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::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();
                let res = match token {
                    "+" => a + b,
                    "-" => a - b,
                    "*" => a * b,
                    "/" => {
                        if b == 0.0 {
                            return Err("division by zero".to_string());
                        }
                        a / b
                    }
                    _ => unreachable!(),
                };
                stack.push(res);
            }
            _ => match token.parse::<f64>() {
                Ok(n) => stack.push(n),
                Err(_) => return Err(format!("unknown token: {}", token)),
            },
        }
    }

    if stack.len() == 1 {
        Ok(stack.pop().unwrap())
    } else {
        Err("leftover operands".to_string())
    }
}
```

The user wants me to implement a Rust function `eval_rpn` that evaluates Reverse Polish Notation expressions.
The function signature is:
```rust
pub fn eval_rpn(tokens: &[&str]) -> Result<f64, String>
```

Requirements:
- Input: slice of string slices `&[&str]`
- Output: `Result<f64, String>`
- Stack-based evaluation
- Tokens are either `f64` parsable numbers or `+`, `-`, `*`, `/`
- For operators, pop `b` (top), then `a` (next), push `a OP b`
- After processing, exactly one value must remain on the stack.
- Error conditions:
  - Too few operands (operator with < 2 values on stack)
  - Leftover operands (> 1 value after processing, or 0 values for empty input)
  - Unknown token (not a valid f64 or known operator)
  - Division by zero (`b == 0.0`)
- Use only standard library.
- The library will be named `challenge` in tests, so I just need to provide `src/lib.rs`.

Let's design 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();
                let res = match *token {
                    "+" => a + b,
                    "-" => a - b,
                    "*" => a * b,
                    "/" => {
                        if b == 0.0 {
                            return Err("division by zero".to_string());
                        }
                        a / b
                    }
                    _ => unreachable!()
                };
                stack.push(res);
            }
            _ => {
                match token.parse::<f64>() {
                    Ok(num) => stack.push(num),
                    Err(_) => return Err(format!("unknown token: {}", token)),
                }
            }
        }
    }
    
    if stack.len() == 1 {
        Ok(stack.pop().unwrap())
    } else {
        Err("leftover operands".to_string())
    }
}
```

Wait, should I handle empty input? The spec says: "leftover operands — more than one value remains after all tokens consumed (or zero tokens / empty input, which leaves no value)."
My code checks `stack.len() == 1`, so if `tokens` is empty, `stack.len()` is 0, which returns `Err("leftover operands".to_string())`. That matches the spec.

Let's double check the error messages. The spec says "any non-empty message", so exact wording doesn't matter much, but I'll keep it descriptive.

One minor point: `token.parse::<f64>()` might fail for strings like "1.0.0" or "foo". That's correctly caught.
What about whitespace? The tokens are given as `&str`, presumably already split. We don't need to trim.
What about negative numbers? `"-5"` parses as `-5.0` correctly.
What about `+5`? Parses as `5.0`.
The spec says tokens are either numbers or `+`, `-`, `*`, `/`. So `+` and `-` are operators. If a token is `+`, it matches the operator arm. If it's `+5`, it goes to the `_` arm and parses as `5.0`. That's fine.

Let's consider edge cases:
- `["1", "2", "+", "3"]` -> stack has `[3.0, 3.0]` -> len 2 -> Err. Correct.
- `["1", "0", "/"]` -> b=0.0 -> Err. Correct.
- `["+"]` -> stack len 0 -> Err. Correct.
- `[]` -> stack len 0 -> Err. Correct.

One thing: `token.parse::<f64>()` might accept things like `inf`, `nan`. The spec says "a number that parses as f64", so it's fine. If tests expect strict numeric parsing, `parse` is standard. I'll stick with `parse`.

Wait, should I return specific error messages? The spec says "any non-empty message". So it's fine.

Let's structure the code properly for `src/lib.rs`.

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

This looks solid. I'll verify against the examples:
- `eval_rpn(&["2", "3", "+"])` -> stack: [2, 3] -> op + -> [5] -> Ok(5.0)
- `eval_rpn(&["5", "1", "2", "+", "4", "*", "+", "3", "-"])` -> 
  5, 1, 2 -> + -> 5, 3 -> *4 -> 5, 12 -> + -> 17 -> -3 -> 14. Ok(14.0)
- `eval_rpn(&["3", "4", "-"])` -> 3, 4 -> - -> -1. Ok(-1.0)
- `eval_rpn(&["1", "+"])` -> stack [1], op + -> len < 2 -> Err
- `eval_rpn(&["1", "2"])` -> stack [1, 2] -> len != 1 -> Err
- `eval_rpn(&["1", "foo", "+"])` -> foo parse fails -> Err
- `eval_rpn(&["1", "0", "/"])` -> b=0.0 -> Err

All match.

One detail: `token.parse::<f64>()` might be considered too permissive if tests use strict number formats, but typically in Rust challenges `parse` is expected. I'll stick with it.

Could there be an issue with `+` or `-` being parsed as numbers if they appear as tokens? The match arm checks for exact `+`, `-`, `*`, `/` first, so they are treated as operators. That's correct.

What about `0.0` vs `-0.0`? `b == 0.0` handles both because `-0.0 == 0.0` is true in IEEE 754. So division by `-0.0` will also trigger the error. That's fine.

The code is straightforward. I will wrap it in the requested format.

One minor po
…[4655 chars elided]…
vide the code block.
All good. 
Output matches. 
Done. 
[Final Check of the Prompt]
"Return `Ok(value)` on success, or `Err(message)` (any non-empty message) on malformed input"
"Use only the standard library."
"Tests live in `tests/` and call it as `challenge::eval_rpn`."
Everything is consistent.
Proceeds. 
[Output Generation] (matches the refined code)
```rust
pub fn eval_rpn(tokens: &[&str]) -> Result<f64, String> {
    let mut stack = 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();
                let res = match token {
                    "+" => a + b,
                    "-" => a - b,
                    "*" => a * b,
                    "/" => {
                        if b == 0.0 {
                            return Err("division by zero".to_string());
                        }
                        a / b
                    }
                    _ => unreachable!(),
                };
                stack.push(res);
            }
            _ => match token.parse::<f64>() {
                Ok(n) => stack.push(n),
                Err(_) => return Err(format!("unknown token: {}", token)),
            },
        }
    }

    if stack.len() == 1 {
        Ok(stack.pop().unwrap())
    } else {
        Err("leftover operands".to_string())
    }
}
```
Looks perfect. Output matches. 
Proceeds. 
[Done] 
(Self-Correction/Note during thought)
Wait, should I handle `tokens` being empty explicitly? The spec says "zero tokens / empty input, which leaves no value" is an error. My code returns `Err("leftover operands".to_string())` for empty input because `stack.len() == 0 != 1`. That's correct and matches the spec's grouping of this case under "leftover operands". I'll keep it.
All good.
Output matches.
Proceeds. 
[Final Output Generation] -> *Proceeds*