DataFrame$tail() (pola-rs#103)

* DataFrame$tail()

* DataFrame_tail PR review

R/dataframe__frame.R

@@ -686,6 +686,19 @@ DataFrame_limit = function(n) {
 }
 
 
+#' Tail a DataFrame
+#' @name DataFrame_tail
+#' @description Get the last n rows.
+#' @param n positive numeric of integer number not larger than 2^32
+#'
+#' @details any number will converted to u32. Negative raises error
+#' @keywords  DataFrame
+#' @return DataFrame
+DataFrame_tail = function(n) {
+  self$lazy()$tail(n)$collect()
+}
+
+
 #' filter DataFrame
 #' @aliases DataFrame_filter
 #' @description DataFrame$filter(bool_expr)

R/extendr-wrappers.R

@@ -830,6 +830,8 @@ LazyFrame$select <- function(exprs) .Call(wrap__LazyFrame__select, self, exprs)
 
 LazyFrame$limit <- function(n) .Call(wrap__LazyFrame__limit, self, n)
 
+LazyFrame$tail <- function(n) .Call(wrap__LazyFrame__tail, self, n)
+
 LazyFrame$filter <- function(expr) .Call(wrap__LazyFrame__filter, self, expr)
 
 LazyFrame$groupby <- function(exprs, maintain_order) .Call(wrap__LazyFrame__groupby, self, exprs, maintain_order)

R/lazyframe__lazy.R

@@ -213,6 +213,7 @@ LazyFrame_limit = function(n) {
   unwrap(.pr$LazyFrame$limit(self,n))
 }
 
+<<<<<<< HEAD
 #' @title First
 #' @description Get the first row of the DataFrame.  
 #' @keywords DataFrame
@@ -299,6 +300,17 @@ LazyFrame_slice = function(offset, length = NULL) {
   unwrap(.pr$LazyFrame$slice(self, offset, length))
 }
 
+#' @title Tail
+#' @description take last n rows of query
+#' @keywords LazyFrame
+#' @param n positive numeric or integer number not larger than 2^32
+#'
+#' @details any number will converted to u32. Negative raises error
+#'
+#' @return A new `LazyFrame` object with applied filter.
+LazyFrame_tail = function(n) {
+  unwrap(.pr$LazyFrame$tail(self,n))
+}
 
 #' @title Lazy_groupby
 #' @description apply groupby on LazyFrame, return LazyGroupBy

src/rust/src/lazy/dataframe.rs

@@ -107,6 +107,10 @@ impl LazyFrame {
         )
     }
 
+    fn tail(&self, n: Robj) -> Result<LazyFrame, String> {
+        Ok(LazyFrame(self.0.clone().tail(robj_to!(u32, n)?)))
+    }
+
     fn filter(&self, expr: &Expr) -> LazyFrame {
         let new_df = self.clone().0.filter(expr.0.clone());
         LazyFrame(new_df)

tests/testthat/test-dataframe.R

@@ -461,4 +461,8 @@ test_that("methods without arguments", {
   a = pl$DataFrame(mtcars)$slice(2, 4)$as_data_frame()
   b = mtcars[3:6,]
   expect_equal(a, b, ignore_attr = TRUE)
-})
+
+  a = as.data.frame(pl$DataFrame(mtcars)$tail(6))
+  b = tail(mtcars)
+  expect_equal(a, b, ignore_attr = TRUE)
+})

tests/testthat/test-lazy.R

@@ -136,8 +136,11 @@ test_that("methods without arguments", {
   a = pl$DataFrame(mtcars)$lazy()$slice(2, 4)$collect()$as_data_frame()
   b = mtcars[3:6,]
   expect_equal(a, b, ignore_attr = TRUE)
-})
 
+  a = pl$DataFrame(mtcars)$lazy()$tail(6)$collect()$as_data_frame()
+  b = tail(mtcars)
+  expect_equal(a, b, ignore_attr = TRUE)
+})
 
 
 #TODO complete tests for lazy