Created
July 25, 2014 14:44
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tokenized_string = "2 + 3 * 4".split() | |
# parsing arithmetic expressions with + and * | |
# A parser returns (parsed_thing, new_index) on success. | |
def fail(): raise Exception("failed to parse") | |
def is_num(x): | |
try: int(x) | |
except ValueError: return False | |
else: return True | |
def parse_expr(tokens, i): | |
print("parse_expr(%s)" % tokens[i:]) | |
if i >= len(tokens): fail() | |
if is_num(tokens[i]): | |
value = int(tokens[i]) | |
new_i = i+1 | |
if tokens[i] == '(': | |
value, new_i = parse_expr(tokens, i+1) | |
if tokens[new_i] != ')': fail() | |
new_i += 1 | |
rest = parse_infix_expr(tokens, new_i, value) | |
if rest is None: return value, new_i | |
return rest | |
# returns (parsed_thing, new_index) on success | |
# returns None on failure | |
def parse_infix_expr(tokens, i, left_arg): | |
print("parse_infix_expr(%s, %s)" % (tokens[i:], left_arg)) | |
if i >= len(tokens): return None | |
if tokens[i] == '+': | |
right_arg, new_i = parse_expr(tokens, i+1) | |
return left_arg + right_arg, new_i | |
if tokens[i] == '*': | |
if i+1 >= len(tokens): fail() | |
if is_num(tokens[i+1]): | |
right_arg = int(tokens[i+1]) | |
new_i = i+2 | |
elif tokens[i+1] == '(': | |
right_arg, new_i = parse_expr(tokens,i+2) | |
if tokens[new_i] != ')': fail() | |
new_i += 1 | |
else: fail() | |
value = left_arg * right_arg | |
print('multiplied: %s * %s = %s' % (left_arg, right_arg, value)) | |
rest = parse_infix_expr(tokens, new_i, value) | |
if rest is None: return value, new_i | |
return rest | |
return None | |
print("loaded parser.py") |
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