Assigning Values to an Array

Please do not add new code, and move existing code to the Arrays task.

Assigning Values to an Array was a programming task. It has been deprecated for reasons that are discussed in its talk page.

In this task, the goal is to assign a value to an element of an array. The value should only replace an existing value, and not insert a new key should the key not exist. If the key does not exist, an error should be returned.