From everything that I have seen about neural networks and genetic algorithms, I have noticed a few things :
NNs are very good at converging to a solution after a certain number of iterations. GAs are good at finding a solution to a problem after a certain number of generations. However there is one time-complexity obstacle at play here, and that is the actual construction of the neural network and genetic algorithm itself. This is where the actual skill and understanding comes in : the consideration of nodes, weights, activation functions etc. For genetic algorithms it's the fitness function, error values etc. This is all determined by the problem domain itself.
My proposal is to find a generalized algorithm that can take the problem statement such as "create a program that simulates a netball game and finds optimal strategies for effective play", and CREATE THE NEURAL NETWORK OR GENETIC ALGORITHM ITSELF using a combination of databases, statistics, classification systems, logic, decision theory, mathematics.
One possible approach to at least partially solve the problem is having a database of problem domains, and an existing neural network and ga for that problem.
The database could have the following attributes :
Problem statement : VARCHAR,
Problem domain : VARCHAR,
numLayers : INT,
NNTree : TREE,
numNodes : INT,
activationFunct : LIST
As the user specifies the program statement, the program must break it up into its elements. For eg , "Netball simulator that learn the effective strategy of the game",
is broken up into Netball [ Rules of game are known ], simulator [ implies 2d or 3d graphics, predefined objects for graphics ], effective [ interpreted as optimal, which influences that activation function used], strategy [ interpreted as emergent behaviour of objects ], game [ interpreted as goal oriented action list ]
What data structures or algorithms are needed for this ?
If someone could solve this problem, he would actually be creating intelligence out of nothing. There is no (and in my humble opinion will never be) a way of automatically discover a way to select a solving problem method. This would presuppose that the problem's solution would already be known. In such case, there is no point to discover the solving problem method...
Hope I helped!
Related
Why specifically is it used?
I know it increases variation which may help explore the problem space, but how much does it increase the probability of finding the optimal solution/configuration in time? And does it do anything else advantageous?
And does it necessarily always help, or are there instances in which it would increase the time taken to find the optimal solution?
As Patrick Trentin said, crossover improve the speed of convergence, because it allows to combine good genes that are already found in the population.
But, for neuro-evolution, crossover is facing the "permutation problem", also known as "the competing convention problem". When two parents are permutations of the same network, then, except in rare cases, their offspring will always have a lower fitness. Because the same part of the network is copied in two different locations, and so the offspring is losing viable genes for one of these two locations.
for example the networks A,B,C,D and D,C,B,A that are permutations of the same network. The offspring can be:
A,B,C,D (copy of parent 1)
D,C,B,A (copy of parent 2)
A,C,B,D OK
A,B,C,A
A,B,B,A
A,B,B,D
A,C,B,A
A,C,C,A
A,C,C,D
D,B,C,A OK
D,C,B,D
D,B,B,A
D,B,B,D
D,B,C,D
D,C,C,A
D,C,C,D
So, for this example, 2/16 of the offspring are copies of the parents. 2/16 are combinations without duplicates. And 12/16 have duplicated genes.
The permutation problem occurs because networks that are permutations one of the other have the same fitness. So, even for an elitist GA, if one is selected as parent, the other will also often be selected as parent.
The permutations may be only partial. In this case, the result is better than for complete permutations, but the offspring will, in a lot of cases, still have a lower fitness than the parents.
To avoid the permutation problem, I heard about similarity based crossover, that compute similarity of neurons and their connected synapses, doing the crossing-over between the most similar neurons instead of a crossing-over based on the locus.
When evolving topology of the networks, some NEAT specialists think the permutation problem is part of a broader problem: "the variable lenght genome problem". And NEAT seems to avoid this problem by speciation of the networks, when two networks are too differents in topology and weights, they aren't allowed to mate. So, NEAT algorithm seems to consider permuted networks as too different, and doesn't allow them to mate.
A website about NEAT also says:
However, in another sense, one could say that the variable length genome problem can never be "solved" because it is inherent in any system that generates different constructions that solve the same problem. For example, both a bird and a bat represent solutions to the problem of flight, yet they are not compatible since they are different conventions of doing the same thing. The same situation can happen in NEAT, where very different structures might arise that do the same thing. Of course, such structures will not mate, avoiding the serious consequence of damaged offspring. Still, it can be said that since disparate representations can exist simultaneously, incompatible genomes are still present and therefore the problem is not "solved." Ultimately, it is subjective whether or not the problem has been solved. It depends on what you would consider a solution. However, it is at least correct to say, "the problem of variable length genomes is avoided."
