Weighted Random

A plugin I created to easily generate weighted random numbers in Unreal Engine.

GitHub

Weighted Random: A Simple Solution for Complex Problems

Randomness is a fundamental aspect of game development, but sometimes you need more control over the distribution of random values. That's where weighted random numbers come in. By assigning probabilities to different outcomes, you can create more nuanced and intentional randomness in your games.

How to Use

Just simply feed the node with your desired weights and it will output a random index based on the weights you provided. Optionally, you can include a RandomStream to get a deterministic random number.

Weighted Random Nodes

Here's the source code. Please check out the plugin on GitHub as well!

// Copyright Epic Games, Inc. All Rights Reserved.
 
#include "WeightedRandomBlueprintLibrary.h"
 
struct FIndexWeightedSampler : public FWeightedRandomSampler
{
public:
	FIndexWeightedSampler() = default;
 
	FIndexWeightedSampler(const int32 IndexRange, const TConstArrayView<float> IndexWeights)
	{
		TotalWeight = 0.0f;
		PerIndexWeight.Reserve(IndexRange);
		for (int32 i = 0; i < IndexRange; ++i)
		{
			if (IndexWeights.Num() > 0)
			{
				PerIndexWeight.Add(IndexWeights.IsValidIndex(i) ? FMath::Max(IndexWeights[i], 0.0f) : 0.0f);
			}
			else
			{
				PerIndexWeight.Add(1.0f);
			}
			TotalWeight += PerIndexWeight.Last();
		}
	}
 
	virtual float GetWeights(TArray<float>& OutWeights) override
	{
		OutWeights = PerIndexWeight;
		return TotalWeight;
	}
 
	TArray<float> PerIndexWeight;
	float TotalWeight = 0.0f;
};
 
void UWeightedRandomBlueprintFunctionLibrary::GenerateWeightedRandomIndex(const TArray<float>& Weights, int32& RandomIndex)
{
	if (Weights.IsEmpty()) return;
 
	auto Sampler = FIndexWeightedSampler(Weights.Num(), Weights);
	Sampler.Initialize();
 
	RandomIndex = Sampler.GetEntryIndex(FMath::FRand(), FMath::FRand());
}
 
void UWeightedRandomBlueprintFunctionLibrary::GenerateWeightedRandomIndexFromStream(const TArray<float>& Weights, const FRandomStream& RandomStream, int32& RandomIndex)
{
	if (Weights.IsEmpty()) return;
 
	auto Sampler = FIndexWeightedSampler(Weights.Num(), Weights);
	Sampler.Initialize();
 
	RandomIndex = Sampler.GetEntryIndex(RandomStream.FRand(), RandomStream.FRand());
}
 
void UWeightedRandomBlueprintFunctionLibrary::GenerateWeightedRandomIndices(const TArray<float>& Weights, const int32 NumIndicesToGenerate, TArray<int32>& RandomIndices)
{
	if (Weights.IsEmpty()) return;
 
	auto Sampler = FIndexWeightedSampler(Weights.Num(), Weights);
	Sampler.Initialize();
 
	RandomIndices.SetNumUninitialized(NumIndicesToGenerate);
	for (int32 i = 0; i < NumIndicesToGenerate; i++)
	{
		const auto RandomIndex = Sampler.GetEntryIndex(FMath::FRand(), FMath::FRand());
		RandomIndices[i] = RandomIndex;
	}
}
 
void UWeightedRandomBlueprintFunctionLibrary::GenerateWeightedRandomIndicesFromStream(const TArray<float>& Weights, const int32 NumIndicesToGenerate, const FRandomStream& RandomStream, TArray<int32>& RandomIndices)
{
	if (Weights.IsEmpty()) return;
 
	auto Sampler = FIndexWeightedSampler(Weights.Num(), Weights);
	Sampler.Initialize();
 
	RandomIndices.SetNumUninitialized(NumIndicesToGenerate);
	for (int32 i = 0; i < NumIndicesToGenerate; i++)
	{
		const auto RandomIndex = Sampler.GetEntryIndex(RandomStream.FRand(), RandomStream.FRand());
		RandomIndices[i] = RandomIndex;
	}
}