jmlr jmlr2005 jmlr2005-8 knowledge-graph by maker-knowledge-mining

8 jmlr-2005-Active Coevolutionary Learning of Deterministic Finite Automata


Source: pdf

Author: Josh Bongard, Hod Lipson

Abstract: This paper describes an active learning approach to the problem of grammatical inference, specifically the inference of deterministic finite automata (DFAs). We refer to the algorithm as the estimation-exploration algorithm (EEA). This approach differs from previous passive and active learning approaches to grammatical inference in that training data is actively proposed by the algorithm, rather than passively receiving training data from some external teacher. Here we show that this algorithm outperforms one version of the most powerful set of algorithms for grammatical inference, evidence driven state merging (EDSM), on randomly-generated DFAs. The performance increase is due to the fact that the EDSM algorithm only works well for DFAs with specific balances (percentage of positive labelings), while the EEA is more consistent over a wider range of balances. Based on this finding we propose a more general method for generating DFAs to be used in the development of future grammatical inference algorithms. Keywords: grammatical inference, evolutionary computation, deterministic finite automata, active learning, system identification

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 This approach differs from previous passive and active learning approaches to grammatical inference in that training data is actively proposed by the algorithm, rather than passively receiving training data from some external teacher. [sent-7, score-0.671]

2 Based on this finding we propose a more general method for generating DFAs to be used in the development of future grammatical inference algorithms. [sent-10, score-0.289]

3 Keywords: grammatical inference, evolutionary computation, deterministic finite automata, active learning, system identification 1. [sent-11, score-0.461]

4 In one class of grammatical inference methods, the system is considered to be some kind of language or classifier, and models are represented as deterministic finite automata (DFA). [sent-15, score-0.476]

5 Both the target system and models take strings of symbols as input (sentences), and produce binary classification as output (labellings), indicating whether that sentence belongs to the language or not. [sent-16, score-0.421]

6 The problem of grammatical inference can also be considered a special instance of the problem of system identification (Ljung, 1999), in which some target system is inferred based solely on input/output data. [sent-17, score-0.419]

7 Grammatical inference methods that employ DFAs as models can be divided into two broad classes: passive and active learning methods. [sent-18, score-0.36]

8 In these iterative active approaches the amount of training data available for inference grows over time, unlike passive approaches, in which a fixed set of training data is used for model construction. [sent-23, score-0.433]

9 However, for real-world usage of grammatical inference algorithms, it is unreasonable to assume that the internal structure of the DFA is known: indeed, this is exactly what is being inferred. [sent-34, score-0.289]

10 In this work we present an active learning algorithm that makes few assumptions about the structure of the target DFA, and in fact outperforms one of the best heuristic methods for grammatical inference, which implicitly assumes that the DFAs are balanced (i. [sent-35, score-0.474]

11 However, like the other passive methods, both heuristic and evolutionary approaches so far assume that some external agent generates either a random or balanced training set2 before inference begins. [sent-41, score-0.443]

12 Despite this active approach to training data generation, these algorithms also require an external agent—an oracle—that can answer equivalence queries: the oracle indicates whether the current model is equivalent to the target DFA and, if it is not, returns 1. [sent-44, score-0.295]

13 1652 ACTIVE C OEVOLUTIONARY L EARNING OF D ETERMINISTIC F INITE AUTOMATA new training data that belongs to the target language but does not belong to the language encoded by the candidate model. [sent-49, score-0.318]

14 Other active learning approaches to language inference also exist, but they all assume completely passive reception of training data: Sempere and Garcia (1993) only require that samples be presented in lexicographic order, and the RPNI (Oncina and Garci´ , 1992, and Lang et al. [sent-54, score-0.4]

15 For example, sentences should be sent to a target system that, during labelling, cause transitions to states that have never or rarely been visited during previous labellings. [sent-57, score-0.347]

