## Turn 1

*   Function: `median(values)`
    *   Current implementation:
        ```python
        def median(values):
            values = sorted(values)
            mid = len(values) // 2
            return values[mid]
        ```
    *   Failing tests:
        1.  `median([3, 1, 2]) == 2` (Current: `sorted([3, 1, 2])` is `[1, 2, 3]`, `mid` is `3 // 2 = 1`, `values[1]` is `2`. This passes.)
        2.  `median([10, 1, 2, 3]) == 2.5` (Current: `sorted([10, 1, 2, 3])` is `[1, 2, 3, 10]`, `mid` is `4 // 2 = 2`, `values[2]` is `3`. This fails. Expected: `(2 + 3) / 2 = 2.5`.)
        3.  `median([]) is None` (Current: `sorted([])` is `[]`, `mid` is `0 // 2 = 0`, `values[0]` raises `IndexError`. This fails. Expected: `None`.)

    *   Handle empty list: Return `None`.
    *   Handle even number of elements: Average the two middle elements.
    *   Handle odd number of elements: Return the middle element.

    ```python
    def median(values):
        if not values:
            return None
        values = sorted(values)
        n = len(values)
        mid = n // 2
        if n % 2 == 0:
            return (values[mid - 1] + values[mid]) / 2
        else:
            return values[mid]
    ```

    *   `median([3, 1, 2])`: `values = [1, 2, 3]`, `n = 3`, `mid = 1`. `n % 2 != 0`, returns `values[1]` which is `2`. (Correct)
    *   `median([10, 1, 2, 3])`: `values = [1, 2, 3, 10]`, `n = 4`, `mid = 2`. `n % 2 == 0`, returns `(values[1] + values[2]) / 2` which is `(2 + 3) / 2 = 2.5`. (Correct)
    *   `median([])`: `if not values` returns `None`. (Correct)

    ```diff
    --- stats.py
    +++ stats.py
    @@ -1,4 +1,8 @@
     def median(values):
    +    if not values:
    +        return None
         values = sorted(values)
    -    mid = len(values) // 2
    -    return values[mid]
    +    n = len(values)
    +    mid = n // 2
    +    if n % 2 == 0:
    +        return (values[mid - 1] + values[mid]) / 2
    +    return values[mid]
    ```

    Wait, let me refine the diff to be cleaner.

    Original:
    ```python
    def median(values):
        values = sorted(values)
        mid = len(values) // 2
        return values[mid]
    ```

    Revised:
    ```python
    def median(values):
        if not values:
            return None
        values = sorted(values)
        n = len(values)
        mid = n // 2
        if n % 2 == 0:
            return (values[mid - 1] + values[mid]) / 2
        return values[mid]
    ```

    Let's double check the diff format.

    ```diff
    --- stats.py
    +++ stats.py
    @@ -1,4 +1,8 @@
     def median(values):
    +    if not values:
    +        return None
         values = sorted(values)
    -    mid = len(values) // 2
    -    return values[mid]
    +    n = len(values)
    +    mid = n // 2
    +    if n % 2 == 0:
    +        return (values[mid - 1] + values[mid]) / 2
    +    return values[mid]
    ```

    Wait, the original code was:
    ```python
    def median(values):
        values = sorted(values)
        mid = len(values) // 2
        return values[mid]
    ```
    Line 1: `def median(values):`
    Line 2: `    values = sorted(values)`
    Line 3: `    mid = len(values) // 2`
    Line 4: `    return values[mid]`

    If I add `if not values: return None` at the top:
    Line 1: `def median(values):`
    Line 2: `    if not values:`
    Line 3: `        return None`
    Line 4: `    values = sorted(values)`
    Line 5: `    n = len(values)`
    Line 6: `    mid = n // 2`
    Line 7: `    if n % 2 == 0:`
    Line 8: `        return (values[mid - 1] + values[mid]) / 2`
    Line 9: `    return values[mid]`

    Diff:
    ```diff
    --- stats.py
    +++ stats.py
    @@ -1,4 +1,9 @@
     def median(values):
    +    if not values:
    +        return None
         values = sorted(values)
    -    mid = len(values) // 2
    -    return values[mid]
    +    n = len(values)
    +    mid = n // 2
    +    if n % 2 == 0:
    +        return (values[mid - 1] + values[mid]) / 2
    +    return values[mid]
    ```

    Check logic again.
    `[1, 2, 3, 10]` -> `n=4`, `mid=2`. `n%2==0` is True. `values[1]` is 2, `values[2]` is 3. `(2+3)/2 = 2.5`. Correct.
    `[1, 2, 3]` -> `n=3`, `mid=1`. `n%2==0` is False. `values[1]` is 2. Correct.
    `[]` -> `if not values` is True. Returns `None`. Correct.