Question

Homework 1:
I didn't complete the either of the extension problems, because I felt that if you make it through the main assignment, the extensions would be simple.

Answer 1

## Tokenizer:
# Convert strings into a list of tokens (strings)
def tokenize(string):
list_test = string.split()
#print "list :", list_test
tokens=[]
for item in list_test:
temp =""
#print "item :", item
for ind in range(len(item)):
if temp == "" and ind == 0 and item[ind] in seps:
tokens.append( item[ind])
#print "1 :", tokens
elif temp == "" and ind > 0 and item[ind] in seps:
if item[ind-1] not in seps:
tokens.append(item[:ind])
#print "2 :", tokens
tokens.append(item[ind])
#print "3 :", tokens
elif temp != "" and ind > 0 and item[ind] in seps:
tokens.append(temp)
temp = ""
#print "n4 :", tokens
tokens.append(item[ind])
#print "n5 :", tokens
else:
temp = temp + item[ind]
#print "temp :", temp
if ind + 1 == len(item):
tokens.append(temp)
#print "4 :", tokens
return tokens

Answer 2

## Parser:

# tokens is a list of tokens
# returns a syntax tree: an instance of {\tt Number}, {\tt Variable},
# or one of the subclasses of {\tt BinaryOp}
def parse(tokens):
def parseExp(index):
if tokens[index] == "(":
leftTree = parseExp(index +1)
rightTree = parseExp(leftTree[-1] +1)
if tokens[leftTree[-1]] == "+":
return (Sum(leftTree[0],rightTree[0]), rightTree[-1] +1)
elif tokens[leftTree[-1]] == "-":
return (Diff(leftTree[0],rightTree[0]), rightTree[-1] +1)
elif tokens[leftTree[-1]] == "*":
return (Prod(leftTree[0],rightTree[0]), rightTree[-1] +1)
elif tokens[leftTree[-1]] == "/":
return (Quot(leftTree[0],rightTree[0]), rightTree[-1] +1)
elif tokens[leftTree[-1]] == "=":
return (Assign(leftTree[0],rightTree[0]), rightTree[-1] +1)
elif numberTok(tokens[index]):
return (Number(tokens[index]), index + 1)
elif variableTok(tokens[index]):
return (Variable(tokens[index]), index +1)
elif tokens[index] == ")":
print "WHY HERE"
pass
(parsedExp, nextIndex) = parseExp(0)
return parsedExp

Answer 3

## Evaluation Statements

## This is far to long to post here, I will post one and you should be able to recreate the rest of them:

class Sum(BinaryOp):
opStr = 'Sum'
def eval(self,env):
if self.left.__class__.__name__ == "Variable":
left = env[self.left.name]
elif self.left.__class__.__name__ == "Number":
left = self.left.value
else:
left = self.left.eval(env)
if self.right.__class__.__name__ == "Variable":
right = env[self.right.name]
elif self.right.__class__.__name__ == "Number":
right = self.right.value
else:
right = self.right.eval(env)
return left + right