py-05-calc
0.100
Challenge · difficulty 5/5
# Arithmetic Expression Evaluator
Implement a file **`solution.py`** containing a function:
```python
def evaluate(expr: str) -> float:
...
```
Evaluate an arithmetic expression given as a string and return its value as a
`float`.
Supported syntax:
- Binary operators `+`, `-`, `*`, `/` with standard precedence
(`*` and `/` bind tighter than `+` and `-`) and left-to-right associativity.
- Parentheses `(` ... `)` for grouping.
- Unary plus and minus, e.g. `-3`, `-(2 + 1)`, `2 * -3`.
- Integer and float literals, e.g. `42`, `3.14`, `.5`, `10.`.
- Arbitrary surrounding / internal whitespace, which is ignored.
Requirements:
- Return the numeric result as a `float`. For example
`evaluate("1 + 2 * 3") == 7.0` and `evaluate("(1 + 2) * 3") == 9.0`.
- Implement a real parser/evaluator that respects precedence and parentheses.
**Do not** use `eval`, `exec`, or similar (the harness may inspect for this).
- Raise `ValueError` on malformed input. Malformed includes: empty / whitespace-only
input, unbalanced parentheses, a missing operand or operator (e.g. `"1 +"`,
`"1 2"`, `"* 3"`), and unexpected characters.
- Division by zero should raise an error (a `ZeroDivisionError` or `ValueError`
is acceptable).
Examples:
```python
evaluate("2 + 3 * 4") # 14.0
evaluate("(2 + 3) * 4") # 20.0
evaluate("10 / 4") # 2.5
evaluate("-3 + 2") # -1.0
evaluate("2 * (1 + -1.5)") # -1.0
```
tests/test_calc.py
import pytest
from solution import evaluate
def test_simple_addition():
assert evaluate("1 + 2") == 3.0
def test_precedence():
assert evaluate("2 + 3 * 4") == 14.0
assert evaluate("1 + 2 * 3") == 7.0
def test_parentheses_override_precedence():
assert evaluate("(2 + 3) * 4") == 20.0
assert evaluate("(1 + 2) * 3") == 9.0
def test_division_is_float():
assert evaluate("10 / 4") == 2.5
assert isinstance(evaluate("4 / 2"), float)
def test_unary_minus():
assert evaluate("-3 + 2") == -1.0
assert evaluate("2 * -3") == -6.0
assert evaluate("-(2 + 1)") == -3.0
def test_float_and_whitespace():
assert evaluate(" 3.14 + .86 ") == pytest.approx(4.0)
assert evaluate("10. / 4") == 2.5
def test_left_associativity():
assert evaluate("10 - 2 - 3") == 5.0
assert evaluate("100 / 5 / 2") == 10.0
def test_nested_parens():
assert evaluate("2 * (1 + (3 - 1) * 2)") == 10.0
def test_malformed_raises_value_error():
for bad in ["", " ", "1 +", "* 3", "1 2", "(1 + 2", "1 + 2)", "1 + * 2", "3 $ 4"]:
with pytest.raises(ValueError):
evaluate(bad)
def test_division_by_zero_raises():
with pytest.raises((ValueError, ZeroDivisionError)):
evaluate("1 / 0")
System prompt
You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.Proposed solution
```python
def evaluate(expr: str) -> float:
def parse_expression(expr):
tokens = tokenize(expr)
return parse_term(tokens)
def parse_term(tokens):
token = tokens.pop(0)
if token == '(':
result = parse_expression(tokens)
if tokens.pop(0) != ')':
raise ValueError("Unbalanced parentheses")
return result
else:
return parse_factor(tokens)
def parse_factor(tokens):
token = tokens.pop(0)
if token == '-':
return -parse_factor(tokens)
elif token == '+':
return parse_factor(tokens)
else:
return parse_number(token)
def parse_number(token):
try:
return float(token)
except ValueError:
raise ValueError(f"Invalid number: {token}")
def tokenize(expr):
tokens = []
number = ''
for char in expr:
if char in '0123456789.':
number += char
else:
if number:
tokens.append(number)
number = ''
if char in '+-*/()':
tokens.append(char)
else:
raise ValueError(f"Unexpected character: {char}")
if number:
