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MathGroup Archive 2007

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Re: programming problem about elements taken

  • To: mathgroup at smc.vnet.net
  • Subject: [mg72497] Re: programming problem about elements taken
  • From: "Ray Koopman" <koopman at sfu.ca>
  • Date: Wed, 3 Jan 2007 01:03:03 -0500 (EST)
  • References: <200612301039.FAA21673@smc.vnet.net><en82hd$6or$1@smc.vnet.net>

Andrzej Kozlowski wrote:
> Consider this list of 5 numbers:
>
>   ls = Sort[Table[Random[], {5}]]
>   {0.165874, 0.256035, 0.556211, 0.811305, 0.865799}
> Suppose we take epsilon = 0.3 and identify the numbers distant by
> less than that from one another. Then
>
> Union[ls, SameTest -> (Abs[#1 - #2] < 0.3 &)]
>   {0.165874, 0.556211, 0.865799}
> gives us just three numbers.
> [...]
> Note that when two numbers are identified it is the smaller one that
> is kept.
> [...]

Is this just an observation, or official-but-not-publicly-documented?
The arguments to SameTest seem always to be reverse-ordered (i.e.,
#2 >= #1), even when the input list is unsorted, and the sequence of
argument pairs seems to be the same regardless of whether the input
list is sorted or unsorted. Can we conclude that Union sorts the input
list before using SameTest, and that when SameTest returns True it is
the second argument that is kept?


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