Mini Interpreter

Lab Python Advanced 30 points

Interpret a tiny language. Each line is let NAME = EXPR or print EXPR, where EXPR is integers and variable names joined by + or - (spaces around operators). Lines run until end. Print error: undefined NAME for unknown variables.

Input

Program lines, then end.

Output

The program's output.

Example

Input

let x = 5
let y = x + 10
print y - 3
print z + 1
let x = x - x
print x
end

Output

12
error: undefined z
0

Sample input available Sample output available

Editor

Python

Hints

Split the expression into tokens and keep a running total and the current sign.

Store variables in a dict.

Solution explanation

Parsing, an environment dict and an evaluation loop are the core of every interpreter, including Python itself.

Show solution

def evaluate(expr, env):
    tokens = expr.split()
    total = 0
    sign = 1
    for token in tokens:
        if token == "+":
            sign = 1
        elif token == "-":
            sign = -1
        else:
            if token.lstrip("-").isdigit():
                value = int(token)
            elif token in env:
                value = env[token]
            else:
                raise NameError(token)
            total += sign * value
    return total


env = {}
while (line := input()) != "end":
    try:
        if line.startswith("let "):
            name, expr = line[4:].split(" = ", 1)
            env[name] = evaluate(expr, env)
        elif line.startswith("print "):
            print(evaluate(line[6:], env))
    except NameError as err:
        print(f"error: undefined {err}")

Output

Answer