CSE 12 – Basic Data Structures Cynthia Bailey Lee Some slides and figures adapted from Paul Kube’s CSE 12 CS2 in Java Peer Instruction Materials by Cynthia Lee is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Based on a work at Permissions beyond the scope of this license may be available at LeeCreative Commons Attribution-NonCommercial 4.0 International Licensehttp://peerinstruction4cs.org

Today’s Topics 1. ASTs 2

READING QUIZ!  A node in the parse tree corresponds to what? A. one symbol in the BNF grammer B. a transition from one FFT to the next C. the index of the symbol in a string

READING QUIZ!  A parse tree where non-essential nodes are not included is called what? A. An Abstract Syntax Tree B. A Generic Parse Tree C. A Universal Interpreter D. All of the above

READING QUIZ!  A map ADT is useful when you want store, retrieve and manipulate data based on keys associated with pieces of data. In relation to parsing, one application is... A. a distance table B. a difference table C. a symbol table D. a sequence table

Compilers and interpreters

Compilers and Interpreters  One of the most important kinds of programs are programs that read other programs and translate them into action  Compilers and interpreters  They both have two main steps in their processing  Syntax analyzer  Semantic evaluator

Compilers and Interpreters  Syntax analyzer:  Recursively builds up an AST  Semantic evaluator:  Takes an AST and does the calculation by traversing the tree

Syntax Analyzer Building Abstract Syntax Trees

Syntax Analyzer: parse() method  static Assmt parse(java.lang.String s)  So we take a string and need to produce a tree node  (the example above produces Assmt, but could be other kinds of nodes)

Our sample grammar := | := = := + | - | := * | / | := | | ( ) := w | x | y | z := 0 | 1 | 2 | 3 | 4

Sample derivation “2” => => => => => 2 := | := = := + | - | := * | / | := | | ( ) := w | x | y | z := 0 | 1 | 2 | 3 | 4

You turn! “2 + w + 3” Which of the following steps can NOT follow immediately after the other in a derivation? A. => + B => C => D => 2 + w + 3 E. Other/none/more := | := = := + | - | := * | / | := | | ( ) := w | x | y | z := 0 | 1 | 2 | 3 | 4

Solution “2 + w + 3” => => + => + => + => + 3 => => => => => => 2 + w + 3 := | := = := + | - | := * | / | := | | ( ) := w | x | y | z := 0 | 1 | 2 | 3 | 4

Your turn! “2 + w * 3” := | := = := + | - | := * | / | := | | ( ) := w | x | y | z := 0 | 1 | 2 | 3 | 4

So we can do parse() on paper…  …how do we do it in code??  static Assmt parse(java.lang.String s)  So we take a string and need to produce a tree node  (the example above produces Assmt, but could be other kinds of nodes)

Semantic Evaluator Traversing Abstract Syntax Trees

Semantic Evaluator  This is really just a traversal of the tree  We have been talking about the “visit” operation as being something like “print the this node’s data variable”  But here it will be to evaluate the expression

Which kind of traversal? (and WHY?) A. Pre-order: 1. “visit” self 2. Traverse left 3. Traverse right B. In-order: 1. Traverse left 2. “visit” self 3. Traverse right C. Post-order: 1. Traverse left 2. Traverse right 3. “visit” self

Ok so we can do eval() on paper…  …how do we do it in code??

Eval of variables