rs-03-rpn

# Evaluate RPN

Implement the library file **`src/lib.rs`** exposing:

```rust
pub fn eval_rpn(tokens: &[&str]) -> Result<f64, String>
```

Evaluate a [Reverse Polish Notation](https://en.wikipedia.org/wiki/Reverse_Polish_notation)
expression given as a slice of tokens. Each token is either:

- a number that parses as `f64`, or
- one of the four binary operators `+`, `-`, `*`, `/`.

Evaluation uses a stack: push numbers; for an operator, pop the top two values
`b` (top) then `a` (next) and push the result of `a OP b`. After consuming all
tokens, exactly one value must remain — that value is the result.

Return `Ok(value)` on success, or `Err(message)` (any non-empty message) on
malformed input:

- **too few operands** — an operator with fewer than two values on the stack.
- **leftover operands** — more than one value remains after all tokens consumed
  (or zero tokens / empty input, which leaves no value).
- **unknown token** — a token that is neither a valid `f64` nor a known operator.
- **division by zero** — a `/` whose right operand `b` is `0.0`.

Note: operator order matters for non-commutative operators. For `["3", "4", "-"]`
the result is `3 - 4 = -1.0`, and for `["8", "2", "/"]` it is `8 / 2 = 4.0`.

Examples:

- `eval_rpn(&["2", "3", "+"])` → `Ok(5.0)`
- `eval_rpn(&["5", "1", "2", "+", "4", "*", "+", "3", "-"])` → `Ok(14.0)` (precedence-via-RPN)
- `eval_rpn(&["3", "4", "-"])` → `Ok(-1.0)`
- `eval_rpn(&["1", "+"])` → `Err(..)` (too few operands)
- `eval_rpn(&["1", "2"])` → `Err(..)` (leftover operands)
- `eval_rpn(&["1", "foo", "+"])` → `Err(..)` (unknown token)
- `eval_rpn(&["1", "0", "/"])` → `Err(..)` (division by zero)

Use only the standard library. Tests live in `tests/` and call it as
`challenge::eval_rpn`.

use challenge::eval_rpn;

#[test]
fn basic_eval() {
    assert_eq!(eval_rpn(&["2", "3", "+"]), Ok(5.0));
    assert_eq!(eval_rpn(&["8", "2", "/"]), Ok(4.0));
    assert_eq!(eval_rpn(&["42"]), Ok(42.0));
}

#[test]
fn operator_order_matters() {
    assert_eq!(eval_rpn(&["3", "4", "-"]), Ok(-1.0));
    assert_eq!(eval_rpn(&["10", "4", "-"]), Ok(6.0));
}

#[test]
fn precedence_via_rpn() {
    // (1 + 2) * 4 + 5 - 3  =  14
    assert_eq!(
        eval_rpn(&["5", "1", "2", "+", "4", "*", "+", "3", "-"]),
        Ok(14.0)
    );
}

#[test]
fn err_too_few_operands() {
    assert!(eval_rpn(&["1", "+"]).is_err());
    assert!(eval_rpn(&["+"]).is_err());
}

#[test]
fn err_leftover_operands() {
    assert!(eval_rpn(&["1", "2"]).is_err());
    assert!(eval_rpn(&["1", "2", "3", "+"]).is_err());
}

#[test]
fn err_empty_input() {
    assert!(eval_rpn(&[]).is_err());
}

#[test]
fn err_unknown_token() {
    assert!(eval_rpn(&["1", "foo", "+"]).is_err());
    assert!(eval_rpn(&["1", "2", "%"]).is_err());
}

#[test]
fn err_division_by_zero() {
    assert!(eval_rpn(&["1", "0", "/"]).is_err());
}

