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During my Interview, I was asked to implement a state machine for a system having 100 states where each state in turn has 100 events, I answered 3 following approaches:
if-else
switch-case
function pointers
If-else is obviously not suited for such a state machine, hence main comparison was between switch-case vs function pointers, here is the comparison as per my understanding:
Speed wise both are almost same.
Switch-case is less modular than function-pointers
Function-pointers has more memory overhead.
Could someone confirm if above understanding is correct ?
There might be a variant of the function pointer approach: a struct which includes a function pointer as well as other information. So you could let one function handle several cases.
Beside of this, I think you are right. Plus, I would consider the overhead concerning memory and speed worth to be considered, but hopefully small enough to be ignored at the end.
I don't know what your interviewers wanted to hear and I hope this is not too off topic but if I were interviewing someone I would give points for knowing of the pros and cons of existing frameworks before justifying rolling your own, especially at that scale.
C++ alternatives (if you can use them, thanks to glglgl for pointing out that you seem to want C) would be:
Boost.MSM although blazingly fast is out of the question at that scale. Reasons are compile time, mpl::vector/list constraints and because you would have one gigantic source file.
Boost.Statecharts can work with 100 states but 100 events per state would max out the mpl::vector/list constraints. Personally if I had 100 events in a state I would try to group them anyway and use custom reactions but that obviously depends on the application.
I don't see any reason why Qt's state machine wouldn't scale that big (please correct me if I'm wrong) but its orders of magnitude slower so I never use it.
The only good C alternative I know of is:
QP which is available in C and C++ and can scale that big, has good organization and is "more than a state-machine" in that it handles event queues, concurrency and memory management etc. Rolling your own may yield better performance (depending on your skill and how much time you put into it) but it should be noted that the memory management of the events is probably going to end up needing more optimization than the state machine implementation it's self. QP does this for you and quite well.
You could specify more detail about your states and events.
Assume your state is continuous integer number. Then you can
Write a table to contain all states and per state handler function on it.
When receiving an event, reference this table and call corresponding handler function.
For each state, write a table that contain all events and its event handler function. Look up this table when processing event on the state.
The time complexity of these 2 table looking up is O(1), and space complexity is O(m*n)
However, how can you have FSM with 100 states and event with 100 types?
I suggest you to simplify your FSM design and the 1~100 number may be parameter of one particular event.
I'm wondering how people test artificial intelligence algorithms in an automated fashion.
One example would be for the Turing Test - say there were a number of submissions for a contest. Is there any conceivable way to score candidates in an automated fashion - other than just having humans test them out.
I've also seen some data sets (obscured images of numbers/letters, groups of photos, etc) that can be fed in and learned over time. What good resources are out there for this.
One challenge I see: you don't want an algorithm that tailors itself to the test data over time, since you are trying to see how well it does in the general case. Are there any techniques to ensure it doesn't do this? Such as giving it a random test each time, or averaging its results over a bunch of random tests.
Basically, given a bunch of algorithms, I want some automated process to feed it data and see how well it "learned" it or can predict new stuff it hasn't seen yet.
This is a complex topic - good AI algorithms are generally the ones which can generalize well to "unseen" data. The simplest method is to have two datasets: a training set and an evaluation set used for measuring the performances. But generally, you want to "tune" your algorithm so you may want 3 datasets, one for learning, one for tuning, and one for evaluation. What defines tuning depends on your algorithm, but a typical example is a model where you have a few hyper-parameters (for example parameters in your Bayesian prior under the Bayesian view of learning) that you would like to tune on a separate dataset. The learning procedure would already have set a value for it (or maybe you hardcoded their value), but having enough data may help so that you can tune them separately.
As for making those separate datasets, there are many ways to do so, for example by dividing the data you have available into subsets used for different purposes. There is a tradeoff to be made because you want as much data as possible for training, but you want enough data for evaluation too (assuming you are in the design phase of your new algorithm/product).
A standard method to do so in a systematic way from a known dataset is cross validation.