16 For this and other reasons, it is not surprising that active learning approaches outperform passive methods: active methods have more control over the collection of training data. [sent-60, score-0.415]

17 However the point of this paper is to demonstrate one reason why active methods outperform passive methods: namely, that they perform well on both balanced and imbalanced DFAs. [sent-61, score-0.314]

18 In such cases, generating random training data is not recommended, because few or no sentences that elucidate the pathways to accepting states will be collected. [sent-66, score-0.291]

19 Secondly, generating balanced training data requires many labellings by the target system until a sufficient number of the minority labellings are collected. [sent-69, score-0.589]

20 024 = 4167 labellings of randomly generated sentences would have to be performed. [sent-71, score-0.303]

21 For the case of imbalanced DFAs, we attribute this performance improvement to the discovery of sufficient minority class training data to produce accurate models: randomly-generated training data contains too little minority class training data. [sent-74, score-0.363]

22 In the next section we briefly describe grammatical inference, as well as describing our method for the inference of target DFAs using active training data generation. [sent-78, score-0.553]

23 We also document an evolutionary and a heuristics-based method for performing grammatical inference using pre-selected training data. [sent-79, score-0.449]

24 Methods In this section we introduce grammatical inference, and outline three methods for approaching the problem: evidence-driven state merging, evolutionary approaches, and the estimation-exploration algorithm. [sent-83, score-0.353]

25 Then, given some sentence made up of a string of symbols taken from the alphabet Σ, and beginning at the start state s, the first symbol is extracted from the sentence, and based on that symbol the sentence transitions to a new state as indicated by T . [sent-86, score-0.377]

26 If the last state visited is a member of F, then the sentence receives a positive classification (the sentence belongs to the language); otherwise, a negative classification is assigned (the sentence does not belong to the language). [sent-90, score-0.511]

27 The quality of a grammatical inference algorithm is viewed as one that can produce some T and F (together referred to as a candidate DFA) that matches the labels of a pool of sentences that have already been labelled by the target DFA (the training set accuracy). [sent-91, score-0.758]

28 The candidate DFA 1654 ACTIVE C OEVOLUTIONARY L EARNING OF D ETERMINISTIC F INITE AUTOMATA should then produce a high classification accuracy when supplied with a different set of unlabelled sentences (test set accuracy). [sent-92, score-0.353]

29 3 Evolutionary Approaches to Grammatical Inference Evolutionary approaches to grammatical inference have also been proposed (Brave, 1996; Luke et al. [sent-120, score-0.289]

30 Generally, an evolutionary algorithm comprises a population of candidate models of the target DFA that compete against each other, and the fitness of a particular model is given by the percentage of training data that it can correctly classify. [sent-122, score-0.466]

31 1 E VOLVING DFA S WITH A F IXED N UMBER OF S TATES Lucas and Reynolds (2005) proposed an evolutionary approach to grammatical inference in which the number of states in candidate models is fixed at 5n/4, where n is believed to be the number of states in the target DFA. [sent-128, score-0.731]

32 Initialization • Create an initial population of candidate models (random, blank, or seeded with prior information) • Create an initial population of candidate tests (random, or seeded with prior information) 3. [sent-132, score-0.348]

33 Exploration Phase • Evolve candidate tests (input sets) • Fitness of a test is the disagreement it causes among good candidate models • Carry out best test on target system, add input/output data to training set 5. [sent-134, score-0.488]

34 • If no model is found, the search space may be inappropriate, or the target system may be inconsistent • If no good test is found, then either: – all good candidate models are perfect; – the search method for finding good tests is failing; or – the target system may be partially unobservable 6. [sent-136, score-0.414]

35 For each state in T , compute the ratio of positive and negative training sentences that terminate at that state: if more positive sentences terminate there, then consider that state an accepting state; otherwise, consider it a rejecting state. [sent-142, score-0.483]

