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Getting the index of an item in a sorted IList

, 25 Dec 2011 CPOL
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A binary search of a sorted IList to retrieve an index
Part of the code I'm working on is a sorted List<someclass>. It is sorted on a property (key) of the class, and we need to access the list to:
0) See if it contains an entry with a particular key.
1) If it does, retrieve or replace the item.
2) If not, add the item in the correct position (to keep it sorted).
 
What is desired is a method that will retrieve the index of the existing item or where the item ought to be. The IndexOf method of List is unsuitable for this, plus because we are assuming a sorted list, we can use a binary search in an effort to optimize the search.
 
Here is an implementation for the simple case where the list contains the same type as the key:
 
public static bool TryGetIndex<T>
(
  this System.Collections.Generic.IList<T> List
,
  T                                        Sought
,
  out int                                  Index
) where T : System.IComparable<T>
{
  bool result = false ;
 
  Index = 0 ;
 
  int lo = 0 ;
  int hi = List.Count - 1 ;
 
  while ( !result && ( lo <= hi ) )
  {
    Index = lo + ( hi - lo ) / 2 ;
 
    int diff = Sought.CompareTo ( List [ Index ] ) ;
 
    if (diff == 0 )
    {
      result = true ;
    }
    else if ( diff < 0 )
    {
      hi = Index - 1 ;
    }
    else
    {
      lo = ++Index ;
    }
  }
 
  return ( result ) ;
}
 
However, that is not the situation I described earlier, so I have this version:
 
public delegate int Comparison<T,U> ( T Op1 , U Op2 ) ;
 
public static bool TryGetIndex<T,U>
(
  this System.Collections.Generic.IList<U> List
,
  T                                        Sought
,
  Comparison<T,U>                          Comparer
,
  out int                                  Index
)
{
  bool result = false ;
 
  Index = 0 ;
 
  int lo = 0 ;
  int hi = List.Count - 1 ;
 
  while ( !result && ( lo <= hi ) )
  {
    Index = lo + ( hi - lo ) / 2 ;
 
    int diff = Comparer ( Sought , List [ Index ] ) ;
 
    if ( diff == 0 )
    {
      result = true ;
    }
    else if ( diff < 0 )
    {
      hi = Index - 1 ;
    }
    else
    {
      lo = ++Index ;
    }
  }
 
  return ( result ) ;
}
 
This requires that you provide a method to perform the comparison. Here's an example where I'm trying to find a DataRowView with a particular value for Name:
 
delegate
(
  string                  Op1
,
  System.Data.DataRowView Op2
)
{
  return ( System.StringComparer.InvariantCultureIgnoreCase.Compare 
  ( 
    Op1 
  , 
    Op2 [ "Name" ]  
  ) ) ;
}
 
Using this code to find the index of an existing item or the index where a non-existent item belongs allows you to add items where they belong to ensure that the list remains sorted. And the binary search is more efficient than a linear search, especially as the list grows large.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

PIEBALDconsult
Software Developer (Senior)
United States United States
BSCS 1992 Wentworth Institute of Technology
 
Originally from the Boston (MA) area. Lived in SoCal for a while. Now in the Phoenix (AZ) area.
 
OpenVMS enthusiast, ISO 8601 evangelist, photographer, opinionated SOB
 
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Comments and Discussions

 
GeneralRe: Have you read the documentation for IList? Edit: Represents... PinmemberPIEBALDconsult12-Jan-12 6:38 
GeneralRe: You get list element by index, like in array, not the list. PinmemberVMAtm12-Jan-12 6:16 
GeneralReason for my vote of 1 This is not about IList - only about... PinmemberVMAtm11-Jan-12 19:44 
GeneralRe: How do you figure that? PinmemberPIEBALDconsult12-Jan-12 5:26 
GeneralIt may be useful to have a search method accept an IComparer... Pinmembersupercat99-Dec-11 12:04 
GeneralRe: I expect that you could write a Comparison&lt;T,U&gt; method... PinmemberPIEBALDconsult10-Dec-11 3:38 

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