![]() ![]() Now the first input symbol ‘ a‘ matches the first leaf node of the tree. The top-down parser will parse the input string ‘abd’ and will start creating the parse tree with the starting symbol ‘ S‘. ![]() The next step is to match the terminals in the production with the terminals of the input string.Ĭonsider the input string provided by the lexical analyzer is ‘abd’ for the following grammar.Now at each step, the parser has to identify which production rule of a non-terminal must be applied in order to derive the input string.To derive the input string, first, a production in grammar with a start symbol is applied.Here the leaf node presents the terminals that match the terminals of the input string. In the top-down approach construction of the parse tree starts from the root node and end up creating the leaf nodes. ![]() Then using leftmost derivation it derives a string that matches the input string. The top-down parser parses the input string and then generates the parse tree for it.Ĭonstruction of the parse tree starts from the root node i.e. Top-down parsing is a method of parsing the input string provided by the lexical analyzer. In this section, we will be learning more about top-down parsing, its key problem, its types and its advantages and disadvantages. Remember the top-down parsing cannot handle the grammar with left recursion and ambiguity.
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