36 4 The Estimation-Exploration Algorithm Both the heuristic and evolutionary method described above assume passive inference: a set of labelled data is presented to the algorithm, and the algorithm produces a candidate model of the target DFA. [sent-150, score-0.506]

37 This new sentence is then supplied to the target system, and the estimation phase then begins again with i + 1 training data points. [sent-154, score-0.43]

38 One population is of candidate models of the target system, where a model’s fitness is determined by its ability to correctly explain observed data from the target system. [sent-160, score-0.391]

39 1, and the specific application to grammatical inference is given below. [sent-169, score-0.289]

40 1, we compare our results against those of Lucas and Reynolds (2005), in which the number of states in a candidate model was fixed to be 5n : we set the maximum number 4 of states in a candidate model to be 2n in order to make less assumptions about the size of the target DFA. [sent-177, score-0.44]

41 Due to the difficulties of devising a similarity metric between the target DFA and a given candidate model, we have chosen to denote similarity between a model and target DFA as the test set accuracy of the candidate model. [sent-184, score-0.485]

42 If it were possible to define a similarity metric between the target and a model DFA, it would be possible to quantitatively determine how well an inference algorithm was doing by periodically measuring the similarity of candidate models against a target DFA. [sent-185, score-0.44]

43 In the exploration phase, for the grammatical inference problem a training item is considered to be an unlabelled binary sentence s . [sent-187, score-0.556]

44 Each sentence is represented as a binary vector of length smax , where smax is the maximum sentence length to be found in the training or test set. [sent-188, score-0.427]

45 2 I NITIALIZATION The algorithm begins inference by generating an unlabelled sentence at random, which is then labelled by the target DFA. [sent-196, score-0.403]

46 In order to generate a pool of competing candidate models, the population of models in the estimation phase is partitioned into two equally-sized, reproductively isolated sub-populations: no candidate model can place offspring into the other sub-population. [sent-199, score-0.434]

47 Starting with an initial population of p candidate models (with p/2 models in each of the two sub-populations), the population is evaluated, fit models are selected, copied and mutated, and less fit models are deleted. [sent-209, score-0.365]

48 The fitness of a candidate model is given by ∑ij=1 |t j − m j | , (1) i where t j is the labelling provided for the jth sentence by the target DFA, and m j is the labelling provided for the same sentence by the model DFA. [sent-212, score-0.786]

49 Then a candidate model that obtains fT = 1 perfectly labels all of the i training sentences seen so far, and models with lower values of fT have lower accuracies. [sent-213, score-0.366]

50 4 E XPLORATION P HASE The exploration phase maintains a population of the same size as that of the estimation phase (p), and evolves candidate sentences for the same number of generations (g). [sent-229, score-0.554]

51 At the end of each generation, 3p pairs of sentences are selected, copied and mutated as described in the previous section: the 4 sentence with higher fitness is copied over the sentence with lower fitness, and the copied sentence is then mutated. [sent-230, score-0.713]

52 1659 B ONGARD AND L IPSON The fitness for a given sentence is set to the amount of disagreement that that sentence causes when labelled by a pool of k candidate models: ∑k c j j=1 |, (2) k where c j is the classification of the candidate sentence by model j. [sent-231, score-0.778]

53 Sentences that do not differentiate between the candidate models—all models produce the same classification—obtain fs = 0 (poorest quality); sentences that produce the maximum classification variance obtain fs = 1 (best quality). [sent-232, score-0.353]

54 In the experiments reported here, however, the number of iterations was set to use exactly the same number of sentence labellings as the benchmark algorithms we compare it to require. [sent-247, score-0.303]

55 This results in the labelling of t sentences by the target DFA, t passes through the estimation phase, and t − 1 passes through the exploration phase (the first sentence proposed for labelling is a random sentence). [sent-249, score-0.772]

56 Although the passive variant of the EEA proposes sentences to the target DFA for labeling, it is considered passive because it does not actively construct training data; rather, it outputs random training data. [sent-260, score-0.669]

