4.3 Calling the Array Example

To call the function avg_of_double requires making an array and then passing it as an argument. Here is an example.

{
  /* The array of values to average.  */
  double nums_to_average[5];
  /* The average, once we compute it.  */
  double average;

  /* Fill in elements of nums_to_average.  */

  nums_to_average[0] = 58.7;
  nums_to_average[1] = 5.1;
  nums_to_average[2] = 7.7;
  nums_to_average[3] = 105.2;
  nums_to_average[4] = -3.14159;

  average = avg_of_double (5, nums_to_average);

  /* …now make use of average… */
}

This shows an array subscripting expression again, this time on the left side of an assignment, storing a value into an element of an array.

It also shows how to declare a local variable that is an array: double nums_to_average[5];. Since this declaration allocates the space for the array, it needs to know the array’s length. You can specify the length with any expression whose value is an integer, but in this declaration the length is a constant, the integer 5.

The name of the array, when used by itself as an expression, stands for the address of the array’s data, and that’s what gets passed to the function avg_of_double in avg_of_double (5, nums_to_average).

We can make the code easier to maintain by avoiding the need to write 5, the array length, when calling avg_of_double. That way, if we change the array to include more elements, we won’t have to change that call. One way to do this is with the sizeof operator:

  average = avg_of_double ((sizeof (nums_to_average)
                            / sizeof (nums_to_average[0])),
                           nums_to_average);

This computes the number of elements in nums_to_average by dividing its total size by the size of one element. See Type Size, for more details of using sizeof.

We don’t show in this example what happens after storing the result of avg_of_double in the variable average. Presumably more code would follow that uses that result somehow. (Why compute the average and not use it?) But that isn’t part of this topic.