[Term Entry] C++ Unordered Map: .key_eq()

* [Entry] C++ unordered map: key_eq() #7146

Added Entry for key_eq() with syntax and examples as
required.

* content tweaks

* Update key-eq.md

* Update key-eq.md

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Author: saumyapandey1998

content/cpp/concepts/unordered-map/terms/key-eq/key-eq.md

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+---
+Title: '.key_eq()'
+Description: 'Returns the key equality comparison function used by the `unordered_map` container.'
+Subjects:
+  - 'Computer Science'
+  - 'Data Scienec'
+Tags:
+  - 'Classes'
+  - 'Map'
+  - 'OOP'
+CatalogContent:
+  - 'learn-example-course'
+  - 'paths/example-path'
+---
+
+**`.key_eq()`** is a member [function](https://www.codecademy.com/resources/docs/cpp/functions) that returns the function (also called a predicate) that the container uses to determine whether two keys are considered equal. This function is essential because, in an `unordered_map`, keys are stored in hash buckets and even if two keys hash to the same bucket, the container still needs to check if they're actually equal. That’s where `.key_eq()` comes in.
+
+## Syntax
+
+```pseudo
+unordered_map_name.key_eq()(args1, args2)
+```
+
+**Parameters:**
+
+- `key1`: The first key to compare.
+- `key2`: The second key to compare.
+
+**Return value:**
+
+- Returns `true` if `key1` and `key2` are considered equal by the map's key comparison function.
+- Returns `false` otherwise.
+
+## Example
+
+In this example, the map's key equality function is used to find and print the population of selected animals by comparing external values to map keys:
+
+```cpp
+#include <iostream>
+#include <unordered_map>
+#include <vector>
+#include <string>
+
+int main() {
+  std::unordered_map<std::string, int> animalPopulations = {
+    {"Tiger", 5574},
+    {"Elephant", 415000},
+    {"Giraffe", 117000},
+    {"Rhinoceros", 27000}
+  };
+
+  // List of animals to look up
+  std::vector<std::string> animalList = {"Tiger", "Giraffe"};
+
+  // Get the equality function used by the map
+  auto eq = animalPopulations.key_eq();
+
+  // Loop through the list
+  for (const std::string& target : animalList) {
+    for (const auto& pair : animalPopulations) {
+      if (eq(pair.first, target)) {
+        std::cout << "There are " << pair.second << " " << pair.first << "s in the wild." << std::endl;
+      }
+    }
+  }
+
+  return 0;
+}
+```
+
+The output of this code is:
+
+```shell
+There are 5574 Tigers in the wild.
+There are 117000 Giraffes in the wild.
+```
+
+## Codebyte Example
+
+In this example, the key equality function is used to check whether foods from a list exist in the map and print their protein type or a not-found message:
+
+```codebyte/cpp
+#include <iostream>
+#include <unordered_map>
+#include <vector>
+#include <string>
+
+int main() {
+  std::unordered_map<std::string, std::string> proteinSource = {
+    {"Chicken", "non-veg"},
+    {"Salmon", "non-veg"},
+    {"Greek Yogurt", "veg"},
+    {"Cottage Cheese", "veg"}
+  };
+
+  // List of foods to look up
+  std::vector<std::string> foodList = {"Chicken", "Greek Yogurt", "Paneer"};
+
+  auto eq = proteinSource.key_eq();
+
+  for (const std::string& food : foodList) {
+    bool found = false;
+    for (const auto& pair : proteinSource) {
+      if (eq(pair.first, food)) {
+        std::cout << pair.first << " is a " << pair.second << " source of protein" << std::endl;
+        found = true;
+        break;
+      }
+    }
+    if (!found) {
+      std::cout << food << " is not found in the protein source list." << std::endl;
+    }
+  }
+
+  return 0;
+}
+```