Arrays and Functions

Arrays and Functions

• Things to note about C-style arrays:
  • An array is not a type
  • An array is a primitive C-style construct that consists of many items stored consecutively and accessed through a single variable name (and indexing)
  • This is actually done by remembering the starting address of an array, and computing an offset
  • The name of an array acts as a special kind of variable -- a pointer -- which stores the starting address of the array
• An array can be passed into a function as a parameter
  • Because an array is not a single item, the array contents are not passed "by value" as we are used to with normal variables
    • The normal meaning of "pass by value" is that the actual argument value is copied into a local formal parameter variable
    • In the case of arrays, just the pointer is copied as a parameter. We'll see this in more detail when we get to pointers
  • When an array is sent into a function, only its starting address is really sent
  • This means the function will always have access to the actual array sent in
  • Returning an array from a function works similarly, but we need pointers to use them well (not yet covered)
• Example function:

    void PrintArray (int arr[], int size)
    {
      for (int i = 0; i < size; i++)
        cout << arr[i] << ' ';
    }
  

  Note that:
  • The varibale arr acts as the local array name inside the function
  • There is no number in the brackets. int [] indicates that this is an array parameter, for an array of type int
  • It's usually a good idea to pass in the array size as well, as another parameter. This helps make a function work for any size array
• Sample call to the above function:

    int list[5] = {2, 4, 6, 8, 10};
  
    PrintArray(list, 5);  // will print: 2 4 6 8 10
  

Using const with array parameters

• Remember: When passing an array into a function, the function will have access to the contents of the original array!
• Some functions that should change the original array:
  • Sort(), Reverse(), SwapElements()
• What if there are functions that should not alter the array contents?
  • PrintArray(), Sum(), Average()
• Put const in front of the array parameter to guarantee that the array contents will not be changed by the function:

    void PrintArray (const int arr[], const int size)
    {
      for (int i = 0; i < size; i++)
        cout << arr[i] << ' ';
    }
  

  Notice that the const on the variable size is a good idea in this example, too. Why?

Here's an example program illustrating a couple of functions that take array parameters.

// example of arrays, and functions using arrays

#include 
using namespace std;

void PrintArray(const int[] , const int);
void SwapElements(int[] , int, int);

int main()
{
   int list1[5] = {2, 6, 9, -10, 12};  // array of 5 integers
   int i;     // for loop indexing

   int list2[20];   // array size 20, uninitialized


   cout << "\nlist1 = ";
   PrintArray(list1, 5);
   cout << "\n";

   // compute the sum of the list elements
   int total = 0;
   for (i = 0; i < 5; i++)
      total = total + list1[i];
 
   cout << "Sum of list1 elements = " << total << '\n';
   
   // initialize the array to 2, 4, 6, 8, 10, ...
   for (i = 0; i < 20; i++)
      list2[i] = 2 * i + 2;

   cout << "\nlist2 = ";
   PrintArray(list2, 20);  // display the contents
   cout << '\n';

   // create a third list, and get user to enter the data
   int list3[10];
   cout << "Enter 10 integer values: ";
   for (i = 0; i < 10; i++)
      cin >> list3[i];   // user enter the items

   cout << "\nlist3 = ";
   PrintArray(list3, 10);  // display the contents
   cout << '\n';
   
   // find the largest element of list3
   int max = list3[0];   // largest so far
   for (i = 1; i < 10; i++)
      if (list3[i] > max)
          max = list3[i];

   cout << "Largest number in list3 is:  " << max << '\n';

   cout << "Swapping elements 4 and 7\n";
   SwapElements(list3, 4, 7);
   cout << "\nlist3 = ";
   PrintArray(list3, 10);
   cout << "\n\n";

   return 0;
}

void PrintArray(const int arr[], const int size)
{
   int i;

   cout << "{ ";
   for (i = 0; i < size-1; i++)
       cout << arr[i] << ", ";

   cout << arr[size-1] << " }";  // print last item
}

void SwapElements(int arr[] , int a, int b)
// swap arr[a] and arr[b]
{
   int temp;
   temp = arr[a];
   arr[a] = arr[b];
   arr[b] = temp;
}