tokens.append(number)
return tokens
if not expr.strip():
raise ValueError("Empty input")
tokens = tokenize(expr.replace(' ', ''))
if not tokens:
raise ValueError("Whitespace-only input")
result = parse_expression(tokens)
if tokens:
raise ValueError("Unbalanced parentheses or malformed input")
return result
```Test output (stdout)
n evaluate
result = parse_expression(tokens)
^^^^^^^^^^^^^^^^^^^^^^^^
solution.py:3: in parse_expression
tokens = tokenize(expr)
^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
expr = ['10', '-', '2', '-', '3']
def tokenize(expr):
tokens = []
number = ''
for char in expr:
if char in '0123456789.':
number += char
else:
if number:
tokens.append(number)
number = ''
if char in '+-*/()':
tokens.append(char)
else:
> raise ValueError(f"Unexpected character: {char}")
E ValueError: Unexpected character: 10
solution.py:44: ValueError
______________________________ test_nested_parens ______________________________
token = '*'
def parse_number(token):
try:
> return float(token)
^^^^^^^^^^^^
E ValueError: could not convert string to float: '*'
solution.py:27: ValueError
During handling of the above exception, another exception occurred:
def test_nested_parens():
> assert evaluate("2 * (1 + (3 - 1) * 2)") == 10.0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
test_calc.py:42:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
solution.py:54: in evaluate
result = parse_expression(tokens)
^^^^^^^^^^^^^^^^^^^^^^^^
solution.py:4: in parse_expression
return parse_term(tokens)
^^^^^^^^^^^^^^^^^^
solution.py:14: in parse_term
return parse_factor(tokens)
^^^^^^^^^^^^^^^^^^^^
solution.py:23: in parse_factor
return parse_number(token)
^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
token = '*'
def parse_number(token):
try:
return float(token)
except ValueError:
> raise ValueError(f"Invalid number: {token}")
E ValueError: Invalid number: *
solution.py:29: ValueError
______________________ test_malformed_raises_value_error _______________________
def test_malformed_raises_value_error():
for bad in ["", " ", "1 +", "* 3", "1 2", "(1 + 2", "1 + 2)", "1 + * 2", "3 $ 4"]:
with pytest.raises(ValueError):
> evaluate(bad)
test_calc.py:48:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
solution.py:54: in evaluate
result = parse_expression(tokens)
^^^^^^^^^^^^^^^^^^^^^^^^
solution.py:4: in parse_expression
return parse_term(tokens)
^^^^^^^^^^^^^^^^^^
solution.py:14: in parse_term
return parse_factor(tokens)
^^^^^^^^^^^^^^^^^^^^
solution.py:21: in parse_factor
return parse_factor(tokens)
^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
tokens = []
def parse_factor(tokens):
> token = tokens.pop(0)
^^^^^^^^^^^^^
E IndexError: pop from empty list
solution.py:17: IndexError
=========================== short test summary info ============================
FAILED test_calc.py::test_simple_addition - ValueError: Unbalanced parenthese...
FAILED test_calc.py::test_precedence - ValueError: Unbalanced parentheses or ...
FAILED test_calc.py::test_parentheses_override_precedence - ValueError: Unbal...
FAILED test_calc.py::test_division_is_float - ValueError: Unexpected characte...
FAILED test_calc.py::test_unary_minus - ValueError: Unbalanced parentheses or...
FAILED test_calc.py::test_float_and_whitespace - ValueError: Unexpected chara...
FAILED test_calc.py::test_left_associativity - ValueError: Unexpected charact...
FAILED test_calc.py::test_nested_parens - ValueError: Invalid number: *
FAILED test_calc.py::test_malformed_raises_value_error - IndexError: pop from...
