parsing to an expression tree in python

Question

I have a genetic program that prints the expression tree to a file (it can easily switch between pre/post/in-fix)

It seems like pre-fix would be the easiest to parse so I am currently using that.

How would I go about parsing this string using python 2.7? For example, how would I parse the string +(*(2,1),*(4,3)) ~~~~ which is 2*1+4*3

f = open('expression_tree.txt', 'r')
input = f.read()
root_node_operator = input[0]

That's about as far as I've gotten. I'm not that familiar with parsing. Thanks!

I have one python program that prints the expression tree data structure and I want to parse it and evaluate it in the next python program.

Or is there a way to pass the expression tree object to the next python program so no parsing is needed? like I have my tree called test_tree in GP.py. Can I somehow get at that from my other file MyBot.py?

Does the output contain anything else besides numbers and basic operations? If yes, what is the name of the genetic program? (Did you mean genetic programming by any chance?) — Matt
– Matt, Commented Dec 12, 2011 at 8:41
What does your “expression tree data structure” look like? Do you want to make a parser from scratch, or use a more powerful library? Why do you want to split the parsing and the evaluation in two files? — poke
– poke, Commented Dec 12, 2011 at 8:49

PaulMcG · Accepted Answer · 2011-12-12 13:27:37Z

For one thing, prefix notation won't require any parentheses - you control the precedence of operations purely by the ordering of elements in the string. For instance, if you wanted 2*(1+4)*3 instead, your prefix expression would become "* * 2 + 1 4 3".

2*1+4*3 would become "+ * 2 1 * 4 3". Using split(), this will give you the list of operators and operands, ['+', '*', '2', '1', '*', '4', '3']. This will take care of the whitespace skipping. Then to evaluate this, recursively walk the list: if you find an operator, get the next two operands from the list, starting at the current position; if you find a constant, return it. Each time you pull something from the list, advance the current position.

opns = {
    '+' : lambda a,b: a+b,
    '-' : lambda a,b: a-b,
    '*' : lambda a,b: a*b,
    '/' : lambda a,b: a/b,
    }

def prefix_eval(expr, posn=0):
    # save current element from expression
    current = expr[posn]

    # advance parsing position
    posn += 1

    if current in ['+','-','*','/']:
        # binary operator, get next two operands
        op1,posn = prefix_eval(expr, posn)
        op2,posn = prefix_eval(expr, posn)

        # evaluate operation from current, on operands
        return opns[current](op1,op2), posn
    else:
        # not an operator, must be a numeric value
        return float(current),posn

print prefix_eval("+ * 2 1 * 4 3".split())[0]
print prefix_eval("* * 2 + 1 4 3".split())[0]

prints

14.0
30.0

kev · Accepted Answer · 2011-12-12 14:25:59Z

2

replace +(*(2,1),*(4,3)) with (+ (* 2 1) (* 4 3)), then pipe to scheme

$ echo '+(*(2,1),*(4,3))' | sed 's/\(.\)(/(\1 /g; s/,/ /g' | scheme | sed -n '/;Value: /s///p'

If you want to use python, please try pyparsing.

answered Dec 12, 2011 at 14:25

kev

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1 Comment

kev Over a year ago

@user1018733: The symbol | is the Unix pipe symbol that is used on the command line. Unix commands alone are powerful, but when you combine them together, you can accomplish complex tasks with ease. Windows cmd also has pipe, but people always use GUI tools in Windows.

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