Discussion #7 1/13

Discussion #7

Recursive Descent Parsing

Discussion #7 2/13

Topics• Recursive Descent Parsing

– Use tables to build recursive descent parsers– Parse tree construction

• Project #2 Parsing

Discussion #7 3/13

Recursive Descent Parsing• Consider an arbitrary production

S xAySz

and assume x is the current (top symbol), i.e.:

S

x

(xAySz, …)

Discussion #7 4/13

Recursive Descent Parsing (continued…)

• Make a method for S (indeed, for every non-terminal) as follows:– For S xAySz

• Attempt to read an x from the input.

• If success, call method A.

• If success, attempt to read a y from the input.

• If success call method S.

• If success attempt to read a z from the input.

• If success, method S reports success!

– If any of the above attempts fail, report failure.

Discussion #7 5/13

Recursive Descent Parsing (continued…)

• Make a method for S (indeed, for every non-terminal) as follows:– For S xAySz | q

• If x is the current input character then– call method A.

– If success, attempt to read a y from the input.

– If success call method S.

– If success attempt to read a z from the input.

– If success, method S reports success!

• Else if q is the current input character then, report success

• Else report error

Discussion #7 6/13

Recursive Descent Parsing (continued…)

• Output from each syntactical class is a left-child, right-sibling parse tree.

• E OEE produces:

E

O E E

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Recursive Descent Parsing Example

• Consider our prefix grammar:

E N | OEEO + | | * | /N 0 | 1 | … | 9

• Design a series of recursive methods:

E() to process N or O, E, E

O() to process +, , *, /

N() to process numbers 0 thru 9

Discussion #7 8/13

Data Structure for Parse Tree

class parseTree{… char value parseTree leftChild parseTree rightSibling…};

Discussion #7 9/13

Initialization: Call to Start Symbol

parseTree buildTree() // build parse tree{ … nextChar = readChar() // read 1st character ptree = E() // start syntactic class if (ptree == error) return error if (nextChar) return error // check for string finished return ptree // return the full parse tree}

parseTree E() // syntactic category E{ parseTree ptree,sibling1,sibling2 if (isCharInString(nextChar, "+-*/")) // E -> OEE {FIRST(OEE)} { ptree = O() // Try to recognize O if (ptree == error) return error sibling1 = E() // Try to recognize E if (sibling1 == error) return error sibling2 = E() // Try to recognize E if (sibling2 == error) return error ptree.rightSibling = sibling1 // Success, link O->E->E ptree.rightSibling.rightSibling = sibling2 } else if (isCharInString(nextChar, "0123456789")) // E -> N { ptree = N() // Try to recognize N if (ptree == error) return error } else return error

return new parseTree('E', ptree, null)} Discussion #7 10/13

Method for E

Discussion #7 11/13

Methods for N and OparseTree N() // syntactic category N{ ptree = new parseTree(nextChar, null, null) nextChar = readChar() return new parseTree('N', ptree, null)}

parseTree O() // syntactic category O{ ptree = new parseTree(nextChar, null, null) nextChar = readChar() return new parseTree('O', ptree, null)}

Discussion #7 12/13

Recursive Descent Parser Execution for +*-748/92

E

O

+

E

O

*

E

O

-

E

N

7

E

N

4

E

N

8

E

O

/

E

N

9

E

N

2

+ * - 7 4 8 / 9 2

E

O E E

* N N

2 3

Discussion #7 13/13

ConstructingRecursive Descent Parsers

• Can be applied to almost all grammars.• Required property: By looking at the look-ahead

symbol, we know which production to process next.• Grammars with this single-symbol look-ahead property

are called single-symbol look-ahead grammars.• Alternatives:

– Backtracking– LR(k) k symbol look ahead– Make grammar have single-symbol look ahead.