fix: $len() should also count null values (#1044)

NEWS.md

@@ -2,6 +2,10 @@
 
 ## polars (development version)
 
+### Bug fixes
+
+* `$len()` now correctly includes `null` values in the count (#1044).
+
 ## Polars R Package 0.16.0
 
 ### Breaking changes

src/rust/src/lazy/dsl.rs

@@ -1727,7 +1727,7 @@ impl RPolarsExpr {
     }
 
     pub fn len(&self) -> Self {
-        self.0.clone().count().into()
+        self.0.clone().len().into()
     }
 
     pub fn slice(&self, offset: &RPolarsExpr, length: Nullable<&RPolarsExpr>) -> Self {

tests/testthat/test-expr_expr.R

@@ -139,6 +139,17 @@ test_that("count + unique + n_unique", {
   )
 })
 
+test_that("$len() and $count() don't have the same behavior for nulls", {
+  expect_equal(
+    pl$DataFrame(x = c(1, 2, NA))$select(pl$col("x")$len())$to_list(),
+    list(x = 3)
+  )
+  expect_equal(
+    pl$DataFrame(x = c(1, 2, NA))$select(pl$col("x")$count())$to_list(),
+    list(x = 2)
+  )
+})
+
 
 test_that("drop_nans drop_nulls", {
   x = c(1.0, 2.0, NaN, NA)