57 This stresses the importance of actively seeking informative sentences for target labeling. [sent-261, score-0.305]

58 (1998), the target DFAs were generated by creating random digraphs with 5n/4, where n is the desired number of active states: an active state is one that is visited by at least one test or training sentence during labelling. [sent-263, score-0.602]

59 In order to fairly compare our active learning method against passive methods, we have elected to equalize the number of labellings performed by the target DFA (i. [sent-274, score-0.499]

60 the number of training sentences that are labelled), and the number of labellings performed by candidate models during inference. [sent-276, score-0.499]

61 In Lucas and Reynolds (2005), the total number of candidate model labellings m is equal to the number of training sentences times the number of mutations considered during hill climbing: m = dStotal × 106 = t × 106 , where d is training set density and t is the number of training sentences. [sent-278, score-0.594]

62 k indicates the number of candidate models output by the estimation phase,3 so pgk(t − 1) indicates how many labellings are performed during the t − 1 passes through the exploration phase. [sent-280, score-0.359]

63 The second term indicates how many labellings are performed during the t passes through the estimation phase: during the first pass there is only one labelling per candidate model; during the second pass there are two labellings per model; and so on. [sent-281, score-0.623]

64 The passive variant disables the exploration phase of the algorithm, so that at each cycle through the algorithm, a random sentence is output to the target DFA for labelling. [sent-288, score-0.537]

65 At the end of each pass through the estimation phase for each variant, the test set accuracy of the best candidate model output by the first sub-population is computed. [sent-293, score-0.296]

66 As can be seen in Figure 1a, the active variant outputs a model consistent with all training and test data after the 93rd pass through the estimation phase. [sent-295, score-0.287]

67 Second, it is noted that the size of the best candidate model increases and decreases over the inference process in the active variant of the EEA. [sent-299, score-0.357]

68 2 C OMPARATIVE P ERFORMANCE A MONG I NFERENCE A LGORITHMS Each of four algorithms—EDSM, Lucas’ method, and the active and passive variants of the EEA— were run against 3720 target DFAs: 1200 with n = 4, 1200 with n = 8, 1200 with n = 16, and 120 with n = 32 states. [sent-317, score-0.351]

69 The total number of training sentences available for inference for each DFA size and training set density is shown in Table 3. [sent-320, score-0.344]

70 Figure 3 reports the percentage of runs of both the passive and active EEA variants that produced a model that correctly classifies all training and test data. [sent-450, score-0.331]

71 6; the minority of DFAs produce less than 40% labellings of their minority classification. [sent-624, score-0.352]

72 First, the evolutionary approach of Lucas and Reynolds (2005) tends to outperform the EDSM variant for smaller target DFAS (n < 32), but the EDSM far outperforms Lucas’ algorithm for larger target DFAs (n = 32). [sent-662, score-0.347]

73 Second, the active EEA variant outperforms all three other algorithms for the smallest size of target DFA (n = 4); is competitive with Lucas’ algorithm for DFAs with n = 8 and n = 16; and is competitive with EDSM on the largest target DFAs (n = 32). [sent-663, score-0.351]

74 Because the passive variant performs the same or less model evaluations than Lucas’ algorithm, and it randomly selects the same number of training sentences, we can conclude that our particular method of evolutionary search is inferior to that proposed by Lucas and Reynolds (2005). [sent-665, score-0.341]

75 For DFAs with n = 32 states, it can be seen that the active EEA generates training strings that achieve a more balanced labelling (lower righthand panel) than the passive EEA variant which outputs random strings for labelling (middle righthand panel). [sent-904, score-0.732]

76 At the outset of inference using the active EEA, training strings are generated at random, because sufficiently accurate models do not yet exist. [sent-906, score-0.338]

77 As inference proceeds, a few training strings are output to the target system that obtain a minority labelling. [sent-907, score-0.391]