Generally when it comes to this sort of thing you have two datasets - one large "training set" which you use to build and tune the algorithm, and a separate smaller "probe set" that you use to evaluate its performance.
#Anon has the right of things - training and what I'll call validation sets. That noted, the bits and pieces I see about developments in this field point at two things:
Bayesian Classifiers: there's something like this probably filtering your email. In short you train the algorithm to make a probabilistic decision if a particular item is part of a group or not (e.g. spam and ham).
Multiple Classifiers: this is the approach that the winning group involved in the Netflix challenge took, whereby it's not about optimizing one particular algorithm (e.g. Bayesian, Genetic Programming, Neural Networks, etc..) by combining several to get a better result.
As for data sets Weka has several available. I haven't explored other libraries for data sets, but mloss.org appears to be a good resource. Finally data.gov offers a lot of sets that provide some interesting opportunities.
Training data sets and test sets are very common for K-means and other clustering algorithms, but to have something that's artificially intelligent without supervised learning (which means having a training set) you are building a "brain" so-to-speak based on:
In chess: all possible future states possible from the current gameState.
In most AI-learning (reinforcement learning) you have a problem where the "agent" is trained by doing the game over and over. Basically you ascribe a value to every state. Then you assign an expected value of each possible action at a state.
So say you have S states and a actions per state (although you might have more possible moves in one state, and not as many in another), then you want to figure out the most-valuable states from s to be in, and the most valuable actions to take.
In order to figure out the value of states and their corresponding actions, you have to iterate the game through. Probabilistically, a certain sequence of states will lead to victory or defeat, and basically you learn which states lead to failure and are "bad states". You also learn which ones are more likely to lead to victory, and these are subsequently "good" states. They each get a mathematical value associated, usually as an expected reward.
Reward from second-last state to a winning state: +10
Reward if entering a losing state: -10
So the states that give negative rewards then give negative rewards backwards, to the state that called the second-last state, and then the state that called the third-last state and so-on.
Eventually, you have a mapping of expected reward based on which state you're in, and based on which action you take. You eventually find the "optimal" sequence of steps to take. This is often referred to as an optimal policy.
It is true of the converse that normal courses of actions that you are stepping-through while deriving the optimal policy are called simply policies and you are always implementing a certain "policy" with respect to Q-Learning.
Usually the way of determining the reward is the interesting part. Suppose I reward you for each state-transition that does not lead to failure. Then the value of walking all the states until I terminated is however many increments I made, however many state transitions I had.
If certain states are extremely unvaluable, then loss is easy to avoid because almost all bad states are avoided.
However, you don't want to discourage discovery of new, potentially more-efficient paths that don't follow just this-one-works, so you want to reward and punish the agent in such a way as to ensure "victory" or "keeping the pole balanced" or whatever as long as possible, but you don't want to be stuck at local maxima and minima for efficiency if failure is too painful, so no new, unexplored routes will be tried. (Although there are many approaches in addition to this one).
So when you ask "how do you test AI algorithms" the best part is is that the testing itself is how many "algorithms" are constructed. The algorithm is designed to test a certain course-of-action (policy). It's much more complicated than
"turn left every half mile"
it's more like
"turn left every half mile if I have turned right 3 times and then turned left 2 times and had a quarter in my left pocket to pay fare... etc etc"
It's very precise.
So the testing is usually actually how the A.I. is being programmed. Most models are just probabilistic representations of what is probably good and probably bad. Calculating every possible state is easier for computers (we thought!) because they can focus on one task for very long periods of time and how much they remember is exactly how much RAM you have. However, we learn by affecting neurons in a probabilistic manner, which is why the memristor is such a great discovery -- it's just like a neuron!
You should look at Neural Networks, it's mindblowing. The first time I read about making a "brain" out of a matrix of fake-neuron synaptic connections... A brain that can "remember" basically rocked my universe.
A.I. research is mostly probabilistic because we don't know how to make "thinking" we just know how to imitate our own inner learning process of try, try again.
What are the relevant differences, in terms of performance and use cases, between simulated annealing (with bean search) and genetic algorithms?
