Read Me

Examples using the pegplusplus library.

Username
--------

This is a simple example using the facilities defined in peg.h.

It shows how LEG parsers can be easily translated to pegplusplus. 
The LEG version (username.leg) was taken from the PEG/LEG distribution.

The parser replaces all occurrences of the string "username" by the current user's 
login name.

Varcalc
-------

This is a more complex example that encapsulates the parser in a class, using the
Parser class defined in pegparser.h.

Varcalc is a calculator with the following features:

    Values are doubles. Valid input formats: 

        17
        2.
        2.33
        .28
        3.21e3
        22e-5

    Operations supported:

        addition/substraction
        multiplication/division
        exponentiation
        grouping (parentheses)
        variable assignment
        printing (print command)

    Named variables: 

        variable names have the format of C identifiers
        assignment is an expression (assignments can be cascaded)

    Operators from highest to lowest precedence and their associativity:

        ^       exponentiation      left
        + -     unary plus/minus    right
        * /     multiply/divide     left
        + -     add/substract       left
        =       assignment          right

Varcalc reads statements. Newline or semi-colon end a statement.

Statements can be:

    <expression>
    print <expression>

Any text from // to the end of the line is a comment.
White space, empty lines and comments are ignored. 
Invalid statements are reported as errors and ignored. 
Valid statements are executed.

Let's assume the following input:

    a = b = 23 + c
    print 2 * a
    hello!
    print c
    print 2^(-1/2) 

Variable c is being used without a previous assignment. This is valid, the variable is
automatically initialized with value 0 and a warning is printed on standard error.

This is the output:

    line 1: defining c = 0      (standard error)
    46                          (standard output)
    line 3: ERROR: hello!       (standard error)
    0                           (standard output)
    0.707107                    (standard output)