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Re: Finding Position in an ordered list

• To: mathgroup at smc.vnet.net
• Subject: [mg57810] Re: Finding Position in an ordered list
• From: "Carl K. Woll" <carlw at u.washington.edu>
• Date: Thu, 9 Jun 2005 05:17:43 -0400 (EDT)
• Organization: University of Washington
• References: <200506060821.EAA12541@smc.vnet.net> <76e8f81805060620315232bc54@mail.gmail.com> <d868f9$bus$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

"János TÓTH" <janostothmeister at gmail.com> wrote in message 
news:d868f9$bus$1 at smc.vnet.net...
> Dear All,
>
> 0. Now I disclose that I am looking for a word in a dictionary.
> 1. Thank for the BinarySearch idea. It is much faster than Position.
> 2. The idea of interpolation in the case of numbers is fantastic.
> 3. However, I tried BinarySearch[{a,b,c},b] without being evaluated.
>
> Thus, I need further help.
>
> Thank for all of you.
>
[snip]

Janos,

Apparently the Combinatorica package BinarySearch function only accepts 
numeric keys. We can modify this function to work with any key as follows:

binarysearch[d_, k_, ordf_:Less] :=
Module[{lo = 1, mid, hi = Length[d], c},
   While[lo <= hi,
      If[(c = d[[mid = Quotient[lo + hi, 2]]]) === k, Return[mid]];
      If[ordf[k, c], hi = mid - 1, lo = mid + 1]];
   Return[lo - 1/2]
]

Then, for your example, we would use the ordering function OrderedQ[{##}]&:

binarysearch[{a, b, c}, b, OrderedQ[{##}] &]
2

Carl Woll 

• References:
  • Finding Position in an ordered list
    • From: janostothmeister@gmail.com