Arrays and Lists

 Properties ◦ Contents are stored in contiguous memory locations ◦ Elements of the array are of the same type  (Perl allows for different types, but then Perl’s strange that way) ◦ Direct access of elements possible using a cardinal indexer  Index is used as the “offset” from the beginning of the array ◦ Operations  Allocation  Accessing

 C# ◦ Initial declaration makes a “null” value array  bool [] booleanArray;  arrayName = new arrayType[allocationSize]; ◦ Contiguous block of memory is allocated  Which is why the allocation size is required up front  Adding elements later cannot extend this memory “block”  Each element in the array is of the “type”  bool or int would be elements of that type value  FileInfo is a reference, so the elements would be of that type, which are references to the actual objects, and not the object per se bool [] booleanArray; FileInfo [] files; booleanArray = new bool[10]; files = new FileInfo[10];

// Read an array element bool b = booleanArray[7]; // Write to an array element booleanArray[0] = false;  Running time is O(1) ◦ Constant, regardless of how many elements are stored  In reality  Managed code  Lookup will take extra running time for integrity (index- bounds) check of element costing performance  Possible to use unmanaged code for access

 Make a larger array and then copy the old over array to the new array ◦ Unassigned values in the new array have the default “0” value ◦ Don’t forget that the old array is still allocated in memory // Create an integer array with three elements int [] fib = new int[3]; fib[0] = 1; fib[1] = 1; fib[2] = 2; // Redimension message to a 10 element array int [] temp = new int[10]; // Copy the fib array to temp fib.CopyTo(temp, 0); // Assign temp to fib fib = temp;

 Good for ◦ storing homogeneous data types ◦ Only accessing them directly  Searching unsorted array ◦ Linear running time  If storing large arrays which are searched frequently ◦ Consider other structures  Sorted array ◦ Binary search can be used to search the array  Running time is O(log N)  Array class has a BinarySearch() method  Multidimensional arrays ◦ Runtime is O(N k ) where “k” is the number of dimension

 Customization of a structure for application ◦ Process array of class Employee with its methods and properties  might want extra functionality not present in array or in order to resize the array if needed  Create a custom data structure with of Employee  Or.. The array can be of type object  This can store any data type (all types derive from object in.Net Framework) .Net Framework has such a structure  System.Collections.ArrayList class  Internal object array  When reading value from ArrayList, cast it to data type being stored

 Flexibility is obtained at the expense of performance ◦ Each array element is a reference to an object

 Performance cost due to indirection and boxing/unboxing of data ◦ Boxing is the conversion a value type to a reference type  Allocate memory from heap  Amount of memory is size needed by the value type plus additional overhead to make this an object  Pointer to virtual method table and pointer to a ‘sync block’  Copy value type’s bit to heap  Return address of the object (reference type) ◦ Unboxing  Retrieve a reference to the value type in an object  CLR checks reference type variable is not null and boxed value of the type desired  Pointer to the value is returned  Array of objects ◦ Potential run time errors  Wrong data type is added to an element at run time  This is not caught at compile time  Bug won’t be caught until testing or actual production run  Typing problem with ArrayList is fixed in.Net Framework with Generics

public class TypeSafeList { T[] innerArray = new T[0]; int currentSize = 0; int capacity = 0; public void Add(T item) { // see if array needs to be resized if (currentSize == capacity) { // resize array capacity = capacity == 0 ? 4 : capacity * 2; // double capacity T[] copy = new T[capacity]; // create newly sized array Array.Copy(innerArray, copy, currentSize); // copy over the array innerArray = copy; // assign innerArray to the new, larger array } innerArray[currentSize] = item; currentSize++; } Type identifier (T) is defined. Requires developer to specify a SINGLE type (aliased as “T”).

public T this[int index] { get { if (index = currentSize) throw new IndexOutOfRangeException(); return innerArray[index]; } set { if (index = currentSize) throw new IndexOutOfRangeException(); innerArray[index] = value; } public override string ToString() { string output = string.Empty; for (int i = 0; i < currentSize - 1; i++) output += innerArray[i] + ", “; return output + innerArray[currentSize - 1]; }

TypeSafeList variableName;  Sample code TypeSafeList fib = new TypeSafeList (); fib.Add(1); for (int i = 2; i < 25; i++) fib.Add(fib[i - 2] + fib[i - 1]); Console.WriteLine(fib.ToString());

 Benefits ◦ Type safety  Can only add elements of “type” declare or derived from declared type  Adding a string to the “fib” example would result in a compile time error rather than run time error ◦ Performance  No type checking required at run time  Boxing/unboxing is eliminated ◦ Reusable  Break the coupling between the data structure and the specific application for which it was created

 Homogeneous array  Self-redimensioning array  List is a “wrapper” for array ◦ Provides read/write access to array ◦ Automatically resizes the array as required ◦ Utilizes Generics  Type is specified at development time  Specified in declaration and instantiation of List  No size is required in declaration  Can specify a default starting size in the constructor  Can use the List.Capacity property to specify  Add() method is used to add an item  Items accessible via an ordinal index

// Create a List of integers List myFavoriteIntegers = new List (); // Create a list of strings List friendsNames = new List ();  Sample // Create a List of integers List powersOf2 = new List (); // Add 6 integers to the List powersOf2.Add(1); powersOf2.Add(2); powersOf2.Add(4); powersOf2.Add(8); powersOf2.Add(16); powersOf2.Add(32); // Change the 2nd List item to 10 powersOf2[1] = 10; // Compute 2^3 + 2^4 int sum = powersOf2[2] + powersOf2[3];

 To find an element in an array, you must process each element in a loop ◦ unless the array is sorted  List has ◦ Contains() method ◦ IndexOf() method ◦ BinarySearch() method ◦ Find() ◦ FindAll() ◦ Sort() ◦ ConvertAll()  Running time of the List’s operations is the same as the standard array’s running time