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### 4.2 An Example with Arrays

A function to take the average of three numbers is very specific and limited. A more general function would take the average of any number of numbers. That requires passing the numbers in an array. An array is an object in memory that contains a series of values of the same data type. This chapter presents the basic concepts and use of arrays through an example; for the full explanation, see Arrays.

Here’s a function definition to take the average of several floating-point numbers, passed as type `double`. The first parameter, `length`, specifies how many numbers are passed. The second parameter, `input_data`, is an array that holds those numbers.

```double
avg_of_double (int length, double input_data[])
{
double sum = 0;
int i;

for (i = 0; i < length; i++)
sum = sum + input_data[i];

return sum / length;
}
```

This introduces the expression to refer to an element of an array: `input_data[i]` means the element at index `i` in `input_data`. The index of the element can be any expression with an integer value; in this case, the expression is `i`. See Accessing Array Elements.

The lowest valid index in an array is 0, not 1, and the highest valid index is one less than the number of elements. (This is known as zero-origin indexing.)

This example also introduces the way to declare that a function parameter is an array. Such declarations are modeled after the syntax for an element of the array. Just as `double foo` declares that `foo` is of type `double`, `double input_data[]` declares that each element of `input_data` is of type `double`. Therefore, `input_data` itself has type “array of `double`.”

When declaring an array parameter, it’s not necessary to say how long the array is. In this case, the parameter `input_data` has no length information. That’s why the function needs another parameter, `length`, for the caller to provide that information to the function `avg_of_double`.

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