Implement apply method

Author: tushushu

tests/test_base.py

@@ -133,6 +133,14 @@ def test_methods_no_arg(
         ('__getitem__', 'int', [1, 2, 3], 1, {'index': 0}),
         ('__getitem__', 'string', ['foo', 'bar', 'baz'], 'foo', {'index': 0}),
 
+        ("apply", "bool", [True, False], [
+         False, True], {"fn": lambda x: x == False},),  # noqa: E712
+        ("apply", "float", [1.0, 2.0], [
+         True, False], {"fn": lambda x: x < 2.0},),
+        ("apply", "int", [1, 2], [3, 5], {"fn": lambda x: x * 2 + 1},),
+        ("apply", "string", ['foo', 'bar'], [
+         True, False], {"fn": lambda x: x != 'bar'},),
+
         ('astype', 'bool', [True, False], [1, 0], {'dtype': 'int'}),
         ('astype', 'bool', [True, False], [1.0, 0.0], {'dtype': 'float'}),
         ('astype', 'bool', [True, False], [True, False], {'dtype': 'bool'}),

ulist/python/ulist/core.py

@@ -161,6 +161,32 @@ def append(self, elem: ELEM) -> None:
         """Adds a new element at the end of the self."""
         self._values.append(elem)
 
+    def apply(
+        self,
+        fn: Callable[[UltraFastList], UltraFastList]
+    ) -> UltraFastList:
+        """Return the result of fn(self).
+
+        Args:
+            fn (Callable[[UltraFastList], UltraFastList]):
+                Function to apply to self.
+
+        Returns:
+            UltraFastList: A ulist object.
+
+        Examples
+        --------
+        >>> import ulist as ul
+        >>> arr = ul.arange(3)
+        >>> arr
+        UltraFastList([0, 1, 2])
+
+        >>> result = arr.apply(lambda x: x * 2)
+        >>> result
+        UltraFastList([0, 2, 4])
+        """
+        return fn(self)
+
     def argmax(self) -> int:
         """Returns the indices of the maximum values of self."""
         assert not isinstance(self._values, (BooleanList, StringList))
@@ -427,8 +453,8 @@ def where(
         self,
         fn: Callable[[UltraFastList], UltraFastList]
     ) -> UltraFastList:
-        """According to the function, return a ulist with elements of self
-        correspondingly.
+        """Calculate the condition by fn(self), and return a ulist
+        with elements of self correspondingly.
 
         Args:
             fn (Callable[[UltraFastList], UltraFastList]):