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Is there a word for function like this...?

  • 18-07-2005 02:32PM
    #1
    davros
    Closed Accounts Posts: 857 ✭✭✭


    Is there a word for a function that satisfies:

    f(f(x)) = f(x)

    As an example of such a function, consider f(x) = x modulo 5

    f(33) = 3
    f(3) = 3

    I'm betting there's a name for this but I have no idea what it is. Any thoughts?


Comments

  • #2
    ecksor
    Moderators, Social & Fun Moderators Posts: 10,501 Mod ✭✭✭✭ecksor


    The word you're looking for is Indempotent.


  • #3
    davros
    Closed Accounts Posts: 857 ✭✭✭davros


    Yep, that's it. From the Wikipedia:
    A unary operation (i.e., a function), is idempotent if, whenever it is applied twice to any element, it gives the same result as if it were applied once. For example, the greatest integer function is idempotent as a function from the set of real numbers to the set of integers.

    Actually, I've often heard that word used in the context of HTTP but I always assumed it meant no change (to a database, say) ever, even on the first invocation.

    Thanks for that, ecksor.


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