Dynamically setting an object field value in Apex

Question

I have searched and read through countless questions & answers here, scoured the interwebs, etc. and I cannot find out whether what I want to do is even doable, much less how. I don't know if I am searching for the wrong terms, looking past the answer, or just being stupid. (FYI: After being a developer on a different large on-premise CRM for almost 5 years, I am only approximately 8 months new to Salesforce; so go easy, please, if I am being stupid.)

Prelude

I have found countless ways to get the value of an sObjectField using a variable. E.g.,

Account a = new Account(Name = 'Test Account');
String n = 'Name';
String result = a.get(n);

// result == 'Test Account'

This can obviously be "genericized" using objects, lists, maps and whatnot; and then with for loops or other iteration, one could get multiple field values when the field names were not previously known.

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Question

Can you do something similar to set a field value using a variable for the field name?

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What I've Tried

a.n = 'New Account Name';

// Results in "Variable does not exist: n" error.

a.set(n) = 'New Account Name';
a.get(n) = 'New Account Name';
a.(a.get(n)) = 'New Account Name';
(sObjectField) a.n = 'New Account Name';
(sObjectField) a.get(n) = 'New Account Name';

// All result in "Expression cannot be assigned" error.

I've used Schema.SObjectType.Account.fields.getMap() and tried all sorts of permutations and combinations of things. (I didn't record all that I tried because I tried hacking at it over and over and can't remember everything.)

I've individually tried...

sObjectField n = Account.Name;
                 Account.Name.getDescribe();
                 Account.Name.getDescribe().getName();
                 Account.Name.getDescribe().getsObjectField();

... all followed by a.n = 'New Account Name';

(which honestly wouldn't really help since the Name part of those would also need to be a variable for it work in a real use case).

And all sorts of things with (), {}, and [] that obviously didn't work.

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Use Case

In a test data factory class I was wanting to iterate a map that has field names and values (where the field names could be different every time) and then create an object with those field names and values. If I can't use a variable to set a field name, I'm going to have to go through and check every possible field name. That obviously makes for MUCH more code; and if we ever add new fields to that object and would want to be able to set those in a test, we would have to add those fields to this code.

Answer 1

The put(key,value) method on SObject does what you're looking for. Slightly modified example from the doc:

String fieldName = 'description';
Account acc = new Account(name = 'test', description = 'old desc');
String oldDesc = (String)acc.put(fieldName, 'new desc');
System.assertEquals('old desc', oldDesc);
System.assertEquals('new desc', acc.description);