PyNetwork

Authors

• @Yoursole1

What is this project?

This is my first attempt at looking into Genetic Algorithms for training a population of "creatures" to optimize a task within a grid. I did this as a short passion project over the first weekend of school, to show as a computer science demo for the IB computer science class I am taking to help people get excited. I hope you find it interesting!

The stages:

• Create a basic network capable of calculating outputs
• Create a genetic algoithm that uses a fitness function
• Test it on basic boolean logic (2 inputs 1 output)
• Test with compositions of boolean logic (3+ inputs)
• Create a GUI that "creatures" can eat "food" squares

Current Version:

The network:

Currently, the network uses a convolutional neural network (CNN Wiki) that takes the grid of cells as an input, and outputs 5 values from 0 to 1 (compressed with a sigmoid activation function). It is also important to note that I used the ReLU activation function for the boolean logic, because its linearity made it better for the task.

Below are the two functions that were used as activation functions.

def sigmoid(value: float):
    return 1 / (1 + math.exp(-1 * value))


def relu(value: float):
    if value > 0:
        return value
    else:
        return 0

The first training example worked through the process of Genetic Learning (GL Wiki). The network had 1 input, 0 hidden layers, and 1 output. The fitness function rewarded networks that divide the input by two, and output 1/2(input). This was easily reached, with the value of the weight converging to 0.5 and the value of the output layer's bias to 0. This makes sense of course, because multiplying the input by 0.5 is dividing by two.

Next, I trained it to learn the average of two numbers. This is non-trivial, unlike the previous training goal. This is because the nature of the transformation between network layers, adding two numbers is something that can only be approximated. This network had the structure 2->10->1 (2 inputs, 10 nodes in the first hidden layer, and 1 output). For this, I used the relu activation function also because averaging two numbers usually results in a number greater than 1. Note that this choice in activation function does remove the possibility of averaging negative numbers, however given that this was just an experiment it wasn't a focus. I trained the network on two random numbers between 1 and 20, and tested using random numbers between 30 and 100. The error was on average +-0.1.

With this success, I moved to boolean logic, which was a final test of this network, which it passed easily.

The Genetic Algorithm:

This algoithm uses a cost function (which acts a fitness function). For the practice examples, the cost function was the standard one used in networks learning with back propagation: The sum of the squared differences between the desired output and the actual output of each output node. For the final one, for each creature I took their distance from the food (smaller = better). This method has issues, because if a creature randomly spawns near food then doesn't move, it will be selected for, but then the next generation simply won't like moving, however it worked decently. For both cost functions, I simply sorted the networks (or creatures), from smallest to largest cost function, then bred the top two to create the next generation.

The breeding works by randomly choosing one of each weight or bias from each network. This ensures that weights and biases don't change their location in the network, which would be unpredictable and inhibit learning. If both networks are extreamly similar, this "recombination" (as it is called in sexual reproduction in life), has little effect because all the weights and biases are similar, so randomly choosing one or the other often doesn't change much.
This is why there is also a chance of mutation (currently set at 1% per weight or bias). The mutation "amount" is a value between -0.5 and 0.5, which is then divided by the epoch (generation number), to make sure large destructive changes aren't made to the network in later stages of evolution. The mutations create genetic diversity, and are essential for "learned behaiviors" to evolve.

Here is the mutation code for the biases:

for i in range(len(self.biases)):
    val1 = copy.deepcopy(self.biases[i])
    val2 = copy.deepcopy(other.biases[i])

    # randomly pick one (recombination in life)
    selected = random.choice([val1, val2])

    # determines if a mutation happens
    mutate = random.random()

    if mutate < mutationRate:
        selected += (random.random() - 0.5) / epoch

        biases[i] = selected

The GUI (World for the creatures):

This is the world in which the creatures (purple squares) live in. They try to walk towards the food (red square). Each purple square has a network associated with it, and the two that get the closest to the food repopulate the next generation. The five possible outputs are up, down, left, right, and no movement. Over time, the squares learned to move in the direction of the red square relatively well, however they never learned to get all the way to it and stop. Rather, they often would walk within 2 or 3 squares of it, and then move in some loop (keeping a relatively close distance throughout).

Future Goals

While this project is over for me, I think that possible interesting additions to this would be adding breeding within a generation, possibly at the expense of one food. This would mean that more complex behaivior, like getting to food faster, and optimizing time collecting food vs time mating, would start to show. I might come back to this at some point, but I will certainly apply what I learned in this project to future things I work on.

Thanks for reading!

Updates

10/8/2022

• Changed "sight" method of the creatures to the one suggested here (https://github.com/greerviau/SnakeAI). They now do even better at finding food