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# Friday, March 7, 2014

From time to time we run into tasks that we would like to solve in LINQ style but unfortunately it either cannot be done or a solution is not efficient.

Note that by LINQ style we do not mean C# query expressions (we have a strong distaste for that syntax) but extension methods defined in System.Linq.Enumerable and other classes.

Here we quote several extension methods that are good for a general use:

1. Select with predicate. This is shorthand of items.Where(...).Select(...):

/// <summary>
/// Projects each element of a sequence into a new form.
/// </summary>
/// <typeparam name="T">A type of elements of source sequence.</typeparam>
/// <typeparam name="R">A type of elements of target sequence.</typeparam>
/// <param name="source">A source sequence.</param>
/// <param name="where">A predicate to filter elements.</param>
/// <param name="selector">A result element selector.</param>
/// <returns>A target sequence.</returns>
public static IEnumerable<R> Select<T, R>(
  this IEnumerable<T> source,
  Func<T, bool> where,
  Func<T, R> selector)
{
  return source.Where(where).Select(selector);
}

2. Select with predicate with source element index passed both into the predicate and into the selector. This one you cannot trivially implement in LINQ:

/// <summary>
/// Projects each element of a sequence into a new form.
/// </summary>
/// <typeparam name="T">A type of elements of source sequence.</typeparam>
/// <typeparam name="R">A type of elements of target sequence.</typeparam>
/// <param name="source">A source sequence.</param>
/// <param name="where">A predicate to filter elements.</param>
/// <param name="selector">A result element selector.</param>
/// <returns>A target sequence.</returns>
public static IEnumerable<R> Select<T, R>(
  this IEnumerable<T> source,
  Func<T, int, bool> where,
  Func<T, int, R> selector)
{
  var index = 0;

  foreach(var value in source)
  {
    if (where(value, index))
    {
      yield return selector(value, index);
    }

    ++index;
  }
}

3. A function with output element as projection of a window of input elements. Such function can be used to get finite difference (operation opposite to a cumulative sum).

/// <summary>
/// Projects a window of source elements in a source sequence into target sequence.
/// Thus
///   target[i] =
///     selector(source[i], source[i - 1], ... source[i - window + 1])
/// </summary>
/// <typeparam name="T">A type of elements of source sequence.</typeparam>
/// <typeparam name="R">A type of elements of target sequence.</typeparam>
/// <param name="source">A source sequence.</param>
/// <param name="window">A size of window.</param>
/// <param name="lookbehind">
/// Indicate whether to produce target if the number of source elements
/// preceeding the current is less than the window size.
/// </param>
/// <param name="lookahead">
/// Indicate whether to produce target if the number of source elements
/// following current is less than the window size.
/// </param>
/// <param name="selector">
/// A selector that derives target element.
/// On input it receives:
///   an array of source elements stored in round-robing fashon;
///   an index of the first element;
///   a number of elements in the array to count.
/// </param>
/// <returns>Returns a sequence of target elements.</returns>
public static IEnumerable<R> Window<T, R>(
  this IEnumerable<T> source,
  int window,
  bool lookbehind,
  bool lookahead,
  Func<T[], int, int, R> selector)
{
  var buffer = new T[window];
  var index = 0;
  var count = 0;

  foreach(var value in source)
  {
    if (count < window)
    {
      buffer[count++] = value;

      if (lookbehind || (count == window))
      {
        yield return selector(buffer, 0, count);
      }
    }
    else
    {
      buffer[index] = value;
      index = index + 1 == window ? 0 : index + 1;

      yield return selector(buffer, index, count);
    }
  }

  if (lookahead)
  {
    while(--count > 0)
    {
      index = index + 1 == window ? 0 : index + 1;

      yield return selector(buffer, index, count);
    }
  }
}

This way a finite difference looks like this:

var diff = input.Window(
  2,
  false,
  false,
  (buffer, index, count) => buffer[index ^ 1] - buffer[index]);

4. A specialization of Window method that returns a enumeration of windows:

/// <summary>
/// Projects a window of source elements in a source sequence into a
/// sequence of window arrays.
/// </summary>
/// <typeparam name="T">A type of elements of source sequence.</typeparam>
/// <typeparam name="R">A type of elements of target sequence.</typeparam>
/// <param name="source">A source sequence.</param>
/// <param name="window">A size of window.</param>
/// <param name="lookbehind">
/// Indicate whether to produce target if the number of source elements
/// preceeding the current is less than the window size.
/// </param>
/// <param name="lookahead">
/// Indicate whether to produce target if the number of source elements
/// following current is less than the window size.
/// </param>
/// <returns>Returns a sequence of windows.</returns>
public static IEnumerable<T[]> Window<T>(
  this IEnumerable<T> source,
  int window,
  bool lookbehind,
  bool lookahead)
{
  return source.Window(
    window,
    lookbehind,
    lookahead,
    (buffer, index, count) =>
    {
      var result = new T[count];

      for(var i = 0; i < count; ++i)
      {
        result[i] = buffer[index];
        index = index + 1 == buffer.Length ? 0 : index + 1;
      }

     return result;
   });
}

Friday, March 7, 2014 2:11:46 PM UTC  #    Comments [3] -
.NET | Tips and tricks
Friday, March 7, 2014 8:28:19 PM UTC
Number 3 and number 4 are very nice, thanks! Will be using them, actually I think I needed something vaguely similar some time ago.

I have been using number 2 already for some time :-) Some ideas are easily borrowed from Ruby or Lisp.

Overall, this trend towards functional programming in C# is the biggest thing what makes me prefer C# to Java so strongly... Very hard to go back from this.

I have the same feeling towards query expressions... But I have to admit there are a couple of things that are way shorter with query expressions than in "fluent syntax." The "let" is probably one of them, without it it's manual coding of yet another projection and anonymous class - "let" does it transparently. And there are a couple more. Probably all of them could be more nicely implemented with extension functions given good design but I've never bothered.
Yuri
Friday, March 7, 2014 8:52:19 PM UTC
Java, does not stand still. V8 has lambda expressions, and something that is analogue to extension functions.

<a href="http://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html">Lambda Expressions</a>

This is a java example:

roster.stream().
filter(p -> p.getGender() == Person.Sex.MALE).
map(p -> p.getEmailAddress()).
forEach(email -> System.out.println(email));
Vladimir Nesterovsky
Friday, March 7, 2014 10:58:09 PM UTC
That's not bad, really. I haven't had a chance yet to look into Java 8, mostly because none of the clients work at that level. But I'll give it a try, thanks for pointing out.

Now I just wish they brought operator overloading into Java...
Yuri
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