ARROW-10640: [C++] An "if_else" kernel to combine two arrays based on a mask

cpp/src/arrow/CMakeLists.txt

@@ -399,6 +399,7 @@ if(ARROW_COMPUTE)
               compute/kernels/scalar_string.cc
               compute/kernels/scalar_validity.cc
               compute/kernels/scalar_fill_null.cc
+              compute/kernels/scalar_if_else.cc
               compute/kernels/util_internal.cc
               compute/kernels/vector_hash.cc
               compute/kernels/vector_nested.cc

cpp/src/arrow/compute/api_scalar.cc

@@ -157,5 +157,10 @@ Result<Datum> FillNull(const Datum& values, const Datum& fill_value, ExecContext
   return CallFunction("fill_null", {values, fill_value}, ctx);
 }
 
+Result<Datum> IfElse(const Datum& cond, const Datum& if_true, const Datum& if_false,
+                     ExecContext* ctx) {
+  return CallFunction("if_else", {cond, if_true, if_false}, ctx);
+}
+
 }  // namespace compute
 }  // namespace arrow

cpp/src/arrow/compute/api_scalar.h

@@ -465,5 +465,21 @@ ARROW_EXPORT
 Result<Datum> FillNull(const Datum& values, const Datum& fill_value,
                        ExecContext* ctx = NULLPTR);
 
+/// \brief IfElse returns elements chosen from `left` or `right`
+/// depending on `cond`. `null` values in `cond` will be promoted to the result
+///
+/// \param[in] cond `Boolean` condition Scalar/ Array
+/// \param[in] left Scalar/ Array
+/// \param[in] right Scalar/ Array
+/// \param[in] ctx the function execution context, optional
+///
+/// \return the resulting datum
+///
+/// \since 5.0.0
+/// \note API not yet finalized
+ARROW_EXPORT
+Result<Datum> IfElse(const Datum& cond, const Datum& left, const Datum& right,
+                     ExecContext* ctx = NULLPTR);
+
 }  // namespace compute
 }  // namespace arrow

cpp/src/arrow/compute/kernels/CMakeLists.txt

@@ -29,6 +29,7 @@ add_arrow_compute_test(scalar_test
                        scalar_string_test.cc
                        scalar_validity_test.cc
                        scalar_fill_null_test.cc
+                       scalar_if_else_test.cc
                        test_util.cc)
 
 add_arrow_benchmark(scalar_arithmetic_benchmark PREFIX "arrow-compute")