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how-to-iterate-and-retrieve-a-multi_index-table

Overview

This guide provides instructions to iterate and retrieve data from a multi-index table.

Reference

See the following code reference:

Before you begin

Make sure you have the following prerequisites in place:

  • An Antelope development environment, for details consult the Get Started Guide,
  • A multi-index testab table instance which stores user objects indexed by the primary key which is of type eosio::name. Consult the section How to instantiate a multi-index table to learn how to set it up.

Procedure

Complete the following steps to iterate, retrieve and print data from the testtab multi-index table.

1. Define The print(...) Action

Add a print action to the testtab multi-index table. The print action takes an account name as a parameter.

[[eosio::action]] void print( name user );

Optionally, for ease of use add the action wrapper definition as well.

[[eosio::action]] void print( name user );

+using print_action = action_wrapper<"print"_n, &multi_index_example::print>;

2. Implement The print(...) Action

Search for the user name in the multi-index table using the primary index. If found, print out the value of field datum. Otherwise raise an error with a custom message. In the contract definition add the following implementation for print action:

  [[eosio::action]] void multi_index_example::print( name user ) {
// searches for the row that corresponds to the user parameter
auto itr = testtab.find(user.value);

// asserts if the row was found for user parameter, if fails use the given message
check( itr != testtab.end(), "user does not exist in table" );

// prints the test_primary and datum fields stored for user parameter
eosio::print_f("Test Table : {%, %}\n", itr->test_primary, itr->datum);
}

3. Put It All Together

The full definition and implementation files for the contract should look like this:

multi_index_example.hpp

#include <eosio/eosio.hpp>
using namespace eosio;

// multi-index example contract class
class [[eosio::contract]] multi_index_example : public contract {
public:
using contract::contract;

// contract class constructor
multi_index_example( name receiver, name code, datastream<const char*> ds ) :
// contract base class contructor
contract(receiver, code, ds),
// instantiate multi-index instance as data member (find it defined below)
testtab(receiver, receiver.value)
{ }

// the row structure of the multi-index table, that is, each row of the table
// will contain an instance of this type of structure
struct [[eosio::table]] test_table {
// this data member stores a name for each row of the multi-index table
name test_primary;
// additional data stored in table row
uint64_t datum;
// mandatory definition for primary key getter
uint64_t primary_key( ) const { return test_primary.value; }
};

// the multi-index type definition, for ease of use define a type alias `test_table_t`,
// based on the multi_index template type, parametarized with a random name and
// the test_table data structure
typedef eosio::multi_index<"testtaba"_n, test_table> test_table_t;

// the multi-index table instance declared as a data member of type test_table_t
test_table_t testtab;

[[eosio::action]] void set( name user );
[[eosio::action]] void print( name user );

using set_action = action_wrapper<"set"_n, &multi_index_example::set>;
using print_action = action_wrapper<"print"_n, &multi_index_example::print>;
};

multi_index_example.cpp

#include <multi_index_example.hpp>

[[eosio::action]] void multi_index_example::set( name user ) {
// check if the user already exists
auto itr = testtab.find(user.value);

if ( itr == testtab.end() ) {
// user is not found in table, use emplace to insert a new row data structure in table
testtab.emplace( _self, [&]( auto& u ) {
u.test_primary = user;
u.secondary = "second"_n;
u.datum = 0;
});
}
}

[[eosio::action]] void multi_index_example::print( name user ) {
// searches for the row that corresponds to the user parameter
auto itr = testtab.find(user.value);

// asserts if the row was found for user parameter, if fails use the given message
check( itr != testtab.end(), "user does not exist in table" );

// prints the test_primary and datum fields stored for user parameter
eosio::print_f("Test Table : {%, %}\n", itr->test_primary, itr->datum);
}
Full example location

A full example project demonstrating the instantiation and usage of multi-index table can be found here.

Summary

In conclusion, the above instructions show how to iterate and retrieve a multi-index table.

Next Steps