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MathGroup Archive 1992

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Re: Evaluating Min[2^(1/2),3^(1/2)]

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Re: Evaluating Min[2^(1/2),3^(1/2)]
  • From: TODD GAYLEY <TGAYLEY at ccit.arizona.edu>
  • Date: Mon, 10 Aug 1992 01:17 MST

Roger B. Kirchner (kirchner at cs.umn.edu) asks:

>     I would like a way to explicitly find the minimum of a list of
>     exact numbers.  Is there a way to get Min[2^(1/2),3^(1/2)] to
>     evaluate to 2^(1/2)?
>     I also want to be able to select an expression in a list which
>     is smallest when any symbols are given numerical values.  I
>     could do this if I knew how to find the subscript of a member
>     of a list which is smallest.

The best way to get Min to work on such expressions is to force them to 
be evaluated numerically:

In[] :=     Min[ N[2^(1/2)], N[3^(1/2)] ]
Out[] =     1.41421

But of course you want 2^(1/2), not 1.4121. You need to evaluate the 
list numerically, find the Min, find its position, and then get that 
element from the original list.
An easy method to get the position of the smallest member of a list is 
to just use Position[list,Min[list]]. Obviously, finding the min and 
then going back into the list to find elements that match it is not very 
efficient, but this method works quite quickly unless the list is very 
large. 
Putting it together, we can define NMin:

   NMin[items__] := Module[ {nlist = N /@ {items}},
                     {items}[[ First at First@Position[nlist,Min[nlist]] ]]
                    ]

(We need the two Firsts to strip off unwanted braces returned by 
Position.)

In[] := NMin[2^(1/2),3^(1/2)]
Out[] = Sqrt[2]

Note that in its present form, NMin is designed to take a sequence of 
values (as in your example), not a list.

--Todd Gayley
--University of Arizona





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