added example of sort_by

Author: venom1204

NEWS.md

@@ -10,7 +10,17 @@
 
 ### NEW FEATURES
 
-1. New `sort_by()` method for data.tables, [#6662](https://github.com/Rdatatable/data.table/issues/6662). It uses `forder()` to improve upon the data.frame method and also match `DT[order(...)]` behavior with respect to locale. Thanks @rikivillalba for the suggestion and PR.
+1. New `sort_by()` method for data.tables, [#6662](https://github.com/Rdatatable/data.table/issues/6662). It uses `forder()` to improve upon the data.frame method and also match `DT[order(...)]` behavior with respect to locale, and allows sorting by expressions or functions. Thanks @rikivillalba for the suggestion and PR.
+
+```r
+    DT = data.table(a = c(1, 2, 1), b = c(3, 1, 2))
+    # sort by column 'b'
+    sort_by(DT, ~b)
+    #    a b
+    # 1: 2 1
+    # 2: 1 2
+    # 3: 1 3
+```
 
 2. `melt()` now supports using `patterns()` with `id.vars`, [#6867](https://github.com/Rdatatable/data.table/issues/6867). Thanks to Toby Dylan Hocking for the suggestion and PR.