In this post, we will understand the differences between top down and bottom-up parsing −
Top-down parsing and bottom-up parsing are different techniques to parse a tree to reach the beginning of the tree. This can be done in two different methods, which have been discussed below.
Top-down Parsing
- It is a parsing technique that looks at the highest level of the parse tree initially, and then works its way down to the parse tree.
- It does this using the rules of grammar.
- It uses left most derivation.
- This parsing method tries to determine the left most derivations for an input string.
- Here, the parsing is done from top-most part to the leaf node of the parse tree, i.e in top-down manner.
- The main decision of this parsing technique is to select the production rule that needs to be used to construct the string.
Bottom-up Parsing
- It is a parsing technique that is just the opposite of top-down parsing, i.e it looks at the lowest level of the parse tree and then works its way up to the parse tree.
- It does this using the rules of grammar.
- It uses right most derivation technique.
- It tries to determine the right most derivations for an input strong.
- It tries to reduce the input string to the start symbol of the grammar.
- This is done by using a production rule that reduces the string to a starting symbol.