diff --git a/.gitignore b/.gitignore
index 97d869a..49f0c5b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -270,3 +270,7 @@ CPack*
 _CPack*
 .cache/
 *.deb
+
+# test files
+test/lib/googletest/googletest/generated/
+jsonlogic_test
diff --git a/src/operations/data/missing.cpp b/src/operations/data/missing.cpp
index ba1ab2e..eca203d 100644
--- a/src/operations/data/missing.cpp
+++ b/src/operations/data/missing.cpp
@@ -1,7 +1,8 @@
 #include "json_logic.h"
 
-#include <set>
 #include <string>
+#include <unordered_set>
+#include <vector>
 
 #include "nlohmann/json.hpp"
 
@@ -17,7 +18,8 @@ namespace json_logic
 				"Expected 1 or more arguments, but received " + std::to_string(values.size())
 			);
 
-        std::set<std::string> missing_keys{};
+        std::vector<std::string> missing_keys{};
+        std::unordered_set<std::string> seen{};
 
         for (const auto& value : values)
         {
@@ -34,7 +36,11 @@ namespace json_logic
             }
             catch (...)
             {
-                missing_keys.insert(key);
+                const auto [_, inserted] = seen.insert(key);
+                if (inserted)
+                {
+                    missing_keys.push_back(key);
+                }
             }
         }
 
diff --git a/src/operations/data/missing_some.cpp b/src/operations/data/missing_some.cpp
index 01ef464..2b43826 100644
--- a/src/operations/data/missing_some.cpp
+++ b/src/operations/data/missing_some.cpp
@@ -1,7 +1,8 @@
 #include "json_logic.h"
 
-#include <set>
 #include <string>
+#include <unordered_set>
+#include <vector>
 
 #include "nlohmann/json.hpp"
 
@@ -23,8 +24,9 @@ namespace json_logic
         if (!search_values.is_array())
             throw JsonLogicException(__FUNCTION__, "Second argument must be an array");
 
-        std::set<std::string> found_keys{};
-        std::set<std::string> missing_keys{};
+        std::vector<std::string> missing_keys{};
+        std::unordered_set<std::string> found_keys{};
+        std::unordered_set<std::string> seen_missing{};
 
         for (const auto& value : search_values)
         {
@@ -43,7 +45,11 @@ namespace json_logic
             }
             catch (...)
             {
-                missing_keys.insert(key);
+                const auto [_, inserted] = seen_missing.insert(key);
+                if (inserted)
+                {
+                    missing_keys.push_back(key);
+                }
             }
         }
 
diff --git a/test/operations/data/missing.cpp b/test/operations/data/missing.cpp
index 658fadf..1e19409 100644
--- a/test/operations/data/missing.cpp
+++ b/test/operations/data/missing.cpp
@@ -1,4 +1,4 @@
-#include <set>
+#include <vector>
 
 #include "gtest/gtest.h"
 
@@ -29,7 +29,7 @@ TEST_F(OperationDataMissing, SimpleKeys)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{"b", "d"};
+    std::vector<std::string> expected_result{"b", "d"};
     EXPECT_EQ(result, expected_result);
 }
 
@@ -52,7 +52,7 @@ TEST_F(OperationDataMissing, SimpleKeysAllPresent)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{};
+    std::vector<std::string> expected_result{};
     EXPECT_EQ(result, expected_result);
 }
 
@@ -73,7 +73,7 @@ TEST_F(OperationDataMissing, SimpleKeysRepeatedKeys)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{"b", "d"};
+    std::vector<std::string> expected_result{"b", "d"};
     EXPECT_EQ(result, expected_result);
 }
 
