The tasks of language translate or processors are primarily not generation but the recognition of sentences and sentence structure. This implies that the generating steps which lead to a sentence must be reconstructed upon reading the sentence and that to a sentence must be retraced. This is generally a very complicated and sometimes even impossible task.
Its complexity intimately depends on the kind of production rules used to define the language. It is the task of theory of syntax analysis to develop recognizing algorithms for languages with rather complicated structural rules. Here, however, our goal is to outline a method for constructing recognizers that are sufficiently simple and efficient to serve in practice.
This implies nothing less than that the computational effort to analyze a sentence must be a linear function of the length of the sentence; in the very worst case the dependency function may be n.log n, where n is the sentence length. Clearly, we cannot be bothered with the problem of finding a recognition algorithm for any given language, but we will work pragmatically in the reverse direction: define an efficient algorithm and then determine the class of languages that can be treated by it.
A first consequence of the basic efficiency requirement is that the choice of every analysis step must depend only on the present state of computation and on a single next symbol being read. Another most important requirement is that no step will have to be revoked later on. These two requirement s are commonly known under the technical term one-symbol look ahead without backtracking.
The basic method to be explained here is called top -down parsing because it consists of trying to reconstruct the generating steps (which in general form a structural tree) from their start symbol to the final sentence, from the top down. Let us start by revisiting Example 1. We are given the sentence, Dogs eat,and we must determine whether or not it belongs to the language.
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This, by definition, is only the case if it can be generated from the start symbol . From the grammatical rules it is evident that it can only be a sentence if it is a subject followed by a predicate. We now divide the remaining task; first, we determine whether or not some initial part of the sentence may be generated from the symbol . This is indeed so since dogs can be directly generated .The symbol dogs is checked off in the input sentence.
