dictionary(loopvar| ExpressionForKeyDefinition ,  ExpressionForLookUpInDictionary )

ACollectionOf1000->collect(ac1| SomeSecondaryCollectionOf1000->dictionary(ss2|ss2.Name,ac1.Name)->first.HomeAddress:=ac1.DeliveryAddress)

When you need to iterate over two lists to match on some common attribute, a dictionary will greatly reduce the work performed by the CPU if the list is read more than written.

1. Dictionaries in OCL use a hash table implementation, which means that they can provide fast lookups of objects based on a value. This is because the hash function can map the key to a unique index in the table, allowing for constant-time access to the value associated with that key.
2. A dictionary helps reduce the time of searching for a particular value in a collection. Instead of searching through the entire collection, the dictionary can perform a hash lookup to quickly locate the desired value.

Overall, using a dictionary in OCL can improve performance by providing fast lookup and efficient searching.