I know that SA can be thought as GA where the population size is only one, but I don't know the key difference between the two.
Also, I am trying to think of a situation where SA will outperform GA or GA will outperform SA. Just one simple example which will help me understand will be enough.
Well strictly speaking, these two things--simulated annealing (SA) and genetic algorithms are neither algorithms nor is their purpose 'data mining'.
Both are meta-heuristics--a couple of levels above 'algorithm' on the abstraction scale. In other words, both terms refer to high-level metaphors--one borrowed from metallurgy and the other from evolutionary biology. In the meta-heuristic taxonomy, SA is a single-state method and GA is a population method (in a sub-class along with PSO, ACO, et al, usually referred to as biologically-inspired meta-heuristics).
These two meta-heuristics are used to solve optimization problems, particularly (though not exclusively) in combinatorial optimization (aka constraint-satisfaction programming). Combinatorial optimization refers to optimization by selecting from among a set of discrete items--in other words, there is no continuous function to minimize. The knapsack problem, traveling salesman problem, cutting stock problem--are all combinatorial optimization problems.
The connection to data mining is that the core of many (most?) supervised Machine Learning (ML) algorithms is the solution of an optimization problem--(Multi-Layer Perceptron and Support Vector Machines for instance).
Any solution technique to solve cap problems, regardless of the algorithm, will consist essentially of these steps (which are typically coded as a single block within a recursive loop):
encode the domain-specific details
in a cost function (it's the
step-wise minimization of the value
returned from this function that
constitutes a 'solution' to the c/o
problem);
evaluate the cost function passing
in an initial 'guess' (to begin
iteration);
based on the value returned from the
cost function, generate a subsequent
candidate solution (or more than
one, depending on the
meta-heuristic) to the cost
function;
evaluate each candidate solution by
passing it in an argument set, to
the cost function;
repeat steps (iii) and (iv) until
either some convergence criterion is
satisfied or a maximum number of
iterations is reached.
Meta-heuristics are directed to step (iii) above; hence, SA and GA differ in how they generate candidate solutions for evaluation by the cost function. In other words, that's the place to look to understand how these two meta-heuristics differ.
Informally, the essence of an algorithm directed to solution of combinatorial optimization is how it handles a candidate solution whose value returned from the cost function is worse than the current best candidate solution (the one that returns the lowest value from the cost function). The simplest way for an optimization algorithm to handle such a candidate solution is to reject it outright--that's what the hill climbing algorithm does. But by doing this, simple hill climbing will always miss a better solution separated from the current solution by a hill. Put another way, a sophisticated optimization algorithm has to include a technique for (temporarily) accepting a candidate solution worse than (i.e., uphill from) the current best solution because an even better solution than the current one might lie along a path through that worse solution.
So how do SA and GA generate candidate solutions?
The essence of SA is usually expressed in terms of the probability that a higher-cost candidate solution will be accepted (the entire expression inside the double parenthesis is an exponent:
p = e((-highCost - lowCost)/temperature)
Or in python:
p = pow(math.e, (-hiCost - loCost) / T)
The 'temperature' term is a variable whose value decays during progress of the optimization--and therefore, the probability that SA will accept a worse solution decreases as iteration number increases.
Put another way, when the algorithm begins iterating, T is very large, which as you can see, causes the algorithm to move to every newly created candidate solution, whether better or worse than the current best solution--i.e., it is doing a random walk in the solution space. As iteration number increases (i.e., as the temperature cools) the algorithm's search of the solution space becomes less permissive, until at T = 0, the behavior is identical to a simple hill-climbing algorithm (i.e., only solutions better than the current best solution are accepted).
