Added required lowercase conversion for all the columns

core/generated-sources/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/rename.kt

@@ -43,15 +43,26 @@ public fun <T, C> DataFrame<T>.rename(cols: Iterable<ColumnReference<C>>): Renam
 public data class RenameClause<T, C>(val df: DataFrame<T>, val columns: ColumnsSelector<T, C>)
 
 public fun <T> DataFrame<T>.renameToCamelCase(): DataFrame<T> {
+    // recursively rename all column groups to camel case
     return rename {
         dfs { it.isColumnGroup() && it.name() matches DELIMITED_STRING_REGEX }
     }.toCamelCase()
+        // recursively rename all other columns to camel case
         .rename {
             dfs { !it.isColumnGroup() && it.name() matches DELIMITED_STRING_REGEX }
         }.toCamelCase()
+
+        // take all frame columns recursively and call renameToCamelCase() on all dataframes inside
         .update {
             dfsOf<AnyFrame>()
         }.with { it.renameToCamelCase() }
+
+        // convert all first chars of all columns to the lowercase
+        .rename {
+            allDfs()
+        }.into {
+            it.name.replaceFirstChar { it.lowercaseChar() }
+        }
 }
 
 public fun <T, C> RenameClause<T, C>.into(vararg newColumns: ColumnReference<*>): DataFrame<T> =

core/generated-sources/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/rename.kt

@@ -55,4 +55,11 @@ class RenameTests {
             }
         }
     }
+
+    @Test
+    fun `rename to camelCase`() {
+        val dfWithUpperCaseColumnNames = dataFrameOf("First_Column", "second_column", "ThirdColumn")(1, 2, 3)
+        val df = dfWithUpperCaseColumnNames.renameToCamelCase()
+        df.columnNames() shouldBe listOf("firstColumn", "secondColumn", "thirdColumn")
+    }
 }

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

@@ -43,15 +43,26 @@ public fun <T, C> DataFrame<T>.rename(cols: Iterable<ColumnReference<C>>): Renam
 public data class RenameClause<T, C>(val df: DataFrame<T>, val columns: ColumnsSelector<T, C>)
 
 public fun <T> DataFrame<T>.renameToCamelCase(): DataFrame<T> {
+    // recursively rename all column groups to camel case
     return rename {
         dfs { it.isColumnGroup() && it.name() matches DELIMITED_STRING_REGEX }
     }.toCamelCase()
+        // recursively rename all other columns to camel case
         .rename {
             dfs { !it.isColumnGroup() && it.name() matches DELIMITED_STRING_REGEX }
         }.toCamelCase()
+
+        // take all frame columns recursively and call renameToCamelCase() on all dataframes inside
         .update {
             dfsOf<AnyFrame>()
         }.with { it.renameToCamelCase() }
+
+        // convert all first chars of all columns to the lowercase
+        .rename {
+            allDfs()
+        }.into {
+            it.name.replaceFirstChar { it.lowercaseChar() }
+        }
 }
 
 public fun <T, C> RenameClause<T, C>.into(vararg newColumns: ColumnReference<*>): DataFrame<T> =

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/rename.kt

@@ -55,4 +55,11 @@ class RenameTests {
             }
         }
     }
+
+    @Test
+    fun `rename to camelCase`() {
+        val dfWithUpperCaseColumnNames = dataFrameOf("First_Column", "second_column", "ThirdColumn")(1, 2, 3)
+        val df = dfWithUpperCaseColumnNames.renameToCamelCase()
+        df.columnNames() shouldBe listOf("firstColumn", "secondColumn", "thirdColumn")
+    }
 }