9 failed, 1 passed in 0.03s
Self-repair attempts
[
{
"total": 10,
"answer": "```python\ndef evaluate(expr: str) -> float:\n def parse_expression(expr):\n tokens = tokenize(expr)\n return parse_term(tokens)\n\n def parse_term(tokens):\n token = tokens.pop(0)\n if token == '(':\n result = parse_expression(tokens)\n if tokens.pop(0) != ')':\n raise ValueError(\"Unbalanced parentheses\")\n return result\n else:\n return parse_factor(tokens)\n\n def parse_factor(tokens):\n token = tokens.pop(0)\n if token == '-':\n return -parse_factor(tokens)\n elif token == '+':\n return parse_factor(tokens)\n else:\n return parse_number(token)\n\n def parse_number(token):\n try:\n return float(token)\n except ValueError:\n raise ValueError(f\"Invalid number: {token}\")\n\n def tokenize(expr):\n tokens = []\n number = ''\n for char in expr:\n if char in '0123456789.':\n number += char\n else:\n if number:\n tokens.append(number)\n number = ''\n if char in '+-*/()':\n tokens.append(char)\n else:\n raise ValueError(f\"Unexpected character: {char}\")\n if number:\n tokens.append(number)\n return tokens\n\n if not expr.strip():\n raise ValueError(\"Empty input\")\n tokens = tokenize(expr.replace(' ', ''))\n if not tokens:\n raise ValueError(\"Whitespace-only input\")\n result = parse_expression(tokens)\n if tokens:\n raise ValueError(\"Unbalanced parentheses or malformed input\")\n return result\n```",
"passed": 1,
"test_error": " ^^^^^^^^^^^^^^^^^^^^^^^^\nsolution.py:4: in parse_expression\n return parse_term(tokens)\n ^^^^^^^^^^^^^^^^^^\nsolution.py:14: in parse_term\n return parse_factor(tokens)\n ^^^^^^^^^^^^^^^^^^^^\nsolution.py:23: in parse_factor\n return parse_number(token)\n ^^^^^^^^^^^^^^^^^^^\n_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ \n\ntoken = '*'\n\n def parse_number(token):\n try:\n return float(token)\n except ValueError:\n> raise ValueError(f\"Invalid number: {token}\")\nE ValueError: Invalid number: *\n\nsolution.py:29: ValueError\n______________________ test_malformed_raises_value_error _______________________\n\n def test_malformed_raises_value_error():\n for bad in [\"\", \" \", \"1 +\", \"* 3\", \"1 2\", \"(1 + 2\", \"1 + 2)\", \"1 + * 2\", \"3 $ 4\"]:\n with pytest.raises(ValueError):\n> evaluate(bad)\n\ntest_calc.py:48: \n_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ \nsolution.py:54: in evaluate\n result = parse_expression(tokens)\n ^^^^^^^^^^^^^^^^^^^^^^^^\nsolution.py:4: in parse_expression\n return parse_term(tokens)\n ^^^^^^^^^^^^^^^^^^\nsolution.py:14: in parse_term\n return parse_factor(tokens)\n ^^^^^^^^^^^^^^^^^^^^\nsolution.py:21: in parse_factor\n return parse_factor(tokens)\n ^^^^^^^^^^^^^^^^^^^^\n_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ \n\ntokens = []\n\n def parse_factor(tokens):\n> token = tokens.pop(0)\n ^^^^^^^^^^^^^\nE IndexError: pop from empty list\n\nsolution.py:17: IndexError\n=========================== short test summary info ============================\nFAILED test_calc.py::test_simple_addition - ValueError: Unbalanced parenthese...\nFAILED test_calc.py::test_precedence - ValueError: Unbalanced parentheses or ...\nFAILED test_calc.py::test_parentheses_override_precedence - ValueError: Unbal...\nFAILED test_calc.py::test_division_is_float - ValueError: Unexpected characte...\nFAILED test_calc.py::test_unary_minus - ValueError: Unbalanced parentheses or...\nFAILED test_calc.py::test_float_and_whitespace - ValueError: Unexpected chara...\nFAILED test_calc.py::test_left_associativity - ValueError: Unexpected charact...\nFAILED test_calc.py::test_nested_parens - ValueError: Invalid number: *\nFAILED test_calc.py::test_malformed_raises_value_error - IndexError: pop from...\n9 failed, 1 passed in 0.03s"
