How-To Iterate Through Key-Value Map

Overview

This how-to provides instructions to iterate through a Key-Value Map (kv map) and read values from it.

Before you begin

Make sure you have the following prerequisites in place:

An EOSIO development environment, for details consult the Get Started Guide
A smart contract named smrtcontract
A user defined type named person, which defines the data stored in the map
A kv map object, name my_map, which stores objects of type person, with unique keys of type int.

Refer to the following reference implementation for your starting point:

smartcontract.hpp file

struct person {
  eosio::name account_name;
  std::string first_name;
  std::string last_name;
};

class [[eosio::contract]] smartcontract : public eosio::contract {

   using my_map_t = eosio::kv::map<"kvmap"_n, int, person>;

   public:
      using contract::contract;
      smartcontract(eosio::name receiver, eosio::name code, eosio::datastream<const char*> ds)
         : contract(receiver, code, ds) {}

   private:
      my_map_t my_map{};
};

Procedure

Complete the following steps to implement an action which iterates through the first N person objects in the kv map and prints their first and last names:

Create a new action iterate, which takes as an input parameter the number of iterations to be executed.
Refer to the following reference implementation to implement an action which iterates through the first iterations_count objects in my_map and prints their first and last names:

smartcontract.hpp file

struct person {
  eosio::name account_name;
  std::string first_name;
  std::string last_name;
};

class [[eosio::contract]] smartcontract : public eosio::contract {

   using my_map_t = eosio::kv::map<"kvmap"_n, int, person>;

   public:
      using contract::contract;
      smartcontract(eosio::name receiver, eosio::name code, eosio::datastream<const char*> ds)
         : contract(receiver, code, ds) {}

      // iterates over the first iterations_count persons in the table
      // and prints their first and last names
      [[eosio::action]]
      void iterate(int iterations_count);

   private:
      my_map_t my_map{};
};

smartcontract.cpp file

// iterates over the first iterations_count persons using
// and prints their first and last names
[[eosio::action]]
void kv_map::iterate(int iterations_count) {

   int current_iteration = 0;
   for ( const auto& person_detail : my_map ) {
      if (current_iteration ++ < iterations_count) {
         eosio::print_f(
            "Person %: {%, %}. ",
            current_iteration,
            person_detail.second().first_name,
            person_detail.second().last_name);
      }
      else {
         break;
      }
   }
}```

## Summary

In conclusion, the above instructions show how to iterate through a `Key-Value Map` (`kv map`) and read values from it.