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.

```rust
pub fn eval_rpn(tokens: &[&str]) -> Result<f64, String> {
    let mut stack: Vec<f64> = Vec::new();

    for token in tokens {
        match token {
            "+" => {
                if stack.len() < 2 {
                    return Err("too few operands".to_string());
                }
                let b = stack.pop().unwrap();
                let a = stack.pop().unwrap();
                stack.push(a + b);
            }
            "-" => {
                if stack.len() < 2 {
                    return Err("too few operands".to_string());
                }
                let b = stack.pop().unwrap();
                let a = stack.pop().unwrap();
                stack.push(a - b);
            }
            "*" => {
                if stack.len() < 2 {
                    return Err("too few operands".to_string());
                }
                let b = stack.pop().unwrap();
                let a = stack.pop().unwrap();
                stack.push(a * b);
            }
            "/" => {
                if stack.len() < 2 {
                    return Err("too few operands".to_string());
                }
                let b = stack.pop().unwrap();
                let a = stack.pop().unwrap();
                if b == 0.0 {
                    return Err("division by zero".to_string());
                }
                stack.push(a / b);
            }
            _ => {
                match token.parse::<f64>() {
                    Ok(value) => stack.push(value),
                    Err(_) => return Err("unknown token".to_string()),
                }
            }
        }
    }

    if stack.is_empty() {
        return Err("leftover operands".to_string());
    }

    if stack.len() > 1 {
        return Err("leftover operands".to_string());
    }

    Ok(stack.pop().unwrap())
}
```

The task is to implement a Reverse Polish Notation (RPN) evaluator in Rust. Let me break down the requirements:

1. Function signature: `pub fn eval_rpn(tokens: &[&str]) -> Result<f64, String>`
2. Tokens can be:
   - Numbers that parse as `f64`
   - Binary operators: `+`, `-`, `*`, `/`
3. Evaluation uses a stack:
   - Push numbers onto the stack
   - For operators, pop two values (b then a), compute `a OP b`, push result
4. Error conditions:
   - Too few operands (operator with < 2 values on stack)
   - Leftover operands (> 1 value remains, or 0 tokens/empty input)
   - Unknown token (not a valid f64 or known operator)
   - Division by zero
5. Use only standard library

Let me write the implementation:

```rust
pub fn eval_rpn(tokens: &[&str]) -> Result<f64, String> {
    let mut stack: Vec<f64> = Vec::new();
    
    for token in tokens {
        match token {
            "+" => {
                if stack.len() < 2 {
                    return Err("too few operands".to_string());
                }
                let b = stack.pop().unwrap();
                let a = stack.pop().unwrap();
                stack.push(a + b);
            }
            "-" => {
                if stack.len() < 2 {
                    return Err("too few operands".to_string());
                }
                let b = stack.pop().unwrap();
                let a = stack.pop().unwrap();
                stack.push(a - b);
            }
            "*" => {
                if stack.len() < 2 {
                    return Err("too few operands".to_string());
                }
                let b = stack.pop().unwrap();
                let a = stack.pop().unwrap();
                stack.push(a * b);
            }
            "/" => {
                if stack.len() < 2 {
                    return Err("too few operands".to_string());
                }
                let b = stack.pop().unwrap();
                let a = stack.pop().unwrap();
                if b == 0.0 {
                    return Err("division by zero".to_string());
                }
                stack.push(a / b);
            }
            _ => {
                // Try to parse as f64
                match token.parse::<f64>() {
                    Ok(value) => stack.push(value),
                    Err(_) => return Err("unknown token".to_string()),
                }
            }
        }
    }
    
    if stack.len() != 1 {
        return Err("leftover operands".to_string());
    }
    
    Ok(stack.pop().unwrap())
}
```

I'm carefully considering the error handling and stack operations. The implementation needs to handle various edge cases precisely, ensuring correct RPN evaluation while catching potential errors.

The key focus is on maintaining the stack's integrity and providing clear error messages for different input scenarios. Each operator requires careful stack manipulation and validation.

The function will systematically process tokens, pushing numeric values and performing arithmetic operations while tracking potential errors like insufficient operands or invalid tokens.

I'll verify the implementation handles edge cases like empty input, division by zero, and unknown tokens correctly. The stack-based approach ensures proper order of operations for Reverse Polish Notation evaluation.

The key validation checks include ensuring exactly one result remains after processing all tokens, preventing leftover operands or incomplete calculations. Each operator requires two operands, and the function will return appropriate error messages for invalid inputs.