},
{
"total": 10,
"answer": "```python\ndef evaluate(expr: str) -> float:\n def parse_expression(expr):\n tokens = tokenize(expr)\n return parse_term(tokens)\n\n def parse_term(tokens):\n token = tokens.pop(0)\n if token == '(':\n result = parse_expression(tokens)\n if tokens.pop(0) != ')':\n raise ValueError(\"Unbalanced parentheses\")\n return result\n else:\n return parse_factor(tokens)\n\n def parse_factor(tokens):\n token = tokens.pop(0)\n if token == '-':\n return -parse_factor(tokens)\n elif token == '+':\n return parse_factor(tokens)\n else:\n return parse_number(token)\n\n def parse_number(token):\n try:\n return float(token)\n except ValueError:\n raise ValueError(f\"Invalid number: {token}\")\n\n def tokenize(expr):\n tokens = []\n number = ''\n for char in expr:\n if char in '0123456789.':\n number += char\n else:\n if number:\n tokens.append(number)\n number = ''\n if char in '+-*/()':\n tokens.append(char)\n else:\n raise ValueError(f\"Unexpected character: {char}\")\n if number:\n tokens.append(number)\n return tokens\n\n if not expr.strip():\n raise ValueError(\"Empty input\")\n tokens = tokenize(expr.replace(' ', ''))\n if not tokens:\n raise ValueError(\"Whitespace-only input\")\n result = parse_expression(tokens)\n if tokens:\n raise ValueError(\"Unbalanced parentheses or malformed input\")\n return result\n\n# Test cases\ndef test_simple_addition():\n assert evaluate(\"2 + 3 * 4\") == 14.0\n\ndef test_precedence():\n assert evaluate(\"(2 + 3) * 4\") == 20.0\n\ndef test_parentheses_override_precedence():\n assert evaluate(\"2 * (1 + -1.5)\") == -1.0\n\ndef test_division_is_float():\n assert evaluate(\"10 / 4\") == 2.5\n\ndef test_unary_minus():\n assert evaluate(\"-3 + 2\") == -1.0\n\ndef test_float_and_whitespace():\n assert evaluate(\"2 * (1 + -1.5)\") == -1.0\n\ndef test_left_associativity():\n assert evaluate(\"2 + 3 * 4\") == 14.0\n\ndef test_nested_parens():\n assert evaluate(\"2 * (1 + (2 * 3)\") == 14.0\n\ndef test_malformed_raises_value_error():\n for bad in [\"\", \" \", \"1 +\", \"* 3\", \"1 2\", \"(1 + 2\", \"1 + 2)\", \"1 + * 2\", \"3 $ 4\"]:\n with pytest.raises(ValueError):\n evaluate(bad)\n\nif __name__ == \"__main__\":\n test_simple_addition()\n test_precedence()\n test_parentheses_override_precedence()\n test_division_is_float()\n test_unary_minus()\n test_float_and_whitespace()\n test_left_associativity()\n test_nested_parens()\n test_malformed_raises_value_error()\n print(\"All tests passed\")\n```",
"passed": 1,
"test_error": " ^^^^^^^^^^^^^^^^^^^^^^^^\nsolution.py:4: in parse_expression\n return parse_term(tokens)\n ^^^^^^^^^^^^^^^^^^\nsolution.py:14: in parse_term\n return parse_factor(tokens)\n ^^^^^^^^^^^^^^^^^^^^\nsolution.py:23: in parse_factor\n return parse_number(token)\n ^^^^^^^^^^^^^^^^^^^\n_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ \n\ntoken = '*'\n\n def parse_number(token):\n try:\n return float(token)\n except ValueError:\n> raise ValueError(f\"Invalid number: {token}\")\nE ValueError: Invalid number: *\n\nsolution.py:29: ValueError\n______________________ test_malformed_raises_value_error _______________________\n\n def test_malformed_raises_value_error():\n for bad in [\"\", \" \", \"1 +\", \"* 3\", \"1 2\", \"(1 + 2\", \"1 + 2)\", \"1 + * 2\", \"3 $ 4\"]:\n with pytest.raises(ValueError):\n> evaluate(bad)\n\ntest_calc.py:48: \n_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ \nsolution.py:54: in evaluate\n result = parse_expression(tokens)\n ^^^^^^^^^^^^^^^^^^^^^^^^\nsolution.py:4: in parse_expression\n return parse_term(tokens)\n ^^^^^^^^^^^^^^^^^^\nsolution.py:14: in parse_term\n return parse_factor(tokens)\n ^^^^^^^^^^^^^^^^^^^^\nsolution.py:21: in parse_factor\n return parse_factor(tokens)\n ^^^^^^^^^^^^^^^^^^^^\n_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ \n\ntokens = []\n\n def parse_factor(tokens):\n> token = tokens.pop(0)\n ^^^^^^^^^^^^^\nE IndexError: pop from empty list\n\nsolution.py:17: IndexError\n=========================== short test summary info ============================\nFAILED test_calc.py::test_simple_addition - ValueError: Unbalanced parenthese...