Map and Reduce in .NET
Linq equivalents of Map and Reduce: If you’re lucky enough to have linq then you don’t need to write your own map and reduce functions. C# 3.5 and Linq already has it albeit under different names.
Map is
Select
:Enumerable.Range(1, 10).Select(x => x + 2);
Reduce is
Aggregate
:Enumerable.Range(1, 10).Aggregate(0, (acc, x) => acc + x);
Filter is
Where
:Enumerable.Range(1, 10).Where(x => x % 2 == 0);
https://www.justinshield.com/2011/06/mapreduce-in-c/
The classes of problem that are well suited for a mapreduce style solution are problems of aggregation. Of extracting data from a dataset. In C#, one could take advantage of LINQ to program in this style.
From the following article: http://codecube.net/2009/02/mapreduce-in-c-using-linq/
the GroupBy method is acting as the map, while the Select method does the job of reducing the intermediate results into the final list of results.
var wordOccurrences = words
.GroupBy(w => w)
.Select(intermediate => new
{
Word = intermediate.Key,
Frequency = intermediate.Sum(w => 1)
})
.Where(w => w.Frequency > 10)
.OrderBy(w => w.Frequency);
For the distributed portion, you could check out DryadLINQ: http://research.microsoft.com/en-us/projects/dryadlinq/default.aspx
Since I never can remember that LINQ calls it Where
, Select
and Aggregate
instead of Filter
, Map
and Reduce
so I created a few extension methods you can use:
IEnumerable<string> myStrings = new List<string>() { "1", "2", "3", "4", "5" };
IEnumerable<int> convertedToInts = myStrings.Map(s => int.Parse(s));
IEnumerable<int> filteredInts = convertedToInts.Filter(i => i <= 3); // Keep 1,2,3
int sumOfAllInts = filteredInts.Reduce((sum, i) => sum + i); // Sum up all ints
Assert.Equal(6, sumOfAllInts); // 1+2+3 is 6
Here are the 3 methods (from https://github.com/cs-util-com/cscore/blob/master/CsCore/PlainNetClassLib/src/Plugins/CsCore/com/csutil/collections/IEnumerableExtensions.cs ):
public static IEnumerable<R> Map<T, R>(this IEnumerable<T> self, Func<T, R> selector) {
return self.Select(selector);
}
public static T Reduce<T>(this IEnumerable<T> self, Func<T, T, T> func) {
return self.Aggregate(func);
}
public static IEnumerable<T> Filter<T>(this IEnumerable<T> self, Func<T, bool> predicate) {
return self.Where(predicate);
}
Some more details from https://github.com/cs-util-com/cscore#ienumerable-extensions :