2012-06-10 Partition - 7 higher order functions

2012-06-10 partition-7-higher-order-functions

# Title: Partition - 7 higher order functions
# Author: Mikael Lundin
# Link: http://blog.mikaellundin.name/2012/06/10/partition-7-higher-order-functions.html

Evens.fs

let evens, odds = [1..10] |> List.partition (fun i -> i % 2 = 0)
 
val odds : int list = [1; 3; 5; 7; 9]
val evens : int list = [2; 4; 6; 8; 10]

Partition.cs

public static IEnumerable<T>[] Partition<T>(Func<T, int> fn, IEnumerable<T> list)
{
    if (list == null)
    {
        throw new ArgumentNullException("list", "Supplied list to Partition is not allowed to be null");
    }
 
    if (fn == null)
    {
        throw new ArgumentNullException("fn", "Supplied function to Partition is not allowed to be null");
    }
 
    // Expand the array arr to fit index i
    Func<IList<T>[], int, IList<T>[]> expand = (arr, i) =>
    {
        var newArray = new IList<T>[i + 1];
        Array.Copy(arr, newArray, arr.Length);
 
        // Initialize new array
        for (var k = arr.Length; k < newArray.Length; k++)
        {
            newArray[k] = new List<T>();
        }
 
        return newArray;
    };
 
    var result = new IList<T>[0];
    foreach (var item in list)
    {
        var index = fn(item);
        if (index < 0)
        {
            throw new IndexOutOfRangeException("Your partition function returned index less than 0");
        }
 
        // new index
        if (index > result.Length - 1)
        {
            // expand array
            result = expand(result, index);
        }
 
        result[index].Add(item);
    }
 
    return result;
}
 
public static IEnumerable<T>[] Partition<T>(this IEnumerable<T> list, Func<T, int> fn)
{
    return Partition(fn, list);
}

People.cs

var people = new List<Person>
{
    new Person(name: "A", age: 12),
    new Person(name: "B", age: 23),
    new Person(name: "C", age: 43),
    new Person(name: "D", age: 65),
    new Person(name: "E", age: 33),
};
 
var parts = people.Partition(p => p.Age < 25 ? 0 : 1);
 
var under25 = parts[0]; // A, B
var over25 = parts[1]; // C, D, E