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I always get confused about how to use the verb "to map" in the context of associating objects with each other, particularly if the mapping is directional.

Is there a distinction between

mapping users to IDs


mapping IDs to users

In the given example, which would be the better way to put it?

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up vote 13 down vote accepted

A map from X to Y would be something (function, table, data-structure) which, given an X (from an X), would allow you to retrieve the (often, but not necessarily, unique) corresponding Y (to a Y).

Something that maps IDs to users would allow you to retrieve a user given his/here ID. A map from users to IDs would be the opposite. A bi-directional mapping would allow you to do the search both ways.

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