Genetic Algorithms are very different. For one thing--and this is a big thing--it generates not a single candidate solution but an entire 'population of them'. It works like this: GA calls the cost function on each member (candidate solution) of the population. It then ranks them, from best to worse, ordered by the value returned from the cost function ('best' has the lowest value). From these ranked values (and their corresponding candidate solutions) the next population is created. New members of the population are created in essentially one of three ways. The first is usually referred to as 'elitism' and in practice usually refers to just taking the highest ranked candidate solutions and passing them straight through--unmodified--to the next generation. The other two ways that new members of the population are usually referred to as 'mutation' and 'crossover'. Mutation usually involves a change in one element in a candidate solution vector from the current population to create a solution vector in the new population, e.g., [4, 5, 1, 0, 2] => [4, 5, 2, 0, 2]. The result of the crossover operation is like what would happen if vectors could have sex--i.e., a new child vector whose elements are comprised of some from each of two parents.
So those are the algorithmic differences between GA and SA. What about the differences in performance?
In practice: (my observations are limited to combinatorial optimization problems) GA nearly always beats SA (returns a lower 'best' return value from the cost function--ie, a value close to the solution space's global minimum), but at a higher computation cost. As far as i am aware, the textbooks and technical publications recite the same conclusion on resolution.
but here's the thing: GA is inherently parallelizable; what's more, it's trivial to do so because the individual "search agents" comprising each population do not need to exchange messages--ie, they work independently of each other. Obviously that means GA computation can be distributed, which means in practice, you can get much better results (closer to the global minimum) and better performance (execution speed).
In what circumstances might SA outperform GA? The general scenario i think would be those optimization problems having a small solution space so that the result from SA and GA are practically the same, yet the execution context (e.g., hundreds of similar problems run in batch mode) favors the faster algorithm (which should always be SA).
It is really difficult to compare the two since they were inspired from different domains..
A Genetic Algorithm maintains a population of possible solutions, and at each step, selects pairs of possible solution, combines them (crossover), and applies some random changes (mutation). The algorithm is based the idea of "survival of the fittest" where the selection process is done according to a fitness criteria (usually in optimization problems it is simply the value of the objective function evaluated using the current solution). The crossover is done in hope that two good solutions, when combined, might give even better solution.
On the other hand, Simulated Annealing only tracks one solution in the space of possible solutions, and at each iteration considers whether to move to a neighboring solution or stay in the current one according to some probabilities (which decays over time). This is different from a heuristic search (say greedy search) in that it doesn't suffer from the problems of local optimum since it can get unstuck from cases where all neighboring solutions are worst the current one.
I'm far from an expert on these algorithms, but I'll try and help out.
I think the biggest difference between the two is the idea of crossover in GA and so any example of a learning task that is better suited to GA than SA is going to hinge on what crossover means in that situation and how it is implemented.
The idea of crossover is that you can meaningfully combine two solutions to produce a better one. I think this only makes sense if the solutions to a problem are structured in some way. I could imagine, for example, in multi-class classification taking two (or many) classifiers that are good at classifying a particular class and combining them by voting to make a much better classifier. Another example might be Genetic Programming, where the solution can be expressed as a tree, but I find it hard to come up with a good example where you could combine two programs to create a better one.
I think it's difficult to come up with a compelling case for one over the other because they really are quite similar algorithms, perhaps having been developed from very different starting points.
I am running a physics simulation and applying a set of movement instructions to a simulated skeleton. I have a multiple sets of instructions for the skeleton consisting of force application to legs, arms, torso etc. and duration of force applied to their respective bone. Each set of instructions (behavior) is developed by testing its effectiveness performing the desired behavior, and then modifying the behavior with a genetic algorithm with other similar behaviors, and testing it again. The skeleton will have an array behaviors in its set list.
I have fitness functions which test for stability, speed, minimization of entropy and force on joints. The problem is that any given behavior will work for a specific context. One behavior works on flat ground, another works if there is a bump in front of the right foot, another if it's in front of the left, and so on. So the fitness of each behavior varies based on the context. Picking a behavior simply on its previous fitness level won't work because that fitness score doesn't apply to this context.
My question is, how do I program to have the skeleton pick the best behavior for the context? Such as picking the best walking behavior for a randomized bumpy terrain.
