Best way to avoid nested loops

Summary:

Hi,
We are making an integration where we are fetching a BIP report, which returns a list of values, and we are fetching a list from an external system through a REST API. For each value that is in the report, we have to check if it exists in the list from the external system, if it does we need to take a certain action, and if it doesn't we take another action.
Right now we are using two loops, we loop over the list of the report, and for each value there we loop over the list from the external system.