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DAG IN COMPILER DESIGN EXAMPLES CODE

If an operand has operators on both sides, the side on which the operator takes this operand is decided by the associativity of those operators. No method can detect and remove ambiguity automatically, but it can be removed by either re-writing the whole grammar without ambiguity, or by setting and following associativity and precedence constraints. Ambiguity in grammar is not good for a compiler construction. The language generated by an ambiguous grammar is said to be inherently ambiguous. AmbiguityĪ grammar G is said to be ambiguous if it has more than one parse tree (left or right derivation) for at least one string.įor the string id + id – id, the above grammar generates two parse trees: The deepest sub-tree is traversed first, therefore the operator in that sub-tree gets precedence over the operator which is in the parent nodes.

In-order traversal gives original input string.Ī parse tree depicts associativity and precedence of operators.

We take the left-most derivation of a + b * c Let us see this by an example from the last topic. The start symbol of the derivation becomes the root of the parse tree. It is convenient to see how strings are derived from the start symbol. Notice that the left-most side non-terminal is always processed first.Ī parse tree is a graphical depiction of a derivation. The sentential form derived from the right-most derivation is called the right-sentential form. If we scan and replace the input with production rules, from right to left, it is known as right-most derivation. The sentential form derived by the left-most derivation is called the left-sentential form. If the sentential form of an input is scanned and replaced from left to right, it is called left-most derivation. To decide which non-terminal to be replaced with production rule, we can have two options.

Deciding the production rule, by which, the non-terminal will be replaced.

Deciding the non-terminal which is to be replaced.

During parsing, we take two decisions for some sentential form of input: DerivationĪ derivation is basically a sequence of production rules, in order to get the input string. Parsers use error recovering strategies, which we will learn later in this chapter.

DAG IN COMPILER DESIGN EXAMPLES CODE

Parsers are expected to parse the whole code even if some errors exist in the program. This way, the parser accomplishes two tasks, i.e., parsing the code, looking for errors and generating a parse tree as the output of the phase. The output of this phase is a parse tree. The parser analyzes the source code (token stream) against the production rules to detect any errors in the code. Syntax AnalyzersĪ syntax analyzer or parser takes the input from a lexical analyzer in the form of token streams. We take the problem of palindrome language, which cannot be described by means of Regular Expression. The strings are derived from the start symbol by repeatedly replacing a non-terminal (initially the start symbol) by the right side of a production, for that non-terminal. One of the non-terminals is designated as the start symbol (S) from where the production begins. Each production consists of a non-terminal called the left side of the production, an arrow, and a sequence of tokens and/or on- terminals, called the right side of the production. The productions of a grammar specify the manner in which the terminals and non-terminals can be combined to form strings.

Terminals are the basic symbols from which strings are formed.Ī set of productions (P). The non-terminals define sets of strings that help define the language generated by the grammar.Ī set of tokens, known as terminal symbols (Σ). Non-terminals are syntactic variables that denote sets of strings. In this section, we will first see the definition of context-free grammar and introduce terminologies used in parsing technology.Ī context-free grammar has four components:Ī set of non-terminals (V). CFG is a helpful tool in describing the syntax of programming languages. It implies that every Regular Grammar is also context-free, but there exists some problems, which are beyond the scope of Regular Grammar. Therefore, this phase uses context-free grammar (CFG), which is recognized by push-down automata.ĬFG, on the other hand, is a superset of Regular Grammar, as depicted below: Regular expressions cannot check balancing tokens, such as parenthesis. But a lexical analyzer cannot check the syntax of a given sentence due to the limitations of the regular expressions. We have seen that a lexical analyzer can identify tokens with the help of regular expressions and pattern rules. In this chapter, we shall learn the basic concepts used in the construction of a parser. Syntax analysis or parsing is the second phase of a compiler.