libExpr

What is it?

That means you can give it something like (as a string):

  "12   3*6 / -(14 3) ^ 2 ^ 3"

And it will convert it to an RPN or Prefix or AST expression object, and then you can evaluate it. (Default is AST) This is as simple as:

require 'libExpr'
exprObj = Expr.new("12   3*6 / -(14 3) ^ 2 ^ 3")
answer = exprObj.eval() 
# Or, if you don't need to repeat the evaluation:
answer = Expr.new("12   3*6 / -(14 3) ^ 2 ^ 3").eval()

That in itself might not be interesting, since you can always use the built-in language eval() (although this one doesn't require any sanitizing of inputs to avoid the inherent security dangers that 'eval()' has).

You can also supply it with expressions that have variables:

  "12   someVar * 6 / (32 - anotherVar)"

And when you evaluate the expression object you can supply a block which the evaluator will use to get any variables, such as:

vars['foo'] = 42
vars['bar'] = 3
require 'libExpr'
someExpr = Expr.new("5*foo-6/bar")
answer = someExpr.eval() { |varname| vars[varname] }
# Returns  5*42-6/3 = 208

# And evaluate again with different vars
vars['foo'] = 100
answer = someExpr.eval() { |varname| vars[varname] }
# Now returns 5*100-6/3 = 498

This can be useful when:

1. Eval is not safe (no need to sanitize inputs here)
2. 'Variable' evaluation is different from ruby variables
3. Expressions are evaluated many times with different variable values
4. 'Variables' may not actually be normal variable notation. I.e.: [`PARAMS@SOME_VAL("DEFAULT")]

This is done with a modified Shunting-Yard Algorithm that can handle Unary/Ternary operations and short-circuiting of operations, so it's also potentially interesting to anyone trying to implement Shunting-Yard for their own application.

Unary Ops?

1. A different operator needs to be used for the unary op vs the binary op. Since libExpr has operator objects, it's easy to create two different objects, and I used a key for hashing ops where a unary op has a space before it, i.e. " -" for unary negative and "-" for binary subtraction.
2. To recognize unary '-' as opposed to binary '-' is actually pretty simple when we realize that a unary '-' unambiguously will always follow either a left paren '(' or another operator (such as '3 -7'). In all other cases it is a binary subtraction. Once we see that, we can just save the last token we saw when we run the Shunting-Yard, and that can tell us whether it's a unary or binary op (if the token supports both)
3. As a final point, when it's a unary op, we obviously only push one operand instead of two.

Finally, short circuiting was only added for AST, because for an AST it's trivial to simply not traverse one side of the tree. It would be interesting to write an algorithm that would intelligently throw out the proper number of operators/operands from an RPN or Prefix stack to accomplish short circuiting, but that's when my development on this project stopped. Use AST if you need short-circuiting.

Last of all, I also added an 'if-then' operator "a -> b" which is effectively logically equivalent to "!a || b". I added this simply because I needed it for a project I was working on.

Format Capabilities

Format Info

RPN (Reverse Polish Notation aka "postfix"):

Fastest (slightly), simplest to view and debug, but missing short-circuit/ternary

Prefix

Converts to RPN then quick conversion to Prefix. Supports short-circuit (for &&,||)

AST

Can do ternary ops. Also fast, though harder to debug/understand since it's a tree

Notes about short-circuit

  (a == 0) || (num/a)

Testing

An example expression

-  1-3   (- var[:n2])    ( (var[:t1] )   ?  (var[:n2])   :- 1)  ^  (  ( - ( var[:n0] ) ) * 10 &((var[:f1] ) ? var[:n2]  : ( - (10) % ( var[:n0] )  )* var[:n2] ) ) 

License:

Documentation?

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