Compiler theory essays 3 (top-down parsing - derivation method)

1, theoretical basis

• Context-free grammar
• Pushdown automata (PDA)

2, parsing mode

• Top-down analysis (-PDA on top of the stack, the lower stack bottom -PDA)
• Bottom-up analysis
  • Priority analysis
  • LR analysis

3、PDA

 

 PDA definitions:

• 

• usage:

  • When reading the input string, and the push-down stack is empty; or read the input string, and the controller reaches certain final state

  • 

     Principle Analysis:

    • Pushdown stack placed inside S (nonterminal as: S-> Sb | c), input with a specific statement as a + b #, compared to the statements in the push-down stack S, if the same, disappear, read head moved back one, continue to compare
    • When pushing the stack is empty, and the input band is empty (the read head to the end) to match the ends.
    • If the push-down stack top element has been completed backtracking all possible states, still can not match the position of the input with the read head, - "do not match, an error, the end.

4, a top down analysis of the disadvantages backtracking

1. If the grammar there is left recursive , there will be an infinite loop
2. If there are multiple production candidate type , select which derivation is completely blind.
3. Back, time & space consumption
4. If the identification statement is wrong, but can not point out exactly what is wrong.

5, the top-down analysis algorithm without backtracking

1. Eliminate left recursion (such as: P-> Pa)