Implement $str$head() and $str$tail()

NEWS.md

@@ -7,6 +7,7 @@
 - `pl$read_ipc()` can read a raw vector of Apache Arrow IPC file (#1072).
 - New method `<DataFrame>$to_raw_ipc()` to serialize a DataFrame to a raw vector
   of Apache Arrow IPC file format (#1072).
+- New methods `$str$head()` and `$str$tail()` (#1074).
 
 ## Polars R Package 0.16.3
 

R/expr__string.R

@@ -1028,3 +1028,73 @@ ExprStr_find = function(pattern, ..., literal = FALSE, strict = TRUE) {
   .pr$Expr$str_find(self, pattern, literal, strict) |>
     unwrap("in $str$find():")
 }
+
+#' Return the first n characters of each string
+#'
+#' @param n Length of the slice (integer or expression). Strings are parsed as
+#' column names. Negative indexing is supported.
+#'
+#' @details
+#' The `n` input is defined in terms of the number of characters in the (UTF-8)
+#' string. A character is defined as a Unicode scalar value. A single character
+#' is represented by a single byte when working with ASCII text, and a maximum
+#' of 4 bytes otherwise.
+#'
+#' When the `n` input is negative, `head()` returns characters up to the `n`th
+#' from the end of the string. For example, if `n = -3`, then all characters
+#' except the last three are returned.
+#'
+#' If the length of the string has fewer than `n` characters, the full string is
+#' returned.
+#'
+#' @return A string Expr
+#'
+#' @examples
+#' df = pl$DataFrame(
+#'   s = c("pear", NA, "papaya", "dragonfruit"),
+#'   n = c(3, 4, -2, -5)
+#' )
+#'
+#' df$with_columns(
+#'   s_head_5 = pl$col("s")$str$head(5),
+#'   s_head_n = pl$col("s")$str$head("n")
+#' )
+ExprStr_head = function(n) {
+  .pr$Expr$str_head(self, n) |>
+    unwrap("in $str$head():")
+}
+
+#' Return the last n characters of each string
+#'
+#' @param n Length of the slice (integer or expression). Strings are parsed as
+#' column names. Negative indexing is supported.
+#'
+#' @details
+#' The `n` input is defined in terms of the number of characters in the (UTF-8)
+#' string. A character is defined as a Unicode scalar value. A single character
+#' is represented by a single byte when working with ASCII text, and a maximum
+#' of 4 bytes otherwise.
+#'
+#' When the `n` input is negative, `tail()` returns characters starting from the
+#' `n`th from the beginning of the string. For example, if `n = -3`, then all
+#' characters except the first three are returned.
+#'
+#' If the length of the string has fewer than `n` characters, the full string is
+#' returned.
+#'
+#' @return A string Expr
+#'
+#' @examples
+#' df = pl$DataFrame(
+#'   s = c("pear", NA, "papaya", "dragonfruit"),
+#'   n = c(3, 4, -2, -5)
+#' )
+#'
+#' df$with_columns(
+#'   s_tail_5 = pl$col("s")$str$tail(5),
+#'   s_tail_n = pl$col("s")$str$tail("n")
+#' )
+ExprStr_tail = function(n) {
+  .pr$Expr$str_tail(self, n) |>
+    unwrap("in $str$tail():")
+}

R/extendr-wrappers.R

@@ -1074,6 +1074,10 @@ RPolarsExpr$str_replace_many <- function(patterns, replace_with, ascii_case_inse
 
 RPolarsExpr$str_find <- function(pat, literal, strict) .Call(wrap__RPolarsExpr__str_find, self, pat, literal, strict)
 
+RPolarsExpr$str_head <- function(n) .Call(wrap__RPolarsExpr__str_head, self, n)
+
+RPolarsExpr$str_tail <- function(n) .Call(wrap__RPolarsExpr__str_tail, self, n)
+
 RPolarsExpr$bin_contains <- function(lit) .Call(wrap__RPolarsExpr__bin_contains, self, lit)
 