Practice exercises

Try writing the following functions (each takes in one or more arrays as a parameter) for practice:

• A function called Sum that takes in an array of type double and returns the sum of its elements
• A function called Average that takes in an integer array and returns the average of it's elements (as a double)
• A function called Reverse that takes in an array (any type) and reverses its contents
• A function called Sort that takes in an array of integers and sorts its contents in ascending order
• A function called Minimum that takes in an array of type double and returns the smallest number in the array

---

Examples of some functions written in lecture class, including Largest, Smallest, Sum, Average

// example of arrays, and functions using arrays

#include 
using namespace std;

void PrintArray(const int[] , const int);
void SwapElements(int[] , int, int);
int Sum(const int arr[], const int size);
double Average(const int arr[], const int size);
int Largest(const int arr[], const int size);
int Smallest(const int arr[], const int size);

int main()
{
   int list1[5] = {2, 6, 9, -10, 12};  // array of 5 integers
   int i;     // for loop indexing

   int list2[20];   // array size 20, uninitialized


   cout << "\nlist1 = ";
   PrintArray(list1, 5);
   cout << "\n";

   // compute the sum of the list elements
   cout << "Sum of list1 elements = " << Sum(list1, 5) << '\n';
   cout << "Average of list1 elements = " << Average(list1, 5) << '\n';
   
   // initialize the array to 2, 4, 6, 8, 10, ...
   for (i = 0; i < 20; i++)
      list2[i] = 2 * i + 2;

   cout << "\nlist2 = ";
   PrintArray(list2, 20);  // display the contents
   cout << '\n';

   cout << "Sum of list2 elements = " << Sum(list2, 20) << '\n';
   cout << "Average of list2 elements = " << Average(list2, 20) << '\n';


   // create a third list, and get user to enter the data
   int list3[10];
   cout << "Enter 10 integer values: ";
   for (i = 0; i < 10; i++)
      cin >> list3[i];   // user enter the items

   cout << "\nlist3 = ";
   PrintArray(list3, 10);  // display the contents
   cout << '\n';
   
   // find the largest element of list3
   cout << "Largest number in list1 is:  " << Largest(list1, 5) << '\n';
   cout << "Largest number in list2 is:  " << Largest(list2, 20) << '\n';
   cout << "Largest number in list3 is:  " << Largest(list3, 10) << '\n';

   cout << "Smallest number in list1 is:  " << Smallest(list1, 5) << '\n';
   cout << "Smallest number in list2 is:  " << Smallest(list2, 20) << '\n';
   cout << "Smallest number in list3 is:  " << Smallest(list3, 10) << '\n';

   cout << "Swapping elements 4 and 7\n";
   SwapElements(list3, 4, 7);
   cout << "\nlist3 = ";
   PrintArray(list3, 10);
   cout << "\n\n";

   return 0;
}

void PrintArray(const int arr[], const int size)
{
   int i;

   cout << "{ ";
   for (i = 0; i < size-1; i++)
       cout << arr[i] << ", ";

   cout << arr[size-1] << " }";  // print last item
}

void SwapElements(int arr[] , int a, int b)
// swap arr[a] and arr[b]
{
   int temp;
   temp = arr[a];
   arr[a] = arr[b];
   arr[b] = temp;
}

int Sum(const int arr[], const int size) 
{
   int total = 0;
   for (int i = 0; i < size; i++)
       total = total + arr[i];

   return total;
}

double Average(const int arr[], const int size)
{
   return  Sum(arr, size) * 1.0 / size;
}

int Largest(const int arr[], const int size)
{
   int max = arr[0];    // the answer SO FAR
   for (int i = 1; i < size; i++)
       if (arr[i] > max)
    max = arr[i];

   return max;  // largest number
}

int Smallest(const int arr[], const int size)
{
   int min = arr[0];    // the answer SO FAR
   for (int i = 1; i < size; i++)
       if (arr[i] < min)
    min = arr[i];

   return min;  // largest number
}