Review: Array

  • Recall:

    • Program = Algorithm + Data Structure

  • Array:

    • The array is one of the fundamental data structures in Computer Science

  • How are arrays used:

    • Arrays are used to organize multiple data of the same type

    • The array size is fixed after creation

    • Arrays provide fast access to the stored data though an index

    • Searching in an unsorted array takes n steps (linear search)

    • Searching in an sorted array takes log(n) steps (binary search)

Adding an element at the end of an array

  • The array size is fixed after its creation

  • To add a new element to the end of an array:

    1. Create a new array with the 1 more element

    2. Copy the elements in the original array tp the new array

    3. Copy the new value in the last element of the new array

    4. Change the array reference to point to the new array

    The code is shown in the next slide

Adding an element at the end of an array

  • Add the value 7 to the array x:

    Algorithm: 

       temp = new int[ x.length + 1 ];

   for ( int i = 0; i < x.length; i++ )
       temp[i] = x[i];

   temp[temp.length-1] = 7;

   x = temp;

Adding an element at the end of an array

  • Add the value 7 to the array x:

    Algorithm: 

       int[] temp = new int[ x.length + 1 ];

   for ( int i = 0; i < x.length; i++ )
       temp[i] = x[i];

   temp[temp.length-1] = 7;

   x = temp;

Adding an element at the end of an array

  • Add the value 7 to the array x:

    Algorithm: 

       int[] temp = new int[ x.length + 1 ];

   for ( int i = 0; i < x.length; i++ )  // Copy
       temp[i] = x[i];

   temp[temp.length-1] = 7;

   x = temp;

Adding an element at the end of an array

  • Add the value 7 to the array x:

    Algorithm: 

       int[] temp = new int[ x.length + 1 ];

   for ( int i = 0; i < x.length; i++ )  // Copy
       temp[i] = x[i];

   temp[temp.length-1] = 7; // Add the new value

   x = temp;

Adding an element at the end of an array

  • Add the value 7 to the array x:

    Algorithm: 

       int[] temp = new int[ x.length + 1 ];

   for ( int i = 0; i < x.length; i++ )  // Copy
       temp[i] = x[i];

   temp[temp.length-1] = 7; // Add the new value

   x = temp;

Adding an element at the end of an array

  • Question: how many data copy statements are executed by this algorithm ?

    Algorithm: 

       int[] temp = new int[ x.length + 1 ];

   for ( int i = 0; i < x.length; i++ )  // Copy
       temp[i] = x[i];

   temp[temp.length-1] = 7; // Add the new value

   x = temp;

Adding an element at the end of an array

  • Question: how many data copy statements are executed by this algorithm ?

    Algorithm: executes   x.length + 1   data copy statements (per addition) 

       int[] temp = new int[ x.length + 1 ];

   for ( int i = 0; i < x.length; i++ )  // Copy
       temp[i] = x[i];       // x.length times

   temp[temp.length-1] = 7; // Add the new value // 1 time

   x = temp;

Deleting the last element from an array

  • The array size is fixed after its creation

  • To delete the last element from an array:

    1. Create a new array with the 1 less element

    2. Copy all except the last elements in the original array to the new array

    3. Change the array reference to point to the new array

    The code is shown in the next slide

Adding an element at the end of an array

  • Delete the last element from the array x:

    Algorithm: 

       temp = new int[ x.length + 1 ];

   for ( int i = 0; i < x.length; i++ )
       temp[i] = x[i];

   temp[temp.length-1] = 7;

   x = temp;

Adding an element at the end of an array

  • Delete the last element from the array x:

    Algorithm: 

       int[] temp = new int[ x.length − 1 ];

   for ( int i = 0; i < x.length; i++ )
       temp[i] = x[i];

   temp[temp.length-1] = 7;

   x = temp;

Adding an element at the end of an array

  • Delete the last element from the array x:

    Algorithm: 

       int[] temp = new int[ x.length − 1 ];

   for ( int i = 0; i < x.length-1; i++ )  // Copy
       temp[i] = x[i];

   temp[temp.length-1] = 7;

   x = temp;

Adding an element at the end of an array

  • Delete the last element from the array x:

    Algorithm: 

       int[] temp = new int[ x.length − 1 ];

   for ( int i = 0; i < x.length-1; i++ )  // Copy
       temp[i] = x[i];

   x = temp;

Adding an element at the end of an array

  • Delete the last element from the array x:

    Algorithm: 

       int[] temp = new int[ x.length − 1 ];

   for ( int i = 0; i < x.length-1; i++ )  // Copy
       temp[i] = x[i];

   x = temp;
   Note: you can delete an element at a different location
         with a similar algorithm

A better way to add and delete elements in arrays

  • Dynamic arrays (a.k.a. ArrayList in Java) consists of:

    1. A (fixed size) array

    2. A count of the actual number of elements stored in the array:

      Schematically:

  • The array is increased only when the add( ) operation encounters a full array

  • The array is reduced when the occupancy drops below a certain threshold