 RPolarsExpr$bin_starts_with <- function(sub) .Call(wrap__RPolarsExpr__bin_starts_with, self, sub)

src/rust/src/lazy/dsl.rs

@@ -2426,6 +2426,15 @@ impl RPolarsExpr {
             _ => Ok(self.0.clone().str().find(pat, strict).into()),
         }
     }
+
+    fn str_head(&self, n: Robj) -> RResult<Self> {
+        Ok(self.0.clone().str().head(robj_to!(PLExprCol, n)?).into())
+    }
+
+    fn str_tail(&self, n: Robj) -> RResult<Self> {
+        Ok(self.0.clone().str().tail(robj_to!(PLExprCol, n)?).into())
+    }
+
     //binary methods
     pub fn bin_contains(&self, lit: Robj) -> RResult<Self> {
         Ok(self

tests/testthat/_snaps/after-wrappers.md

@@ -414,29 +414,30 @@
       [271] "str_count_matches"          "str_ends_with"             
       [273] "str_explode"                "str_extract"               
       [275] "str_extract_all"            "str_extract_groups"        
-      [277] "str_find"                   "str_hex_decode"            
-      [279] "str_hex_encode"             "str_json_decode"           
-      [281] "str_json_path_match"        "str_len_bytes"             
-      [283] "str_len_chars"              "str_pad_end"               
-      [285] "str_pad_start"              "str_replace"               
-      [287] "str_replace_all"            "str_replace_many"          
-      [289] "str_reverse"                "str_slice"                 
-      [291] "str_split"                  "str_split_exact"           
-      [293] "str_splitn"                 "str_starts_with"           
-      [295] "str_strip_chars"            "str_strip_chars_end"       
-      [297] "str_strip_chars_start"      "str_to_date"               
-      [299] "str_to_datetime"            "str_to_integer"            
-      [301] "str_to_lowercase"           "str_to_time"               
-      [303] "str_to_titlecase"           "str_to_uppercase"          
-      [305] "str_zfill"                  "struct_field_by_name"      
-      [307] "struct_rename_fields"       "sub"                       
-      [309] "sum"                        "tail"                      
-      [311] "tan"                        "tanh"                      
-      [313] "timestamp"                  "to_physical"               
-      [315] "top_k"                      "unique"                    
-      [317] "unique_counts"              "unique_stable"             
-      [319] "upper_bound"                "value_counts"              
-      [321] "var"                        "xor"                       
+      [277] "str_find"                   "str_head"                  
+      [279] "str_hex_decode"             "str_hex_encode"            
+      [281] "str_json_decode"            "str_json_path_match"       
+      [283] "str_len_bytes"              "str_len_chars"             
+      [285] "str_pad_end"                "str_pad_start"             
+      [287] "str_replace"                "str_replace_all"           
+      [289] "str_replace_many"           "str_reverse"               
+      [291] "str_slice"                  "str_split"                 
+      [293] "str_split_exact"            "str_splitn"                
+      [295] "str_starts_with"            "str_strip_chars"           
+      [297] "str_strip_chars_end"        "str_strip_chars_start"     
+      [299] "str_tail"                   "str_to_date"               
+      [301] "str_to_datetime"            "str_to_integer"            
+      [303] "str_to_lowercase"           "str_to_time"               
+      [305] "str_to_titlecase"           "str_to_uppercase"          
+      [307] "str_zfill"                  "struct_field_by_name"      
+      [309] "struct_rename_fields"       "sub"                       
+      [311] "sum"                        "tail"                      
+      [313] "tan"                        "tanh"                      
+      [315] "timestamp"                  "to_physical"               
+      [317] "top_k"                      "unique"                    
+      [319] "unique_counts"              "unique_stable"             
+      [321] "upper_bound"                "value_counts"              
+      [323] "var"                        "xor"                       
 
 # public and private methods of each class When
 

tests/testthat/test-expr_string.R

@@ -884,3 +884,40 @@ test_that("str$find() works", {
     test$select(lit = pl$col("s")$str$find("(?iAa", strict = TRUE, literal = TRUE))
   )
 })
+
+test_that("$str$head() works", {
+  df = pl$DataFrame(
+    s = c("pear", NA, "papaya", "dragonfruit"),
+    n = c(3, 4, -2, -5)
+  )
+
+  expect_equal(
+    df$select(
+      s_head_5 = pl$col("s")$str$head(5),
+      s_head_n = pl$col("s")$str$head("n")
+    )$to_list(),
+    list(
+      s_head_5 = c("pear", NA, "papay", "drago"),
+      s_head_n = c("pea", NA, "papa", "dragon")
+    )
+  )
+})
+
+
+test_that("$str$tail() works", {
+  df = pl$DataFrame(
+    s = c("pear", NA, "papaya", "dragonfruit"),
+    n = c(3, 4, -2, -5)
+  )
+
+  expect_equal(
+    df$select(
+      s_tail_5 = pl$col("s")$str$tail(5),
+      s_tail_n = pl$col("s")$str$tail("n")
+    )$to_list(),
+    list(
+      s_tail_5 = c("pear", NA, "apaya", "fruit"),
+      s_tail_n = c("ear", NA, "paya", "nfruit")
+    )
+  )
+})