In a different answer I've given to this question, I assumed that the "terrain" information you have for your model was very approximate and large-grained, e.g., "smooth and flat", "rough", "rocky", etc. and perhaps only at a grid level. However, if the world model is in fact very detailed, such as from a simulated version of a 3-D laser range scanner, then algorithmic and computational path/motion planning approaches from robotics are likely to be more useful than a machine-learning classifier system.
PATH/MOTION PLANNING METHODS
There are a fairly large number of path and motion planning methods, including some perhaps more suited to walking/locomotion, but a few of the more general ones worth mentioning are:
Visibility graphs
Potential Fields
Sampling-based methods
The general solution approach would be use a path planning method to determine the walking trajectory that your skeleton should follow to avoid obstacles, and then use your GA-based controller to achieve the appropriate motion. This is very much at the core of robotics: sense the world and determine actions and motor control required to achieve some goal(s).
Also, a quick literature search turned up the following papers and a book as a source of ideas and starting points for further investigation. The paper on legged robot motion planning may be especially useful as it discusses several motion planning strategies.
Reading Suggestions
Steven Michael LaValle (2006). Planning Algorithms, Cambridge University Press.
Kris Hauser, Timothy Bretl, Jean-Claude Latombe, Kensuke Harada, Brian Wilcox (2008). "Motion Planning for Legged Robots on Varied Terrain", The International Journal of Robotics Research, Vol. 27, No. 11-12, 1325-1349,
DOI: 10.1177/0278364908098447
Guilherme N. DeSouza and Avinash C. Kak (2002). "Vision for Mobile Robot Navigation: A Survey", IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 24, No. 2, February, pp 237-267.
Why not test the behaviors against a randomized bumpy terrain? Just set the parameters of the GA so that it's a little forgiving, and won't condemn a behavior for one or two failures.
You have two problems:
Bipedal locomotion without senses is very difficult. I've seen good robotic locomotion over rough terrain without senses, but never with only two legs. So the best solution you can possibly find this way might not be very good.
Running a GA is as much art as science. There are a lot of knobs you can turn, and it's hard to find parameters that will allow novelty to grow without drowning it in noise.
Starting simple (e.g. crawling) will help with both of these.
EDIT:
Wait... you're training it over and over on the same randomized terrain? Well no wonder you're having trouble! It's optimizing for that particular layout of rocks and bumps, which is much easier than generalizing. Depending on how your GA works, you might get some benefit from making the course really long, but a better solution is to randomize the terrain for every pass. When it can no longer exploit specific features of the terrain, it will have an evolutionary incentive to generalize. Since this is a more difficult problem it will not learn as quickly as it did before, and it might not be able to get very good at all with its current parameters; be prepared to tinker.
There are three aspects to my answer: (1) control theory, (2) sensing, and (3) merging sensing and action.
CONTROL THEORY
The answer to your problem depends partially on what kind of control scheme you are using: is it feed-forward or feedback control? If the latter, what simulated real-time sensors do you have other than terrain information?
Simply having terrain information and incorporating it into your control strategy would not mean you are using feedback control. It is possible to use such information to select a feed-forward strategy, which seems closest to the problem that you have described.
SENSING
Whether you are using feed-forward or feedback control, you need to represent the terrain information and any other sensory data as an input space for your control system. Part of training your GA-based motion controller should be moving your skeleton through a broad range of random terrain in order to learn feature detectors. The feature detectors classify the terrain scenarios by segmenting the input space into regions critical to deciding what is the best action policy, i.e., what control behavior to employ.
How to best represent the input space depends on the level of granularity of the terrain information you have for your simulation. If it's just a discrete space of terrain type and/or obstacles in some grid space, you may be able to present it directly to your GA without transformation. If, however, the data is in a continuous space such as terrain type and obstacles at arbitrary range/direction, you may need to transform it into a space from which it may be easier to infer spatial relationships, such as coarse-coded range and direction, e.g., near, mid, far and forward, left-forward, left, etc. Gaussian and fuzzy classifiers can be useful for the latter approach, but discrete-valued coding can also work.
