[RESOLVED] Filtering array items according to a dynamically created predicate

[Eri523]

I just encountered the requirement to filter array items according to a dynamically created predicate, i.e. one determined by user input. I didn't find anything really enlightening on MSDN (the sample on Array::FindAll<T> only demonstrates hard-coded predicates) or CodeGuru, so I rolled my own. It involves an extra class created for the sole purpose of representing the predicate. It goes something like this:

// Test1.cpp: Hauptprojektdatei.

#include "stdafx.h"

using namespace System;
using namespace System::IO;
using namespace System::Text::RegularExpressions;

// Regular expression predicate class

public ref class RegexPredicate
{
public:
  RegexPredicate(String ^strRegex, RegexOptions opt) : m_regex(gcnew Regex(strRegex, opt))
  {
    m_pred = gcnew Predicate<String ^>(this, &RegexPredicate::PredFunc);
  }

  initonly Predicate<String ^> ^m_pred;

private:
  bool PredFunc(String ^s)
  {
    return m_regex->IsMatch(s);
  }

  initonly Regex ^m_regex;
};

int main(array<System::String ^> ^args)
{
  if (args->Length != 1) {
    Console::WriteLine("Usage: Invoke this program with a dir-style file name match pattern as the only\n"
      "command line argument");
#ifdef _DEBUG
    Console::WriteLine();
    Console::WriteLine("Hit enter to continue...");
    Console::ReadLine();
#endif
    return 1;
  }

  // Build an array of all file names without absolute path

  array<FileInfo ^> ^afi = (gcnew DirectoryInfo("."))->GetFiles();
  array<String ^> ^astrFileNames = gcnew array<String ^>(afi->Length);
  for (int i = 0; i < afi->Length; ++i)
    astrFileNames[i] = afi[i]->Name;

  // Construct regex predicate object based on the dir-style wildcard pattern converted to a regex

  RegexPredicate ^rxp =
    gcnew RegexPredicate(String::Concat("^", args[0]->Replace(".", "\\.")->Replace("?", ".?")->Replace("*", ".*"), "$"),
    RegexOptions::IgnoreCase);

  // Filter the file names according to the predicate

  array<String ^> ^astrFilteredFileNames = Array::FindAll(astrFileNames, rxp->m_pred);

  // List the filtered file names

  for (int i = 0; i < astrFilteredFileNames->Length; ++i)
    Console::WriteLine(astrFilteredFileNames[i]);
  Console::WriteLine();
  Console::WriteLine("{0} out of {1} files matched the predicate", astrFilteredFileNames->Length, astrFileNames->Length);

#ifdef _DEBUG
  Console::WriteLine();
  Console::WriteLine("Hit enter to continue...");
  Console::ReadLine();
#endif
  return 0;
}

You can see, the predicate class holds the raw predicate (i.e. the delegate) and the predicate function, along with the data needed to determine the match. It works like a charm and is nicely encapsulated, however, since this certainly is something that's required in a vast lot of programs, I wonder whether there is any more straightforward and/or elegant way to achieve this.

Of course the above code is just a demo program. Even my XP already has a dir command and the classes in the System::IO namespace do a good job filtering file names as well. This is just a model of what I need to do (and am currently doing that way) in another app.

Any comments are welcome.