/// <summary>
/// Returns a random subset of an array with a supplied number of
/// elements (subsetCount). The elements in the subset are in the
/// same order as in the original array. O(count).
/// NOTE: this method needs to generate one random number for each
/// element of the original array. If subsetCount is significantly
/// smaller than count, it is more efficient to use
/// <see cref="CreateSmallRandomSubsetIndexArray"/> or
/// <see cref="CreateSmallRandomSubsetIndexArrayLong"/> or
/// <see cref="CreateSmallRandomOrderedSubsetIndexArray"/> or
/// <see cref="CreateSmallRandomOrderedSubsetIndexArrayLong"/>.
/// </summary>
public static T[] CreateRandomSubsetOfSize <T>(
this T[] array, long subsetCount, IRandomUniform rnd = null)
{
if (rnd == null)
{
rnd = new RandomSystem();
}
long count = array.LongLength;
if (!(subsetCount >= 0 && subsetCount <= count))
{
throw new ArgumentOutOfRangeException(nameof(subsetCount));
}
var subset = new T[subsetCount];
long si = 0;
for (int ai = 0; ai < count && si < subsetCount; ai++)
{
var p = (double)(subsetCount - si) / (double)(count - ai);
if (rnd.UniformDouble() <= p)
{
subset[si++] = array[ai];
}
}
return(subset);
}
/// <summary>
/// Returns a random subset of the enumeration with a supplied number of
/// elements (subsetCount). The elements in the subset are in the
/// same order as in the input. O(count).
/// NOTE: The number of elements of the Enumerable need to be calculated, in case of true enumerations
/// the implementation of .Count() results in a second evaluation of the enumerable.
/// </summary>
public static T[] CreateRandomSubsetOfSize <T>(
this IEnumerable <T> input, long subsetCount, IRandomUniform rnd = null)
{
if (rnd == null)
{
rnd = new RandomSystem();
}
long count = input.Count();
if (!(subsetCount >= 0 && subsetCount <= count))
{
throw new ArgumentOutOfRangeException(nameof(subsetCount));
}
var subset = new T[subsetCount];
long si = 0, ai = 0;
foreach (var a in input)
{
if (ai < count && si < subsetCount)
{
var p = (double)(subsetCount - si) / (double)(count - ai++);
if (rnd.UniformDouble() <= p)
{
subset[si++] = a;
}
}
else
{
break;
}
}
return(subset);
}
/// <summary>
/// Enumerates elements in random order.
/// </summary>
public static IEnumerable <T> RandomOrder <T>(this IEnumerable <T> self, IRandomUniform rnd = null)
{
var tmp = self.ToArray();
if (rnd == null)
{
rnd = new RandomSystem();
}
var perm = rnd.CreatePermutationArray(tmp.Length);
return(perm.Select(index => tmp[index]));
}
/// <summary>
/// Yields each element with propability p.
/// </summary>
public static IEnumerable <R> TakeRandomly <T, R>(this IEnumerable <T> self, Func <T, R> selector, double p,
IRandomUniform random = null)
{
if (self == null)
{
throw new ArgumentNullException();
}
if (p < 0 || p > 1)
{
throw new ArgumentOutOfRangeException();
}
if (random == null)
{
random = new RandomSystem();
}
foreach (var s in self)
{
if (random.UniformDouble() <= p)
{
yield return(selector(s));
}
}
}