MERGING SENSING AND ACTION
Using one of the input-space-encoding approaches above, you have a few options for how to connect behavior selection search space and motion control search space:
Separate the two spaces into two learning problems and use a separate GA to evolve the parameters of a standard multi-layer perceptron neural network. The latter would have your sensor data (perhaps transformed) as inputs and your set of skeleton behaviors as outputs. Instead of using back-propagation or some other ANN-learning method to learn the network weights, your GA could use some fitness function to evolve the parameters over a series of simulated trials, e.g., fitness = distance traveled in a fixed time period toward point B starting from point A. This should evolve over successive generations from completely random selection of behaviors to something more coordinated and useful.
Merge the two search spaces (behavior selection and skeleton motor control) by linking a multi-layer perceptron network as described in (1) above into the existing GA-based controller framework that you have, using the skeleton behavior set as the linkage. The parameter space that will be evolved will be both the neural network weights and whatever your existing controller parameter space is. Assuming that you are using a multi-objective genetic algorithm, such as the NSGA-II algorithm, (since you have multiple fitness functions), the fitness functions would be stability, speed, minimization of entropy, force on joints, etc, plus some fitness function(s) targeted at learning the behavior-selection policy, e.g., distance moved toward point B starting from point A in a fixed time period.
The difference between this approach and (1) above is that you may be able to learn both better coordination of behaviors and finer-grain motor control since the parameter space is likely to be better explored when the two problems are merged as opposed to being separate. The downside is that it may take much longer to converge on reasonable parameter solutions(s), and not all aspects of motor control may be learned as well as they would if the two learning problems were kept separate.
Given that you already have working evolved solutions for the motor control problem, you are probably better off using approach (1) to learn the behavior-selection model with a separate GA. Also, there are many alternatives to the hybrid GA-ANN scheme I described above for learning the latter model, including not learning a model at all and instead using a path planning algorithm as described in a separate answer from me. I simply offered this approach since you are already familiar with GA-based machine learning.
The action selection problem is a robust area of research in both machine learning and autonomous robotics. It's probably well-worth reading up on this topic in itself to gain better perspective and insight into your current problem, and you may be able to devise a simpler strategy than anything I've suggested so far by viewing your problem through the lens of this paradigm.
You're using a genetic algorithm to modify the behavior, so that must mean you have devised a fitness function for each combination of factors. Is that your question?
If yes, the answer depends on what metrics you use to define best walking behavior:
Maximize stability
Maximize speed
Minimize forces on joints
Minimize energy or entropy production
Or do you just try a bunch of parameters, record the values, and then let the genetic algorithm drive you to the best solution?
If each behavior works well in one context and not another, I'd try quantifying how to sense and interpolate between contexts and blend the strategies to see if that would help.
It sounds like at this point you have just a classification problem. You want to map some knowledge about what you are currently walking on to one of a set of classes. Knowing the class of the terrain allows you to then invoke the proper subroutine. Is this correct?
If so, then there are a wide array of classification engines that you can use including neural networks, Bayesian networks, decision trees, nearest neighbor, etc. In order to pick the best fit, we will need more information about your problem.
First, what kind of input or sensory data do you have available to help you identify the behavior class you should invoke? Second, can you describe the circumstances in which you will be training this classifier and what the circumstances are during runtime when you deploy it, such as any limits on computational resources or requirements of robustness to noise?
EDIT: Since you have a fixed number of classes, and you have some parameterized model for generating all possible terrains, I would consider using k-means clustering. The principle is as follows. You cluster a whole bunch of terrains into k different classes, where each cluster is associated with one of your specialized subroutines that performs best for that cluster of terrains. Then when a new terrain comes in, it will probably fall near one of these clusters. You then invoke the corresponding specialized subroutine to navigate that terrain.
Do this offline: Generate enough random terrains to sufficiently sample the parameter space, map these terrains to your sensory space (but remember which points in sensory space correspond to which terrains), and then run k-means clustering on this sensory space corpus where k is the number of classes you want to learn. Your distance function between a class representative C and a point P in sensory space would be simply the fitness function of letting algorithm C navigate the terrain that generated P. You would then get a partitioning of your sensory space into k clusters, each cluster mapping to the best subroutine that you've got. Each cluster will have a representative point in sensory space.
