dhruv9vats commented on a change in pull request #12162:
URL: https://github.com/apache/arrow/pull/12162#discussion_r785451341



##########
File path: cpp/src/arrow/compute/kernels/scalar_nested.cc
##########
@@ -429,6 +430,97 @@ const FunctionDoc make_struct_doc{"Wrap Arrays into a 
StructArray",
                                   {"*args"},
                                   "MakeStructOptions"};
 
+struct MapArrayLookupFunctor {
+  static Status ExecMapArray(KernelContext* ctx, const ExecBatch& batch, 
Datum* out) {
+    const auto& options = OptionsWrapper<MapArrayLookupOptions>::Get(ctx);
+
+    MapArray map_array(batch[0].array());
+
+    // Offset differences will tell the number of Strcut = {key, value} pairs
+    // present in the current list.
+    // const std::shared_ptr<arrow::Buffer> offsets = 
map_array.value_offsets();
+
+    std::shared_ptr<arrow::Array> keys = map_array.keys();
+    std::shared_ptr<arrow::Array> items = map_array.items();
+
+    const auto& query_key = options.query_key;
+    const auto& occurence = options.occurence;
+
+    std::unique_ptr<ArrayBuilder> builder;
+    RETURN_NOT_OK(
+        MakeBuilder(ctx->memory_pool(), map_array.map_type()->item_type(), 
&builder));
+
+    int32_t last_key_idx_checked = 0;
+
+    // aka, number of {key, value} pairs in the current map
+    int32_t list_struct_len;
+    bool found_one_key = false;
+    for (int32_t map_array_idx = 0; map_array_idx < map_array.length(); 
++map_array_idx) {
+      // Number of Struct('s) = {key, value} in the list at the current index
+      list_struct_len = map_array.value_length(map_array_idx);

Review comment:
       The length of the current `List`, aka, the number of `keys` to check in 
the `keys` `arrow::Array` above (which is _continous_ in memory and are 
separated by offsets).
   Have I understood this correctly, and is using the 
`map_array.value_length(map_array_idx)`, the correct way to obtain it?




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