Edit: To answer your comment.
You may be right for similarity based crossover, I'm not sure it totally avoids the permutation problem.
About the ultimate goal of crossover, without considering the permutation problem, I'm not sure it is useful for the evolution of neural networks, but my thought is: if we divide a neural network in several parts, each part contributes to the fitness, so two networks with a high fitness may have different good parts. And combining these parts should create an even better network. Some offspring will of course inherit the bad parts, but some other offspring will inherit the good parts.
Like Ray suggested, it could be useful to experiment the evolution of neural networks with and without crossover. As there is randomness in the evolution, the problem is to run a large number of tests, to compute the average evolution speed.
About evolving something else than a neural network, I found a paper that says an algorithm using crossover outperforms a mutation-only algorithm for solving the all-pairs shortest path problem (APSP).
Edit 2:
Even if the permutation problem seems to be only applicable to some particular problems like neuro-evolution, I don't think we can say the same about crossover, because maybe we are missing something about the problems that don't seem to be suitable for crossover.
I found a free version of the paper about similarity based crossover for neuro-evolution, and it shows that:
an algorithm using a naive crossover performs worse than a mutation-only algorithm.
using similarity based crossover it performs better than a mutation-only algorithm for all tested cases.
NEAT algorithm sometimes performs better than a mutation-only algorithm.
Crossover is complex and I think there is a lack of studies that compare it with mutation-only algorithms, maybe because its usefulness highly depends:
of its engineering, in function of particular problems like the permutation problem. So of the type of crossover we use (similarity based, single point, uniform, edge recombination, etc...).
And of the mating algorithm. For example, this paper shows that a gendered genetic algorithm strongly outperforms a non-gendered genetic algorithm for solving the TSP. For solving two other problems, the algorithm doesn't strongly outperforms, but it is better than the non-gendered GA. In this experiment, males are selected on their fitness, and females are selected on their ability to produce a good offspring. Unfortunately, this study doesn't compare the results with a mutation-only algorithm.
I am looking for some comprehensive description. I couldn't find it via browsing as things are more clustered on the web and its not in my scope currently.
Classification and evolutionary computing is comparing oranges to apples. Let me explain:
Classification is a type of problem, where the goal is to determine a label given some input. (Typical example, given pixel values, determine image label).
Evolutionary computing is a family of algorithms to solve different types of problems. They work with a "population" of candidates (imagine a set of different neural networks trying to solve a given problem). Somehow you evaluate how good each candidate is in the given task (typically using a "fitness function", but there are other methods). Then a new generation of candidates is produced, taking the best candidates from the previous generation as a model, and including mutations and cross-over (that is, introducing changes). Repeat until happy.
Evolutionary computing can absolutely be used for classification! But there are examples where it is used in different ways. You may use evolutionary computing to create an artificial neural network controlling a robot (in this case, inputs are sensor values, outputs are commands for actuators). Or to create original content free of a given goal, as in Picbreeder.
Classification may be solved using evolutionary computation (maybe this is why you where confused in the first place) but other techniques are also common. You can use decision trees, or notably deep-learning (based on backpropagation).
Deep-learning based on backpropagation may sound similar to evolutionary computation, but it is quite different. Here you have only one artificial neural network, and a clear rule (backpropagation) telling you which changes to introduce every iteration.
Hope this helps to complement other answers!
Classification algorithms and evolutionary computing are different approaches. However, they are related in some ways.
Classification algorithms aim to identify the class label of new instances. They are trained with some labeled instances. For example, recognition of digits is a classification algorithm.
Evolutionary algorithms are used to find out the minimum or maximum solution of an optimization problem. They randomly explore the solution space of the given problem. They can find a good solution in a reasonable time and are not able to find the global optimum in all problems.
In some classification approaches, evolutionary algorithms are used to find out the optimal value of the parameters.
