How to modify data in a multi-index table

Overview

This guide provides instructions to modify data in a multi-index table.

Reference

See the following code reference:

• The multi-index class.
• The multi-index::modify(...) method.

Before you begin

Make sure you have the following prerequisites in place:

• An Antelope development environment, for details consult the Documentation Portal.
• 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 modify data in the testtab multi-index table.

1. Define The mod(...) Action

Add a mod action to the testtab multi-index table. The mod action takes as input parameters a user of type eosio::name and a value of type uint32_t. The mod action updates the user object datum data member with the uint32_t value.

[[eosio::action]] void mod( name user, uint32_t value );

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

[[eosio::action]] void mod( name user, uint32_t value );

+using mod_action = action_wrapper<"mod"_n, &multi_index_example::mod>;

2. Find The User You Want To Modify

Use the multi-index find(...) method to locate the user object you want to modify. The targeted user is searched based on its account name.

[[eosio::action]] void multi_index_example::mod( name user, uint32_t value ) {
  auto itr = testtab.find(user.value);
}

3. Yield Error If User Not Found

If the user object you want to update is not found then raise an error message by using the eosio::check method.

[[eosio::action]] void multi_index_example::mod( name user, uint32_t value ) {
  auto itr = testtab.find(user.value);
+  check( itr != testtab.end(), "user does not exist in table" );
}

4. Update The User If Found

If the user object you want to update is found, the eosio::check method will do nothing and the iterator itr will be pointing at the object which you want to update. Use the multi-index::modify(...) method to update the user object datum data member with the value parameter.

[[eosio::action]] void multi_index_example::mod( name user, uint32_t value ) {
  // check if the user already exists
  auto itr = testtab.find(user.value);
  check( itr != testtab.end(), "user does not exist in table" );

+  testtab.modify( itr, _self, [&]( auto& row ) {
+    row.datum = value;
+  });
}

Now you have implemented a new action mod. Call mod to update the datum data member for the user object identified by the user name parameter.

[[info | 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 modify data in a multi-index table.

Next Steps

• You can insert data into the multi-index table.
• You can delete data from the multi-index table.