[
  {
    "total": 1,
    "answer": "```rust\npub fn eval_rpn(tokens: &[&str]) -> Result<f64, String> {\n    let mut stack: Vec<f64> = Vec::new();\n\n    for token in tokens {\n        match token {\n            \"+\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a + b);\n            }\n            \"-\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a - b);\n            }\n            \"*\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a * b);\n            }\n            \"/\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                if b == 0.0 {\n                    return Err(\"division by zero\".to_string());\n                }\n                stack.push(a / b);\n            }\n            _ => {\n                match token.parse::<f64>() {\n                    Ok(value) => stack.push(value),\n                    Err(_) => return Err(\"unknown token\".to_string()),\n                }\n            }\n        }\n    }\n\n    if stack.is_empty() {\n        return Err(\"leftover operands\".to_string());\n    }\n\n    if stack.len() > 1 {\n        return Err(\"leftover operands\".to_string());\n    }\n\n    Ok(stack.pop().unwrap())\n}\n```",
    "passed": 0,
    "reasoning": "The task is to implement a Reverse Polish Notation (RPN) evaluator in Rust. Let me break down the requirements:\n\n1. Function signature: `pub fn eval_rpn(tokens: &[&str]) -> Result<f64, String>`\n2. Tokens can be:\n   - Numbers that parse as `f64`\n   - Binary operators: `+`, `-`, `*`, `/`\n3. Evaluation uses a stack:\n   - Push numbers onto the stack\n   - For operators, pop two values (b then a), compute `a OP b`, push result\n4. Error conditions:\n   - Too few operands (operator with < 2 values on stack)\n   - Leftover operands (> 1 value remains, or 0 tokens/empty input)\n   - Unknown token (not a valid f64 or known operator)\n   - Division by zero\n5. Use only standard library\n\nLet me write the implementation:\n\n```rust\npub fn eval_rpn(tokens: &[&str]) -> Result<f64, String> {\n    let mut stack: Vec<f64> = Vec::new();\n    \n    for token in tokens {\n        match token {\n            \"+\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a + b);\n            }\n            \"-\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a - b);\n            }\n            \"*\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a * b);\n            }\n            \"/\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                if b == 0.0 {\n                    return Err(\"division by zero\".to_string());\n                }\n                stack.push(a / b);\n            }\n            _ => {\n                // Try to parse as f64\n                match token.parse::<f64>() {\n                    Ok(value) => stack.push(value),\n                    Err(_) => return Err(\"unknown token\".to_string()),\n                }\n            }\n        }\n    }\n    \n    if stack.len() != 1 {\n        return Err(\"leftover operands\".to_string());\n    }\n    \n    Ok(stack.pop().unwrap())\n}\n```\n\nI'm carefully considering the error handling and stack operations. The implementation needs to handle various edge cases precisely, ensuring correct RPN evaluation while catching potential errors.\n\nThe key focus is on maintaining the stack's integrity and providing clear error messages for different input scenarios. Each operator requires careful stack manipulation and validation.\n\nThe function will systematically process tokens, pushing numeric values and performing arithmetic operations while tracking potential errors like insufficient operands or invalid tokens.\n\nI'll verify the implementation handles edge cases like empty input, division by zero, and unknown tokens correctly. The stack-based approach ensures proper order of operations for Reverse Polish Notation evaluation.\n\nThe key validation checks include ensuring exactly one result remains after processing all tokens, preventing leftover operands or incomplete calculations. Each operator requires two operands, and the function will return appropriate error messages for invalid inputs.\n",
