I'm facing a challenge with a recursive-descent parser I've created for evaluating arithmetic expressions. The parsing logic seems fine overall, but when I test expressions with mixed operators, I get unexpected results. For example, when processing the expression "2 * 3 + 4 * 5", the output comes out to be 46 instead of the expected 26. I've attached my parser code below for reference:
```python
def parse_expression(ts):
value = parse_term(ts)
while ts.peek() == '+':
ts.next()
rhs = parse_term(ts)
value = value + rhs
return value
def parse_term(ts):
value = parse_factor(ts)
while ts.peek() == '*':
ts.next()
rhs = parse_expression(ts)
value = value * rhs
return value
def parse_factor(ts):
tok = ts.peek()
if tok.isdigit():
ts.next()
return int(tok)
if tok == '(':
ts.next()
value = parse_expression(ts)
ts.next()
return value
```
Could someone help me identify where my structural error might be in handling the operator precedence?
3 Answers
The issue seems to arise from how `parse_term` calls `parse_expression` whenever it encounters a multiplication operator. Instead, it should be calling `parse_factor`. This results in the multiplication being handled after encountering the next addition or higher priority operation, which is likely causing your output to calculate from the end towards the start instead of honoring the intended order, leading to the result of 46 instead of 26.
You mentioned that the tokenizer works fine, but it might be helpful to show the full code for a clearer perspective. Sometimes the issue can be there or in the way tokens are being processed leading to unexpected results.
If you run the parser through a Python debugger, you can step through the execution one line at a time. This will let you see where the calculations go wrong and the state of your variables. It's instrumental for figuring out why the logic breaks down! But, check if your method for handling multiplication is being executed correctly with regard to operation precedence.

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