78 This indicates that training strings with high fitness at least elicit a minority labelling from some of the candidate models, and since the models are now somewhat accurate, this increases the probability of obtaining a new minority labelling from the target system. [sent-910, score-0.786]

79 As inference proceeds, minority labellings are extracted with increasing probability, allowing for the better inference of imbalanced DFAs. [sent-911, score-0.409]

80 This advantage of active training data generation, compared to passive training data collection, is also supported by the results reported in Figure 3. [sent-912, score-0.343]

81 Clearly, the active EEA discovers models consistent with all training and test data more often than the passive EEA. [sent-913, score-0.35]

82 This requires rethinking how grammatical inference algorithms are compared: rather than simply providing them with training data already collected from a DFA, the algorithms should be free to request training data labelling, as is done in the active learning community (Baram et al. [sent-916, score-0.499]

83 1 Intelligent Testing The results reported here support the claim that useful experiments (in the domain of grammatical inference, binary sentences) are those that elicit informative responses from the target system. [sent-919, score-0.356]

84 Here we have stressed that one informative type of test are training data belonging to the minority class of an imbalanced DFA: automatically and actively generating such informative tests helps the algorithm to outperform other methods that rely on passively generated random training data. [sent-921, score-0.299]

85 There may be other kinds of informative sentences in grammatical inference that are unknowingly being favored by the active EEA variant. [sent-926, score-0.606]

86 It is clear that states closer4 to the start state in the transition function T will be visited more often than distant states, and longer sentences have a higher probability of reaching these distant states than shorter sentences do. [sent-928, score-0.457]

87 So, we predict that the active EEA variant will propose, on average, longer training sentences than a passive algorithm will. [sent-930, score-0.485]

88 Whether longer sentences truly are more informative than shorter ones, and whether longer sentences are actually favored by the active EEA variant has not yet been verified. [sent-931, score-0.488]

89 In grammatical inference, unbalanced DFAs are less observable than balanced DFAs: there are either less states that produce the minority labelling than states that produce the majority labelling, or minority labelling states are more distant from the start state than majority labelling states. [sent-933, score-0.992]

90 It follows then that passive grammatical inference approaches are inappropriate in these cases, for one of two reasons: either a balanced training set is assumed, in which case the minority class is grossly over-represented in the training data; or random training 4. [sent-934, score-0.692]

91 An active approach to grammatical inference, like the estimation-exploration algorithm, actively generates a training set that falls between these two extremes, and accelerates inference. [sent-937, score-0.401]

92 Also, acquiring a balanced training set will require a large number of target labellings in order to obtain enough of the minority labellings. [sent-939, score-0.426]

93 Furthermore, our method may be useful for indicating what kinds of training data is most useful for the inference of particular kinds of languages, by simply observing what kinds of sentences are generated by our method. [sent-941, score-0.327]

94 Formulating time complexities for both methods as a function of both DFA size and balance as well as allowed number of target labellings requires further investigation. [sent-964, score-0.289]

95 It was shown that the EEA performs better on DFAs with differing balances by actively extracting minority labellings from the target DFA. [sent-975, score-0.513]

96 In order to better gauge the inference ability of grammatical inference methods, we introduce a more general method for generating DFAs with specific sizes and balances. [sent-976, score-0.364]

97 Our algorithm also allows for continual expansion and compression of candidate models over time, in response to new training data: expansion allows for the accommodation of new training data, and compression usually leads to greater test set accuracy. [sent-978, score-0.295]

98 In many approaches to grammatical inference, either a balanced training set is assumed, or random training data is generated. [sent-984, score-0.348]

99 One possible approach would be to evolve test sequences that cause the candidate models to disagree most in the class probabilities they predict the target system will output for that sequence. [sent-989, score-0.305]

100 Learning deterministic finite automata with a smart state labelling evolutionary algorithm. [sent-1098, score-0.364]


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