    "test_error": "error[E0308]: mismatched types\n --> src/lib.rs:6:13\n  |\n5 |         match token {\n  |               ----- this expression has type `&&str`\n6 |             \"+\" => {\n  |             ^^^ expected `&&str`, found `&str`\n  |\n  = note: expected reference `&&_`\n             found reference `&'static _`\nhelp: consider dereferencing to access the inner value using the `Deref` trait\n  |\n5 |         match *token {\n  |               +\n\nerror[E0308]: mismatched types\n  --> src/lib.rs:14:13\n   |\n 5 |         match token {\n   |               ----- this expression has type `&&str`\n...\n14 |             \"-\" => {\n   |             ^^^ expected `&&str`, found `&str`\n   |\n   = note: expected reference `&&_`\n              found reference `&'static _`\nhelp: consider dereferencing to access the inner value using the `Deref` trait\n   |\n 5 |         match *token {\n   |               +\n\nerror[E0308]: mismatched types\n  --> src/lib.rs:22:13\n   |\n 5 |         match token {\n   |               ----- this expression has type `&&str`\n...\n22 |             \"*\" => {\n   |             ^^^ expected `&&str`, found `&str`\n   |\n   = note: expected reference `&&_`\n              found reference `&'static _`\nhelp: consider dereferencing to access the inner value using the `Deref` trait\n   |\n 5 |         match *token {\n   |               +\n\nerror[E0308]: mismatched types\n  --> src/lib.rs:30:13\n   |\n 5 |         match token {\n   |               ----- this expression has type `&&str`\n...\n30 |             \"/\" => {\n   |             ^^^ expected `&&str`, found `&str`\n   |\n   = note: expected reference `&&_`\n              found reference `&'static _`\nhelp: consider dereferencing to access the inner value using the `Deref` trait\n   |\n 5 |         match *token {\n   |               +\n\nFor more information about this error, try `rustc --explain E0308`.\nerror: could not compile `challenge` (lib test) due to 4 previous errors\nerror: could not compile `challenge` (lib) due to 4 previous errors"
  },
  {
    "total": 8,
    "answer": "```rust\npub fn eval_rpn(tokens: &[&str]) -> Result<f64, String> {\n    let mut stack: Vec<f64> = Vec::new();\n\n    for token in tokens {\n        match *token {\n            \"+\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a + b);\n            }\n            \"-\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a - b);\n            }\n            \"*\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a * b);\n            }\n            \"/\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                if b == 0.0 {\n                    return Err(\"division by zero\".to_string());\n                }\n                stack.push(a / b);\n            }\n            _ => {\n                match token.parse::<f64>() {\n                    Ok(value) => stack.push(value),\n                    Err(_) => return Err(\"unknown token\".to_string()),\n                }\n            }\n        }\n    }\n\n    if stack.is_empty() || stack.len() > 1 {\n        return Err(\"leftover operands\".to_string());\n    }\n\n    Ok(stack.pop().unwrap())\n}\n```",
    "passed": 8,
    "reasoning": "The error is clear - `token` is of type `&&str` (because it's `&[&str]` and we're iterating over it), but the match arms expect `&str`. I need to dereference `token` once to get `&str`.\n\nLet me fix this by changing `match token` to `match *token`.\n\nAlso, I should check the error message for \"leftover operands\" - the problem says \"more than one value remains after all tokens consumed (or zero tokens / empty input, which leaves no value)\". So I need to handle both cases:\n- Empty input (stack is empty)\n- More than one value remains\n\nBoth should return \"leftover operands\" error.\n\nLet me write the corrected code:\n\n```rust\npub fn eval_rpn(tokens: &[&str]) -> Result<f64, String> {\n    let mut stack: Vec<f64> = Vec::new();\n\n    for token in tokens {\n        match *token {\n            \"+\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a + b);\n            }\n            \"-\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a - b);\n            }\n            \"*\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                stack.push(a * b);\n            }\n            \"/\" => {\n                if stack.len() < 2 {\n                    return Err(\"too few operands\".to_string());\n                }\n                let b = stack.pop().unwrap();\n                let a = stack.pop().unwrap();\n                if b == 0.0 {\n                    return Err(\"division by zero\".to_string());\n                }\n                stack.push(a / b);\n            }\n            _ => {\n                match token.parse::<f64>() {\n                    Ok(value) => stack.push(value),\n                    Err(_) => return Err(\"unknown token\".to_string()),\n                }\n            }\n        }\n    }\n\n    if stack.is_empty() || stack.len() > 1 {\n        return Err(\"leftover operands\".to_string());\n    }\n\n    Ok(stack.pop().unwrap())\n}\n```\n\nI'll complete the division operation by handling the zero check and pushing the result. The final validation ensures exactly one value remains on the stack.\n",
    "test_error": ""
  }
]