init research

dataframe/docs/StardustDocs/topics/count.md

@@ -0,0 +1,37 @@
+[//]: # (title: count)
+
+<!---IMPORT org.jetbrains.kotlinx.dataframe.samples.api.Analyze-->
+
+Counts the number of rows.
+
+<!---FUN count-->
+
+```kotlin
+df.count()
+```
+
+<!---END-->
+
+Pass a [row condition](DataRow.md#row-conditions) to count only the number of rows that satisfy that condition:
+
+<!---FUN countCondition-->
+
+```kotlin
+df.count { age > 15 }
+```
+
+<!---END-->
+
+When `count` is used in [`groupBy`](groupBy.md#aggregation) or [`pivot`](pivot.md#aggregation) aggregations,
+it counts rows for every data group:
+
+<!---FUN countAggregation-->
+
+```kotlin
+df.groupBy { city }.count()
+df.pivot { city }.count { age > 18 }
+df.pivot { name.firstName }.groupBy { name.lastName }.count()
+```
+
+<inline-frame src="resources/org.jetbrains.kotlinx.dataframe.samples.api.Analyze.countAggregation.html" width="100%"/>
+<!---END-->