I am currently studying a doctoral thesis in control theory. At the end of every chapter there is a simulation of a relative-with-the-subject problem. I have finished the theory,but for further understanding I would like to reproduce the simulations. The first simulation is as follows :
The solution of the problem concludes in a system of differential equations whose right hand side consists of functions with unknown parameters. The author states the following : "We will use neural networks with one hidden layer,sigmoid basis functions and 5 weights in the external layer in order to approximate every parameter of the unknown functions.More specifically, the weights of the hidden layer are selected through iterative trials and are kept stable during the simulation." And then he states the logic with which he selects the initial values of the unknown parameters and then shows the results of the simulation.
Could anyone give me a lead on where to look and what I need to know in order to solve this specific problem myself in MATLAB (since this is the environment I am most familiar with)? Because the results of a google search are chaotic since I don't really know what I'm looking for.
If you need any more info,feel free to ask!
You can try MATLAB's Neural Network Toolbox. This gives you an nice UI where you can configure the network, train it with data to find the parameter values and test for performance. No coding involved.
Or, you can program it by hand. Since you are working with one hidden layer, it should be very simple. I am sure any machine learning or neural net (NN) textbook would have one example of it. You can also look into GitHib for projects. There should be many NN projects there, in case you are looking to salvage code from existing project.
Most importantly, you should start by learning about NN, if you haven't done that already. NN with single hidden layer is easy to implement once you understand the equations for the forward and back propagation.
I am trying to make a neural network for approximation of some unkown function (for my neural network course). The problem is that this function has very many variables but many of them are not important (for example in [f(x,y,z) = x+y] z is not important). How could I design (and learn) network for this kind of problem?
To be more specific the function is an evaluation function for some board game with unkown rules and I need to somehow learn this rules by experience of the agent. After each move the score is given to the agent so actually it needs to find how to get max score.
I tried to pass the neighborhood of the agent to the network but there are too many variables which are not important for the score and agent is finding very local solutions.
If you have a sufficient amount of data, your ANN should be able to ignore the noisy inputs. You also may want to try other learning approaches like scaled conjugate gradient or simple heuristics like momentum or early stopping so your ANN isn't over learning the training data.
If you think there may be multiple, local solutions, and you think you can get enough training data, then you could try a "mixture of experts" approach. If you go with a mixture of experts, you should use ANNs that are too "small" to solve the entire problem to force it to use multiple experts.
So, you are given a set of states and actions and your target values are the score after the action is applied to the state? If this problem gets any hairier, it will sound like a reinforcement learning problem.
Does this game have discrete actions? Does it have a discrete state space? If so, maybe a decision tree would be worth trying?
Why do we use neural networks? It's biologic. Aren't there any more solutions that're more "suitable" for computers?
In other words: Why do we use the human brain as a model for inspiration for artifical intelligence?
Neural networks aren't really very biological. They resemble, at a very general level, the architecture of neurons, but it's a great exaggeration to say that they work "just like the brain" (an exaggeration that's encouraged by some neural-net advocates, alas).
Neural nets are mostly used for fuzzy, difficult problems that don't yield to traditional algorithmic approaches. IOWs, there are more "suitable" solutions for computers, but sometimes those solutions don't work, and in those cases one approach is a neural network.
Why do we use neural networks?
Because they're simple to construct, and often appear to be a good approach to certain classes of problems, such as pattern recognition.
Aren't there any more solutions that're more "suitable" for computers?
Yes, implementations that more closely match a computer's architecture can be more suitable for the computer, but then can be less suitable for an effective solution.
Why do we use the human brain as a model for inspiration for artifical intelligence?
Because our brain is the superior example we have of something intelligent.
Neural Networks are still used for two reasons.
They are easy to understand for people who don't want to delve into the math of a more complicated algorithm.
They have a really good name. I mean when you role into a CEO's office to sell him your model which would you rather say, Neural Network or Support Vector Machine. When he asks how it works you can just say "just like the neurons in your brain", which is something most people understand. If you try and explain a support vector machine Mr. CEO is going to be lost (Not because he is dumb but because SVMs are harder to understand).
Sometimes they are still useful however I think that the training time is often just too long.
I don't understand the question. Neural nets are suitable for certain functions, and not others. The same is true for various other sorts of classes of algorithms, regardless of what they might have been inspired by.
If we have a good many inputs to something, and we want some outputs, and we have a set of example inputs with known desired outputs, and we don't want to calculate a function ourselves, neural nets are excellent. We feed in the example inputs, compare the output to the example outputs, and adjust the inner workings of the NN in an automatic fashion, to make the NN output closer to the desired output.
