuePair<*>>.name: String @JvmName("NameValuePairAny_nam
 public val ColumnsContainer<NameValuePair<*>>.value: DataColumn<*> @JvmName("NameValuePairAny_value") get() = this["value"]
 public val DataRow<NameValuePair<*>>.value: Any? @JvmName("NameValuePairAny_value") get() = this["value"]
 
+// endregion
+
 public inline fun <reified R> AnyRow.namedValuesOf(): List<NameValuePair<R>> =
     values().zip(columnNames()).filter { it.first is R }.map { NameValuePair(it.second, it.first as R) }
 
@@ -106,11 +109,11 @@ public fun <T> DataRow<T>.relative(relativeIndices: Iterable<Int>): DataFrame<T>
     getRows(relativeIndices.mapNotNull { (index + it).let { if (it >= 0 && it < df().rowsCount()) it else null } })
 
 public fun <T> DataRow<T>.relative(relativeIndices: IntRange): DataFrame<T> =
-    getRows((relativeIndices.start + index).coerceIn(df().indices)..(relativeIndices.endInclusive + index).coerceIn(df().indices))
+    getRows((relativeIndices.first + index).coerceIn(df().indices)..(relativeIndices.last + index).coerceIn(df().indices))
 
 public fun <T> DataRow<T>.movingAverage(k: Int, expression: RowExpression<T, Number>): Double {
     var count = 0
-    return backwardIterable().take(k).sumByDouble {
+    return backwardIterable().take(k).sumOf {
         count++
         expression(it).toDouble()
     } / count

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/columns/ColumnKind.kt

@@ -1,7 +1,13 @@
 package org.jetbrains.kotlinx.dataframe.columns
 
 public enum class ColumnKind {
-    Value,
-    Group,
-    Frame
+    Value {
+        override fun toString(): String = "ValueColumn"
+    },
+    Group {
+        override fun toString(): String = "ColumnGroup"
+    },
+    Frame {
+        override fun toString(): String = "FrameColumn"
+    }
 }

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/json.kt

@@ -62,7 +62,6 @@ public fun DataRow.Companion.readJson(file: File, header: List<String> = emptyLi
 public fun DataFrame.Companion.readJson(path: String, header: List<String> = emptyList()): AnyFrame = readJson(asURL(path), header)
 public fun DataRow.Companion.readJson(path: String, header: List<String> = emptyList()): AnyRow = DataFrame.readJson(path, header).single()
 
-@Suppress("UNCHECKED_CAST")
 public fun DataFrame.Companion.readJson(url: URL, header: List<String> = emptyList()): AnyFrame = catchHttpResponse(url) { readJson(it, header) }
 public fun DataRow.Companion.readJson(url: URL, header: List<String> = emptyList()): AnyRow = DataFrame.readJson(url, header).single()
 

tests/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/get.kt

@@ -1,7 +1,9 @@
 package org.jetbrains.kotlinx.dataframe.api
 
 import io.kotest.assertions.throwables.shouldThrow
+import io.kotest.assertions.throwables.shouldThrowAny
 import io.kotest.matchers.shouldBe
+import io.kotest.matchers.string.shouldContain
 import org.jetbrains.kotlinx.dataframe.DataFrame
 import org.jetbrains.kotlinx.dataframe.annotations.DataSchema
 import org.junit.Test
@@ -78,4 +80,13 @@ class GetTests {
         df[x][0].a shouldBe 1
         df[1][x].a shouldBe 3
     }
+
+    @Test
+    fun `throw meaningful exception when traverse columns in DataRow`() {
+        val df = dataFrameOf("a")(null)
+        val throwable = shouldThrowAny {
+            df[0].getColumnGroup("a")
+        }
+        throwable.message shouldContain "Cannot cast null value of a ValueColumn to"
+    }
 }