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Arrays as parameters of functions

In passing an array as a parameter to a function it is passed as a reference parameter. What is actually passed is the address of its first element. Since arrays are passed by reference this means that if the function changes the value of an element in an array that is a parameter of the function then the corresponding actual array of the call will have that element changed.

Though an array is passed as a reference parameter an & is not used to denote a reference parameter. However it must be indicated to the compiler that this parameter is an array by appending [] to the formal parameter name. Thus to declare an array of real values as a parameter requires the parameter to be specified as follows:

..., float A[],...

This is the same as a normal array declaration but the size of the array is not specified. This is illustrated in the following example which returns the average value of the first n elements in a real array.

float meanarray(int n,        // IN no of elements
                float A[])    // IN array parameter
     // This function returns the average value of
     // the first n elements in the array A which
     // is assumed to have >= n elements.
  {
    float sum = 0.0;     // local variable to
                         // accumulate sum
    int i;               // local loop control
    for (i = 0; i < n; i++)
        sum += A[i];

    return sum/n;
  }  // end meanarray
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If when this function was called the value given for the parameter n was greater than the number of elements in the actual array replacing the parameter A then an incorrect result would be returned.
The function meanarray could be used as follows:

const int NE = 100;
float average, data[NE];
int i, m;

cout << "Enter no of data items (no more than "
     << NE << "): ";
cin >> m;

for (i = 0; i < m; i++)
   cin >> data[i];

average = meanarray(m, data);

An array can also be an output parameter, consider the following example in which the function addarray adds two arrays together to produce a third array whose elements are the sum of the corresponding elements in the original two arrays.

void addarray(int size,         // IN size of arrays
              const float A[],  // IN input array
              const float B[],  // IN input array
              float C[])        // OUT result array

     // Takes two arrays of the same size as input
     // parameters and outputs an array whose elements
     // are the sum of the corresponding elements in
     // the two input arrays.

  {
    int i;     // local control variable
    for (i = 0; i < size; i++)
         C[i] = A[i] + B[i];

  } // End of addarray
Download program.
The function addarray could be used as follows:
float one[50], two[50], three[50];
           .
           .
addarray(20, one, two, three);

Note that the parameter size could have been replaced with any value up to the size that was declared for the arrays that were used as actual parameters. In the example above the value of 20 was used which means that only the first 20 elements of the array three are set.

Also note that the input parameters A and B have been declared in the function head as being of type const float. Since they are input parameters they should not be changed by the function and declaring them as constant arrays prevents the function from changing them.


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