russell/spec/aula/lsa

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Introduction to Bison.
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1. Get started!

a) Check the available files:
- Makefile : makefile to build the parser.
- scanner.flex : the lexical analyser (scanner) in flex
- parser.bison: the parser in bison
Note: the given scanner & grammar only deal
with numbers and the "+" operator.
- common.c, common.h : common definitions / routines
- ex1.txt ex2.txt ex3.txt : samples

b) Compile the parser by typing "make" in the command line.
The generated executable file is called "parser".

c) Execute the parser for exemplo1.txt exemplo2.txt exemplo3.txt
e.g. "./parser exemplo1.txt".
You should get errors for all of them.
Modify these files until the parser reports no errors.

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2. You may have noticed during compilation (step 1b)
that bison reports "1 shift/reduce conflict"
(if you missed it, recompile the parser using "make clean all").
The conflict comes from the ambiguity of "expr ADD_TOKEN expr"
(why is it ambiguous?).

To overcome this conflict add
%left ADD_TOKEN
to the parser specification (on top) and re-compile.

This change specifies left associativity for sum expressions
and gets rid of the ambiguity.

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3. Complete the grammar to also handle the binary operators
-, *, and /.

Use %left as in step 2 to ensure there are no grammar conflicts.


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4. Now we will use semantic actions to associate a "double"
value to each expression, and print these values.

Proceed in the following steps:

a) In parser.bison add at the top

%define api.value.type { double }

This will set "double" as the type of expressions.

b) Modify scanner.flex to assign the value of a number to
the special variable 'yylval' as part of the actions in the
NUMBER_TOKEN rule. You may use the C library function 'atof'
to convert the 'yytext' string to a number.

c) In parser.bison encode semantic actions as follows:

NUMBER_TOKEN { $$ = $1; }
expr SUM_TOKEN expr { $$ = $1 + $3;}
... etc ...

d) Use a semantic action for 'prog' to print the value of each
expression read from the input

e) If the values printed are wrong look at the order of the
"%left" directives for the operators.
This order dictates the precedence (lowest to highest) for
the operators.

5. Extensions (outside class):

a) Allow single-line comments begginning with "#".

b) Handle expressions between "(" and ")".

c) Allow variables identified by "a" to "z" in expressions, and allow
attributions of the form "var = expr".
You may assume all variables are initially 0.
Suggestion: use an array to hold variable values.

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