init research

dataframe/docs/StardustDocs/topics/cumSum.md

@@ -0,0 +1,32 @@
+[//]: # (title: cumSum)
+
+<!---IMPORT org.jetbrains.kotlinx.dataframe.samples.api.Analyze-->
+
+Computes the cumulative sum of values in the selected columns.
+
+```text
+cumSum(skipNA = true) [ { columns } ]
+```
+
+Returns a [`DataFrame`](DataFrame.md) or [`DataColumn`](DataColumn.md) containing the cumulative sum.
+
+See [column selectors](ColumnSelectors.md) for how to select the columns for this operation.
+
+**Parameters:**
+* `skipNA` — when `true`, ignores [`NA` values](nanAndNa.md#na) (`null` or `NaN`). 
+  When `false`, all values after first `NA` will be `NaN` (for `Double` and `Float` columns) or `null` (for integer columns).
+
+**Available for:**
+* [`DataFrame`](DataFrame.md)
+* [`DataColumn`](DataColumn.md)
+* [`GroupBy DataFrame`](groupBy.md#transformation) — cumulative sum per every data group
+
+<!---FUN cumSum-->
+
+```kotlin
+df.cumSum { weight }
+df.weight.cumSum()
+df.groupBy { city }.cumSum { weight }.concat()
+```
+
+<!---END-->