Now during runtime: You will get some unlabeled point in sensory space. Use a different distance function to find the closest representative point to this new incoming point. That tells you what class the terrain is.
Note that the success of this method depends on the quality of the mapping from the parameter space of terrain generation to sensory space, from sensory space to your fitness functions, and the eventual distance function you use to compare points in sensory space.
Note also that if you had enough memory, instead of only using the k representative sensory points to tell you which class an unlabeled sensory point belongs to, you might go through your training set and label all points with the learned class. Then during runtime you pick the nearest neighbor, and conclude that your unlabeled point in sensory space is in the same class as that neighbor.
In a rule system, or any reasoning system that deduces facts via forward-chaining inference rules, how would you prune "unnecessary" branches? I'm not sure what the formal terminology is, but I'm just trying to understand how people are able to limit their train-of-thought when reasoning over problems, whereas all semantic reasoners I've seen appear unable to do this.
For example, in John McCarthy's paper An Example for Natural Language Understanding and the AI Problems It Raises, he describes potential problems in getting a program to intelligently answer questions about a news article in the New York Times. In section 4, "The Need For Nonmonotonic Reasoning", he discusses the use of Occam's Razer to restrict the inclusion of facts when reasoning about the story. The sample story he uses is one about robbers who victimize a furniture store owner.
If a program were asked to form a "minimal completion" of the story in predicate calculus, it might need to include facts not directly mentioned in the original story. However, it would also need some way of knowing when to limit its chain of deduction, so as not to include irrelevant details. For example, it might want to include the exact number of police involved in the case, which the article omits, but it won't want to include the fact that each police officer has a mother.
Good Question.
From your Question i think what you refer to as 'pruning' is a model-building step performed ex ante--ie, to limit the inputs available to the algorithm to build the model. The term 'pruning' when used in Machine Learning refers to something different--an ex post step, after model construction and that operates upon the model itself and not on the available inputs. (There could be a second meaning in the ML domain, for the term 'pruning.' of, but i'm not aware of it.) In other words, pruning is indeed literally a technique to "limit its chain of deduction" as you put it, but it does so ex post, by excision of components of a complete (working) model, and not by limiting the inputs used to create that model.
On the other hand, isolating or limiting the inputs available for model construction--which is what i think you might have had in mind--is indeed a key Machine Learning theme; it's clearly a factor responsible for the superior performance of many of the more recent ML algorithms--for instance, Support Vector Machines (the insight that underlies SVM is construction of the maximum-margin hyperplane from only a small subset of the data, i.e, the 'support vectors'), and Multi-Adaptive Regression Splines (a regression technique in which no attempt is made to fit the data by "drawing a single continuous curve through it", instead, discrete section of the data are fit, one by one, using a bounded linear equation for each portion, ie., the 'splines', so the predicate step of optimal partitioning of the data is obviously the crux of this algorithm).
What problem is solving by pruning?
At least w/r/t specific ML algorithms i have actually coded and used--Decision Trees, MARS, and Neural Networks--pruning is performed on an initially over-fit model (a model that fits the training data so closely that it is unable to generalize (accurately predict new instances). In each instance, pruning involves removing marginal nodes (DT, NN) or terms in the regression equation (MARS) one by one.
Second, why is pruning necessary/desirable?
Isn't it better to just accurately set the convergence/splitting criteria? That won't always help. Pruning works from "the bottom up"; the model is constructed from the top down, so tuning the model (to achieve the same benefit as pruning) eliminates not just one or more decision nodes but also the child nodes that (like trimming a tree closer to the trunk). So eliminating a marginal node might also eliminate one or more strong nodes subordinate to that marginal node--but the modeler would never know that because his/her tuning eliminated further node creation at that marginal node. Pruning works from the other direction--from the most subordinate (lowest-level) child nodes upward in the direction of the root node.