This sort of function derivation is very useful in various forms of pattern recognition and general classification. It isn't a panacea, of course. It has no explanatory power (in that you can't look at the innards to see why it classifies something in a particular way), it doesn't offer guarantees of correctness within certain limits, validating how well it works is difficult, and gathering enough examples for training and validation can be expensive or even impossible. The trick is to know when to use a NN and what sort to use.
There are, of course, people who oversell the things as some sort of super solution or even an explanation of human thought, and you might be reacting to them.
Neural network are only "inspired" by the neural structure of our brain, but they are not even close to the complexity of the behaviour of a real neuron (to date there is no neuron model that captures the complexity of a SINGLE neuron, don't even think about a neuronal population...)
Although "neural", machine "learning" and other "pseudo-bio" (like "genetic algorithms") terms are very "cool", that does not mean that they are actually based on real biological processes.
Just that they may very approximatively remind of a biological situation.
NB: of course this does not make them useless! They're very very important in many fields!
Neural networks have been around for a while, and originally were developed to model as close an understanding as we had at the time to the way neurons work in the brain. They represent a network of neurons, hence "neural network." Since computers and brains are very different hardware-wise, implementing anything like a brain with a computer is going to be rather clunky. However, as others have stated so far, neural networks can be useful for some things that are vague such as pattern recognition, facial recognition, and other similar uses. They are also still useful as a basic model of how neurons connect and are often used in Cognitive Science and other fields of artificial intelligence to try to understand how small parts of the complex human brain might make simple decisions. Unfortunately, once a neural network "learns" something, it is very difficult to understand how it actually makes its decisions.
There are, of course, many misuses of neural networks and in most non-research applications, other algorithms have been developed that are much more accurate. If a piece of business software proudly proclaims it uses a neural network, chances are it probably doesn't need it, and might be using it to inefficiently perform a task that could be performed in a much easier way. Unless the software is actually "learning" on the fly, which is very rare, neural networks are pretty much useless. And even when the software is "learning", sometimes neural networks aren't the best way to go.
While I admit, I tinker with Neural Networks because of my hopes in creating high level AI, however, you can look at a Neural Network as being more than just just an artificial representation of a human brain, but as a Mathematical construct.
For example Let's say you have a function y = f(x) or more abstractly y = f(x1, x2, ..., xn-1, xn), Neural networks themselves act as functions, or even a set of functions, taking in a large input and producing some output [y1, y2, ..., yn-1, yn] = f(x1, x2, ..., xn-1, xn)
Furthermore, they are not static, but instead can continue adapting and learning and eventually extrapolate(predict) interesting things. Their abstractness can even result in them coming up with unique solutions to problems that haven't haven't been thought up yet. For example the TDGammon program learned to play backgammon and beat the world champion. The world champion stated that the program play a unique end game that he had never seen. (that's pretty awesome if you ask me considering the complexity of NNs)
And then when you look at recurrent neural networks (i.e. can have internal feedback loops, or pipe their output back into their input, while consuming new input) they can solve even more interesting problems, and map even more complex functions.
In a nutshell Neural Networks are like a very very abstract high dimensional function and capable of mapping/learning very interesting things that would be otherwise impossible to program programmatically. For example, the energy needed to calculate the total net Forces of Gravity on a large number of objects is intense (you have to calculate it for each object, and against each object), but once a neural network learns how to map it they can do these complex calculations that would run in exponential or combinatoric? time in polynomial time. Just look at how fast your brain processes physics data, spatial data/ images / sound when you dream. That's the potential computation power of Neural Networks. And to also mention the way they store data is very clever as well (in synaptics patterns, i.e. memories)
Artificial intelligence is a branch of computer science devoted to making computers more 'biologic.' This is useful when you want a computer to do human(biologic) things like play chess, or imitate casual conversation.
Human brains are much more efficient and powerful in some ways than the most powerful computers, so it makes sense to try to imitate a biological way of processing information.
Most neural networks I'm aware of are nothing more than flexible interpolators. Backpropagating of errors is easy and fast, here are some possible uses :
Classification of data
Some games (modern backgammon AIs beat the best players in the world, the evaluation function is a neural net)
Pattern recognition (OCR ?)
There is nothing particularly related to human intelligence. There are other uses of neural nets, I have seen an implementation of associative memory which allowed for degradation without (much) data loss, pretty much like the brain which sees some neurons die with time.