README.md

Writing eval() for simple math expressions

Rather than starting with lexing and parsing, this week we just assume we have those bits already working, so that we can jump straight to eval().

video:

recording of the meeting

diagrams:

diagrams used during the meeting:

whiteboard notes:

matheval.py

#!/usr/bin/env python


def add(a,b):
    return a + b

def multiply(a,b):
    return a * b


def flookup(toktype):
    if toktype == "plus":
        return add
    elif toktype == "mult":
        return multiply


def matheval(node):
    if node["token"]["type"] == "num":
        return float(node["token"]["text"])
    else:
        f = flookup(node["token"]["type"])
        l = matheval(node["left"])
        r = matheval(node["right"])
        return f(l,r)


t1 = { "type": "num", "text": "1" }
tp = { "type": "plus", "text": "+" }
t2 = { "type": "num",  "text": "2" }
tm = { "type": "mult", "text": "*" }
t3 = { "type": "num",  "text": "3" }

n1 = { "token": t1, "left": None, "right": None }
np = { "token": tp, "left": None, "right": None }
n2 = { "token": t2, "left": None, "right": None }
nm = { "token": tm, "left": None, "right": None }
n3 = { "token": t3, "left": None, "right": None }


#     *
#    / \
#   +   3
#  / \
# 1   2

tree = nm
nm["left"] = np
nm["right"] = n3
np["left"] = n1
np["right"] = n2

print matheval(tree)  # 9


#   +
#  / \
# 1   *
#    / \
#   2   3

tree = np
np["left"] = n1
np["right"] = nm
nm["left"] = n2
nm["right"] = n3

print matheval(tree)  # 7