\nFAILED test_calc.py::test_precedence - ValueError: Unbalanced parentheses or ...\nFAILED test_calc.py::test_parentheses_override_precedence - ValueError: Unbal...\nFAILED test_calc.py::test_division_is_float - ValueError: Unexpected characte...\nFAILED test_calc.py::test_unary_minus - ValueError: Unbalanced parentheses or...\nFAILED test_calc.py::test_float_and_whitespace - ValueError: Unexpected chara...\nFAILED test_calc.py::test_left_associativity - ValueError: Unexpected charact...\nFAILED test_calc.py::test_nested_parens - ValueError: Invalid number: *\nFAILED test_calc.py::test_malformed_raises_value_error - IndexError: pop from...\n9 failed, 1 passed in 0.04s"
},
{
"total": 10,
"answer": "```python\ndef evaluate(expr: str) -> float:\n def parse_expression(expr):\n tokens = tokenize(expr)\n return parse_term(tokens)\n\n def parse_term(tokens):\n token = tokens.pop(0)\n if token == '(':\n result = parse_expression(tokens)\n if tokens.pop(0) != ')':\n raise ValueError(\"Unbalanced parentheses\")\n return result\n else:\n return parse_factor(tokens)\n\n def parse_factor(tokens):\n token = tokens.pop(0)\n if token == '-':\n return -parse_factor(tokens)\n elif token == '+':\n return parse_factor(tokens)\n else:\n return parse_number(token)\n\n def parse_number(token):\n try:\n return float(token)\n except ValueError:\n raise ValueError(f\"Invalid number: {token}\")\n\n def tokenize(expr):\n tokens = []\n number = ''\n for char in expr:\n if char in '0123456789.':\n number += char\n else:\n if number:\n tokens.append(number)\n number = ''\n if char in '+-*/()':\n tokens.append(char)\n else:\n raise ValueError(f\"Unexpected character: {char}\")\n if number:\n tokens.append(number)\n return tokens\n\n if not expr.strip():\n raise ValueError(\"Empty input\")\n tokens = tokenize(expr.replace(' ', ''))\n if not tokens:\n raise ValueError(\"Whitespace-only input\")\n result = parse_expression(tokens)\n if tokens:\n raise ValueError(\"Unbalanced parentheses or malformed input\")\n return result\n\n# Test cases\ndef test_simple_addition():\n assert evaluate(\"2 + 3 * 4\") == 14.0\n\ndef test_precedence():\n assert evaluate(\"(2 + 3) * 4\") == 20.0\n\ndef test_parentheses_override_precedence():\n assert evaluate(\"2 * (1 + -1.5)\") == -1.0\n\ndef test_division_is_float():\n assert evaluate(\"10 / 4\") == 2.5\n\ndef test_unary_minus():\n assert evaluate(\"-3 + 2\") == -1.0\n\ndef test_float_and_whitespace():\n assert evaluate(\"2 * (1 + -1.5)\") == -1.0\n\ndef test_left_associativity():\n assert evaluate(\"2 + 3 * 4\") == 14.0\n\ndef test_nested_parens():\n assert evaluate(\"2 * (1 + (2 * 3)\") == 14.0\n\ndef test_malformed_raises_value_error():\n for bad in [\"\", \" \", \"1 +\", \"* 3\", \"1 2\", \"(1 + 2\", \"1 + 2)\", \"1 + * 2\", \"3 $ 4\"]:\n with pytest.raises(ValueError):\n evaluate(bad)\n\nif __name__ == \"__main__\":\n test_simple_addition()\n test_precedence()\n test_parentheses_override_precedence()\n test_division_is_float()\n test_unary_minus()\n test_float_and_whitespace()\n test_left_associativity()\n test_nested_parens()\n test_malformed_raises_value_error()\n print(\"All tests passed\")\n```",
"passed": 1,
"test_error": ""
}
]