Maps of sObjects

List<Account> accounts = [SELECT Id, Name, Industry FROM Account];
Map<Id, Account> accountsById = new Map<Id, Account>(accounts);

Id someAccountId = accounts[0].Id;
System.debug(accountsById.get(someAccountId).Name);

Passing a List<Account> directly into new Map<Id, Account>(...) automatically builds a map keyed by each record's Id — no manual loop needed. This is such a common pattern that Apex has built-in support for it.

// DON'T DO THIS — a query inside a loop
for (Opportunity opp : opportunities) {
    Account acc = [SELECT Name FROM Account WHERE Id = :opp.AccountId]; // a query per iteration!
    System.debug(acc.Name);
}

This is exactly the "SOQL-in-a-loop" governor limit problem from Module 7's "Loops and Governor Limits" lesson — a separate query runs for every single opportunity, quickly hitting the 100-query limit with enough records.

List<Opportunity> opportunities = [SELECT Id, AccountId FROM Opportunity];

Set<Id> accountIds = new Set<Id>();
for (Opportunity opp : opportunities) {
    accountIds.add(opp.AccountId);
}

Map<Id, Account> accountsById = new Map<Id, Account>(
    [SELECT Id, Name FROM Account WHERE Id IN :accountIds]
);

for (Opportunity opp : opportunities) {
    Account relatedAccount = accountsById.get(opp.AccountId);
    System.debug(relatedAccount.Name);
}

One query collects the needed Account Ids (using a Set to deduplicate, from this module's first lesson), one query fetches those Accounts, and the Map makes every subsequent lookup instant — regardless of how many opportunities there are. This exact pattern reappears constantly in later modules on triggers and bulkification.