- Questions on the Homework?
- Read a line of text: fgets (never "gets")
- Split the line into tokens.
- Parse the tokens into an abstract syntax tree
- Evaluate the AST
Draw a diagram on the board, with an example.
- We have a line of text.
- We need a list of tokens.
- Work character by character and build up current token.
- When the current character doesn't match the previous token,
push the previous token and start a new one.
- This is a state machine.
Parser and AST
- An Abstract Syntax tree encodes the expression as it will be evaluated.
- For an arithemtic expression:
- Numbers are leaves.
- Internal nodes are operators.
- Order of operations is encoded in the structure; the root note is evaluated
- In general, this can be done efficiently with a stack.
- Our code uses a less efficient method: scan the whole input for the root,
split there, and recurse on the subtrees.
Evaluating an AST
- AST can be evaluated.
- This is a simple recursive function.
Clang Check and Valgrind
- Clang is a compiler; clang-check compiles and spits out the warnings.
- Valgrind is a tool for finding memory errors.
- Warnings from either of these may or may not indicate a bug in your
program, but fixing them tends to be a good idea.