Recursive Descent Parsing Lesson 16 Compiler Design Learning Monkey

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Recursive Descent Parsing • In this class, We discuss Recursive Descent Parsing. • The reader should have prior knowledge of syntax analysis basics. Click Here. • The recursive descent parsing is a top-down method. • The recursive descent parsing uses backtracking. • First, We will take a simple example and understand recursive descent parsing. • The concept is extended to the example CFG used previously to identify statements and expressions. • The below example is simple context-free grammar. • S – aAb • A – ab | a • In our example, S and A are non-terminals. • In recursive descent parsing, we write a function for each non-terminal. • The below example shows the function for non-terminal S. • We write a loop to choose each production in the non-terminal. • In the loop, we write a condition to check the productions. • The second loop will take the symbols in the production one by one and check the conditions. • We call the function related to non-terminal if the symbol is non-terminal. • Suppose the symbol is terminal check the match for the input symbol. If input matches, move one step on input. • We stop checking the current production if the input is not matched. We take the next production and continue. • If all the productions fail, we stop and return an error message. • Here we are going back and taking the next production is called backtracking. • Understanding Back Tracking with an example: • The below example shows the backtracking example. • The input string is chosen, cad. • The first tree checks for the input symbol ‘c’ and its matches. • The second tree is expanding with the production S – ab. • The production S – ab is not matching the input string. • So backtrack and take the next production. • After backtracking, the production S – a is matching to input cad. • Example Expression Grammar. • The below example shows the CFG to identify expressions. • We write functions for each non-terminal. • Problem with Recursive Descent Parsing: • In our previous class, we wrote CFG to find four different pascal language statements. • Similarly, Think complexity to writing CFG for identifying all the statements symbols in the C programming language. • How many functions should be written? • Recursively calling many functions lead to space constraint. • Link for playlists: • / @learningmonkey • • Link for our website: https://learningmonkey.in • Follow us on Facebook @ / learningmonkey • Follow us on Instagram @ / learningmonkey1 • Follow us on Twitter @ / _learningmonkey • Mail us @ [email protected]

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