Constructing a Shift-Reduce Parser involves implementing a parsing algorithm that shifts tokens onto a stack and then reduces them using production rules. Since a full implementation can be quite extensive, I'll provide you with a basic example in Python and explain each line of code. Note that this example will use a simple grammar for illustration purposes.
**Example Grammar**:
```
E -> E + T | T
T -> T * F | F
F -> ( E ) | id
```
**Python Shift-Reduce Parser Example**:
```python
# Input string to be parsed
input_string = "id + id * id"
# Define grammar rules as a dictionary
grammar = {
"E": [["E", "+", "T"], ["T"]],
"T": [["T", "*", "F"], ["F"]],
"F": [["(", "E", ")"], ["id"]]
}
# Define terminals and non-terminals
terminals = set(["+", "*", "(", ")", "id"])
non_terminals = set(["E", "T", "F"])
# Initialize the stack and add the starting symbol
stack = ["$"] # $ is the end-marker
stack.append("E")
# Initialize the input pointer
input_ptr = 0
# Parsing algorithm
while len(stack) > 0:
top = stack[-1]
current_token = input_string[input_ptr]
if top in terminals:
if top == current_token:
stack.pop()
input_ptr += 1
else:
print("Error: Unexpected token")
break
elif top in non_terminals:
if current_token in terminals:
production_found = False
for production in grammar[top]:
if production[0] == current_token:
stack.pop()
for symbol in reversed(production[1:]):
if symbol != "ε": # ε is epsilon, representing an empty string
stack.append(symbol)
production_found = True
break
if not production_found:
print("Error: No production found")
break
else:
print("Error: Unexpected token")
break
else:
print("Error: Invalid symbol")
break
if len(stack) == 0 and input_ptr == len(input_string):
print("Parsing successful")
else:
print("Parsing failed")
```
**Explanation**:
1. We start by defining the input string and the grammar rules.
2. Terminals and non-terminals are defined separately.
3. The stack is initialized with the end-marker `$` and the starting symbol `E`.
4. We start parsing using a loop that continues until the stack is empty.
5. For each iteration, we check the top of the stack and the current input token.
6. If the top is a terminal, we compare it with the current token. If they match, we pop the stack and advance the input pointer.
7. If the top is a non-terminal and the current token is a terminal, we find a production that matches and replace the non-terminal with the right-hand side of the production.
8. If the top is a non-terminal and the current token is not a terminal, we report an error.
9. If the top is not a terminal or a non-terminal, we report an error.
10. After the loop, we check if both the stack is empty and the input pointer has reached the end of the input string to determine parsing success or failure.
Please note that this is a simplified example. Real-world parsers would involve error handling, precedence rules, and more complex grammars.
Comments
Post a Comment