@@ -100,6 +100,48 @@ TEST_F(OperationDataMissing, NestedKeys)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{"a.y", "b.z"};
+    std::vector<std::string> expected_result{"a.y", "b.z"};
+    EXPECT_EQ(result, expected_result);
+}
+
+TEST_F(OperationDataMissing, OrderPreservationNonAlphabetic)
+{
+    const auto logic = R"(
+        {
+        "missing": ["zebra", "apple", "banana", "dog", "cat"]
+        }
+    )"_json;
+
+    const auto data = R"(
+        {
+        "apple": 1,
+        "dog": 4
+        }
+    )"_json;
+
+    const auto result = json_logic_->Apply(logic, data);
+
+    std::vector<std::string> expected_result{"zebra", "banana", "cat"};
+    EXPECT_EQ(result, expected_result);
+}
+
+TEST_F(OperationDataMissing, OrderPreservationReversedKeys)
+{
+    const auto logic = R"(
+        {
+        "missing": ["z", "y", "x", "w", "v"]
+        }
+    )"_json;
+
+    const auto data = R"(
+        {
+        "y": 2,
+        "w": 4
+        }
+    )"_json;
+
+    const auto result = json_logic_->Apply(logic, data);
+
+    std::vector<std::string> expected_result{"z", "x", "v"};
     EXPECT_EQ(result, expected_result);
 }
diff --git a/test/operations/data/missing_some.cpp b/test/operations/data/missing_some.cpp
index 9773ca0..b81f292 100644
--- a/test/operations/data/missing_some.cpp
+++ b/test/operations/data/missing_some.cpp
@@ -1,4 +1,4 @@
-#include <set>
+#include <vector>
 
 #include "gtest/gtest.h"
 
@@ -29,7 +29,7 @@ TEST_F(OperationDataMissingSome, SimpleKeysFoundMinimum)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{};
+    std::vector<std::string> expected_result{};
     EXPECT_EQ(result, expected_result);
 }
 
@@ -50,7 +50,7 @@ TEST_F(OperationDataMissingSome, SimpleKeysNotFoundMinimum)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{"b", "d"};
+    std::vector<std::string> expected_result{"b", "d"};
     EXPECT_EQ(result, expected_result);
 }
 
@@ -71,7 +71,7 @@ TEST_F(OperationDataMissingSome, SimpleRepeatedKeysFoundMinimum)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{};
+    std::vector<std::string> expected_result{};
     EXPECT_EQ(result, expected_result);
 }
 
@@ -92,7 +92,7 @@ TEST_F(OperationDataMissingSome, SimpleRepeatedKeysNotFoundMinimum)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{"b", "d"};
+    std::vector<std::string> expected_result{"b", "d"};
     EXPECT_EQ(result, expected_result);
 }
 
@@ -119,7 +119,7 @@ TEST_F(OperationDataMissingSome, NestedKeysFoundMinimum)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{};
+    std::vector<std::string> expected_result{};
     EXPECT_EQ(result, expected_result);
 }
 
@@ -146,6 +146,48 @@ TEST_F(OperationDataMissingSome, NestedKeysNotFoundMinimum)
 
     const auto result = json_logic_->Apply(logic, data);
 
-    std::set<std::string> expected_result{"a.y", "b.z"};
+    std::vector<std::string> expected_result{"a.y", "b.z"};
+    EXPECT_EQ(result, expected_result);
+}
+
+TEST_F(OperationDataMissingSome, OrderPreservationNonAlphabetic)
+{
+    const auto logic = R"(
+        {
+        "missing_some": [3, ["zebra", "dog", "apple", "banana", "cat"]]
+        }
+    )"_json;
+
+    const auto data = R"(
+        {
+        "dog": 4,
+        "apple": 1
+        }
+    )"_json;
+
+    const auto result = json_logic_->Apply(logic, data);
+
+    std::vector<std::string> expected_result{"zebra", "banana", "cat"};
+    EXPECT_EQ(result, expected_result);
+}
+
+TEST_F(OperationDataMissingSome, OrderPreservationReversedKeys)
+{
+    const auto logic = R"(
+        {
+        "missing_some": [3, ["z", "y", "x", "w", "v"]]
+        }
+    )"_json;
+
+    const auto data = R"(
+        {
+        "y": 2,
+        "w": 4
+        }
+    )"_json;
+
+    const auto result = json_logic_->Apply(logic, data);
+
+    std::vector<std::string> expected_result{"z", "x", "v"};
     EXPECT_EQ(result, expected_result);
 }