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Pass by value or by reference

What a function really receives: a copy or access to the caller's data. Aliases, side effects on a list passed as a parameter, mutable and immutable types, input and output parameters.

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  • Pass by value: what is it?

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    A way of passing arguments where the function receives a copy of the argument: changing the parameter changes nothing for the caller.

    This is how C works: for a function to change a variable of the caller, you pass it the variable's address.

  • What does this program print?

    ```python
    def add_one(n):
    n = n + 1
    return n

    x = 5
    y = add_one(x)
    print(x, y)
    `

    • 5 6
    • 6 6
    • 5 5
    • 6 5
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    5 6

    n = n + 1 makes the local name n refer to a new integer, 6: x still refers to 5. Only the returned value, stored in y, leaves the function.

  • An … parameter is only read by the function; an … parameter lets it pass a result to the caller; an input-output parameter is read then modified.

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    An input parameter is only read by the function; an output parameter lets it pass a result to the caller; an input-output parameter is read then modified.

    In Python, a list passed to a function that sorts it in place is an input-output parameter; an integer can only be an input, and the result comes back through return.

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