evalexpr @ 42

This is one of the numerous projects done during the 42 piscine in July 2015.
It was completed in a week and can solve basic mathematical expressions. The
evaluator runs on recursive descent parsing
explained here. I absolutely
recommend you read that article as it helped me tons while making this.
This was an assignment, so some things are not handled on purpose (see below).

Compiling

Run make, an executable called eval_expr should compile directly. Please
submit an issue if you run into any.

Usage

./eval_expr "expression"

Example:

$ ./eval_expr "1 + 2 * (3 - 42) % 5"
-2

Subject constraints

Please note the evaluator does not have error-handling of any kind. The
subject states that all input must be valid, so no error checking is
performed.

A valid expression:

• must only contain the operators +, -, *, /, and %
• must only have integer values
• can contain parentheses, but each group must be properly closed
• can contain spaces

Since we are doing whole number arithmetic, divisions are euclidian keeping
only the quotient, while the remainder can be obtained with a modulo operation.