How to Efficiently Iterate Through Nested Maps in C ?

Iterating through Nested Maps in C

Consider a scenario where you have a nested map in C . Specifically, your map is a std::map>. This map structure holds data like this:

m["name1"]["value1"] = "data1";
m["name1"]["value2"] = "data2";
m["name2"]["value1"] = "data1";
m["name2"]["value2"] = "data2";
m["name3"]["value1"] = "data1";
m["name3"]["value2"] = "data2";

To effectively iterate through this nested map and access the various values, you can utilize the following techniques:

Using C 11 Ranged for Loops:

With C 11, you can employ ranged-based for loops to simplify the iteration process. This approach avoids unnecessary copies and provides a concise syntax.

std::map<std::string, std::map<std::string, std::string>> mymap;

for (auto const &ent1 : mymap) {
  // ent1.first is the first key
  for (auto const &ent2 : ent1.second) {
    // ent2.first is the second key
    // ent2.second is the data
  }
}

Using Explicit Definition of Reference Variables:

Alternatively, you can enhance readability by explicitly defining reference variables for the keys and values. While this approach generates more code, it explicitly defines variables for better clarity.

for (auto const &ent1 : mymap) {
  auto const &outer_key = ent1.first;
  auto const &inner_map = ent1.second;
  for (auto const &ent2 : inner_map) {
    auto const &inner_key = ent2.first;
    auto const &inner_value = ent2.second;
  }
}

Using Structured Bindings in C 17:

In C 17, structured bindings offer a further simplified and concise method of iterating through nested maps.

for (auto const &[outer_key, inner_map] : mymap) {
  for (auto const &[inner_key, inner_value] : inner_map) {
    // Access outer_key, inner_key and inner_value directly
  }
}

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