Julia – 23 – funzioni – 3

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Funzioni con numero di argomenti variabile (varargs)
It is often convenient to be able to write functions taking an arbitrary number of arguments. Such functions are traditionally known as “varargs” functions, which is short for “variable number of arguments”. You can define a varargs function by following the last argument with an ellipsis:

The variables a and b are bound to the first two argument values as usual, and the variable x is bound to an iterable collection of the zero or more values passed to bar after its first two arguments:

In all these cases, x is bound to a tuple of the trailing values passed to bar.

It is possible to constrain the number of values passed as a variable argument; this will be discussed later in Parametrically-constrained Varargs methods [prossimamente].

On the flip side, it is often handy to “splice” the values contained in an iterable collection into a function call as individual arguments. To do this, one also uses ... but in the function call instead:

In this case a tuple of values is spliced into a varargs call precisely where the variable number of arguments go. This need not be the case, however:

Furthermore, the iterable object spliced into a function call need not be a tuple:

Also, the function that arguments are spliced into need not be a varargs function (although it often is):

As you can see, if the wrong number of elements are in the spliced container, then the function call will fail, just as it would if too many arguments were given explicitly.

Argomenti opzionali
In many cases, function arguments have sensible default values and therefore might not need to be passed explicitly in every call. For example, the library function parse(T, num, base) interprets a string as a number in some base. The base argument defaults to 10. This behavior can be expressed concisely as:

function parse(type, num, base=10)

With this definition, the function can be called with either two or three arguments, and 10 is automatically passed when a third argument is not specified:

Optional arguments are actually just a convenient syntax for writing multiple method definitions with different numbers of arguments (see Note on Optional and keyword Arguments [prossimamente]).


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