Top down parsing attempts to find the left most derivation for a given string. Bottom up parsing attempts to reduce the input string to first symbol of the grammer. Top down parsing uses leftmost derivation. Bottom up parsing uses the rightmost derivation.
If you grammar is simple enough, it makes sense to write a top-down parser. If it’s complex (such as grammars of most programming languages), then you might have to use a bottom-up parser to successfully accept the input.
Each approach can be quite simple—the top-down approach goes from the general to the specific, and the bottom-up approach begins at the specific and moves to the general. These methods are possible approaches for a wide range of endeavors, such as goal setting, budgeting, and forecasting.
Which parsing technique is more efficient *?
LR Parser. The LR parser is a non-recursive, shift-reduce, bottom-up parser. It uses a wide class of context-free grammar which makes it the most efficient syntax analysis technique.
Which is the efficient top down parser?
Some of the parsers that use top-down parsing include: Definite clause grammar parsers. Recursive descent parser. Predictive parser.
Bottom-up (or shift-reduce) parsers work by “shifting” symbols onto a stack until the top of the stack contains a right-hand side of a production. … This process continues until the string has been “reduced” to the start symbol of the grammar.
BOTTOM-UP PARSING constructs a parse tree for an input string beginning at the leaves and working up towards the root. To do so, bottom-up parsing tries to find a rightmost derivation of a given string backwards.
Bottom Up Parsing has the following advantages over top-down parsing. Attribute computation is easy. Since choices are made only at the end of a rule, shared prefixes are unproblematic. Because of this, there is usually no need to modify grammar rules.