A couple of weeks ago I was writing some code to initialize List<T> but my technique was very verbose and it seemed to be a distraction from what I was trying to do. Plus it seemed that every one else was writing about getting their func on.
So I looked back at some code that Nick Parker had written:
1: public class Builder2: {3: public static TType Create<TType>(Action<TType> actionOnTType) where TType : new()4: {5: var item = new TType();6: actionOnTType(item);7: return item;8: }9: }
And tweaked it to initialize a list:
1: public class Builder2: {3: public static List<TType> CreateList<TType>(long size, Func<TType> initExpression)4: {5: var items = new List<TType>();6: for (long i = 0; i < size; i++)7: {8: TType item = initExpression();9: items.Add(item);10: }11:12: return items;13: }14: }
Now I can initialize a list like this:
1: List<Door> doors = Builder.CreateList(100, () => new Door {IsOpen = false});
Just for the record the Func initialization above could handle much more complex scenarios if needed. My usage is very simple.
Another variation on this is to create an extension method that does the same initialization like this:
1: public static void Init<TType>(this IList<TType> values, int size, Func<TType> initExpression)2: {3: for (int i = 0; i < size; i++)4: {5: TType item = initExpression();6: values.Add(item);7: }8: }
And here is how you would use the extension method version:
1: var doors = new List<Door>();2: doors.Init(10, () => new Door());
Thanks to Nick Parker for some suggestions as I was working on this.
Something I would like to figure out is how to do this based on IEnumerable instead of IList. If you have ideas regarding implementing this on IEnumerable please add them as comments.
Also, if you want to see where I was using this, pull down the subversion source here: http://subversion.assembla.com/svn/solon-tools/trunk/Puzzles
Chris, you can’t do the same thing with an IEnumerable because member assignment is not defined in that interface. If you want something a little more generic you can convert your code to ICollection pretty easily but I’m afraid that will be as far as you can go.
Since I’m a compulsive suggestion provider, I couldn’t pass the opportunity to tweak your code. Changing the extension method to return the original collection and the delegate to accept the current index (maybe just an overloaded version of the same method):
public ICollection void Init(this ICollection values, int size,
Func initExpression)
{
for (int i = 0; i < size; i++)
{
T item = initExpression(i);
values.Add(item);
}
return values;
}
Then you could write something like:
var doors = new List().Init(10, i => new Door{ Name = “Door #” + i });
I’d probably also rename the method to Add since the list might not be empty to begin with.
Comment by Sergio Pereira — December 6, 2008 @ 2:26 pm |
For what it’s worth, F#’s List.init also accepts the index in its generator.
How about Builder.Init with deferred execution:
public static IEnumerable Init(long size, Func generator)
{
for (long i = 0; i new Door { Name = “Door #”+i }).ToList();
Or without an intermediate data structure:
foreach(var sq in Builder.Init(10, i => new { Num = i, IsEven = (0 == i % 2) }))
…
And for fun, an interesting take on an example from PuzzleMath.cs:
public static IEnumerable Squares {
get {
for (long i = 0, sq = 0; sq <= long.MaxValue; sq = (++i) * i)
yield return sq;
}
}
public static IEnumerable FindPerfectSquares(long max)
{
return Squares.Where(i => i <= max);
}
Cheers ~
Keith
Comment by Keith Dahlby — January 7, 2009 @ 1:29 pm |
Apparently WordPress thought I was trying to post HTML… Trying again.
How about Builder.Init with deferred execution:
public static IEnumerable<T> Init<T>(long size, Func<long, T> generator)
{
for (long i = 0; i < size; i++)
yield return generator(i);
}
Which could be used to build a List…
var doors = Builder.Init(10, i => new Door { Name = “Door #”+i }).ToList();
Or without an intermediate data structure:
foreach(var sq in Builder.Init(10, i => new { Num = i, IsEven = (0 == i % 2) }))
…
And for fun, an interesting take on an example from PuzzleMath.cs:
public static IEnumerable<long> Squares {
get {
for (long i = 0, sq = 0; sq FindPerfectSquares(long max)
{
return Squares.Where(i => i <= max);
}
Cheers ~
Keith
Comment by Keith Dahlby — January 7, 2009 @ 1:36 pm |
Missed one…
And for fun, an interesting take on an example from PuzzleMath.cs:
public static IEnumerable<long> Squares {
get {
for (long i = 0, sq = 0; sq <= long.MaxValue; sq = (++i) * i)
yield return sq;
}
}
public static IEnumerable<long> FindPerfectSquares(long max)
{
return Squares.Where(i => i <= max);
}
Comment by Keith Dahlby — January 7, 2009 @ 1:38 pm |