Sorting/Searching and File I/O

1
Sorting/Searching and File I/O

• Sorting
• Searching
• CLI File Input
• CLI File Output
• GUI File Chooser
• Reading for this lecture LL 10.4-10.5, 10.8

2
Sorting

• Sorting is the process of arranging a list of
  items in a particular order
• The sorting process is based on specific value(s)
• Sorting a list of test scores in ascending
  numeric order
• Sorting a list of people alphabetically by last
  name
• There are many algorithms, which vary in
  efficiency, for sorting a list of items
• We will examine two specific algorithms
• Selection Sort
• Insertion Sort

3
Selection Sort

• The approach of Selection Sort
• Select a value and put it in its final place in
  the list
• Repeat for all other values
• In more detail
• Find the smallest value in the list
• Switch it with the value in the first position
• Find the next smallest value in the list
• Switch it with the value in the second position
• Repeat until all values are in their proper places

4
Selection Sort

• An example
• original 3 9 6 1 2
• smallest is 1 1 9 6 3 2
• smallest is 2 1 2 6 3 9
• smallest is 3 1 2 3 6 9
• smallest is 6 1 2 3 6 9
• Each time, the smallest remaining value is found
  and exchanged with the element in the "next"
  position to be filled

5
Swapping Two Values

• The processing of the selection sort algorithm
  includes the swapping of two values
• Swapping requires three assignment statements and
  a temporary storage location of the same type as
  the data being swapped
• int first 1, second 2
• int temp first
• first second // 2 now
• second temp // 1 now

6
Polymorphism in Sorting

• Recall that a class that implements the
  Comparable interface defines a compareTo method
  that returns the relative order of its objects
• We can use polymorphism to develop a generic sort
  for any set of Comparable objects
• The sorting method accepts as a parameter an
  array of Comparable objects
• That way, one method can be used to sort a group
  of People, or Books, or whatever as long as the
  class implements Comparable

7
Selection Sort

• The sorting method doesn't "care" what type of
  object it is sorting, it just needs to be able to
  call the compareTo method of that object
• That is guaranteed by using Comparable as the
  parameter type passed to the sorting method
• Each Comparable class has a compareTo method that
  determines what it means for one object of that
  class to be less than another
• See PhoneList.java (page 505)
• See Sorting.java (page 506), specifically the
  selectionSort method
• See Contact.java (page 507-508)

8
Insertion Sort

• The approach of Insertion Sort
• Pick any item and insert it into its proper place
  in a sorted sublist
• Repeat until all items have been inserted
• In more detail
• Consider the first item to be a sorted sublist
  (of one item)
• Insert the second item into the sorted sublist,
  shifting the first item as needed to make room to
  insert the new addition
• Insert the third item into the sorted sublist (of
  two items), shifting items as necessary
• Repeat until all values are inserted into their
  proper positions

9
Insertion Sort

• An example
• original 3 9 6 1 2
• insert 9 3 9 6 1 2
• insert 6 3 6 9 1 2
• insert 1 1 3 6 9 2
• insert 2 1 2 3 6 9
• See Sorting.java (page 506-507), specifically the
  insertionSort method

10
Comparing Sorts

• The Selection and Insertion sort algorithms are
  similar in efficiency
• They both have outer loops that scan all
  elements, and inner loops that compare the value
  of the outer loop with almost all values in the
  list
• Approximately n2 number of comparisons are made
  to sort a list of size n
• We therefore say that these sorts are of order n2
• Other sorts are more efficient order n log2 n

11
Searching

• Searching is the process of finding a target
  element within a group of items called the search
  pool
• The target may or may not be in the search pool
• We want to perform the search efficiently,
  minimizing the number of comparisons
• Let's look at two classic searching approaches
  linear search and binary search
• As we did with sorting, we'll implement the
  searches with polymorphic Comparable parameters

12
Linear Search

• A linear search begins at one end of a list and
  examines each element in turn
• Eventually, either the item is found or the end
  of the list is encountered
• See PhoneList2.java (page 512-513)
• See Searching.java (page 514-515), specifically
  the linearSearch method

13
Binary Search

• A binary search assumes the list of items in the
  search pool is sorted
• It eliminates a large part of the search pool
  with a single comparison
• A binary search first examines the middle element
  of the list -- if it matches the target, the
  search is over
• If it doesn't, only half of the remaining
  elements need be searched
• Since they are sorted, the target can only be in
  one half of the other

14
Binary Search

• The process continues by comparing the target to
  the middle element of the remaining viable
  candidates
• Each comparison eliminates approximately half of
  the remaining data
• Eventually, the target is found or there are no
  remaining viable candidates (and the target has
  not been found)
• See PhoneList2.java (page 512-513)
• See Searching.java (page 514-515), specifically
  the binarySearch method

15
Binary Versus Linear Search

• The efficiency of binary search is good for the
  retrieval of data from a sorted group
• However, the group must be sorted initially
• As items are added to the group, it must be kept
  in sorted order
• The sorting process creates inefficiency
• If you add data to a group much more often than
  you search it, it may be worse to use a binary
  search than a linear search

16
CLI File Input

• In a CLI, we want the user to select a file
  within a directory system so that its contents
  can be read and processed
• However, we must rely on the user typing in the
  file name (including any required path name)
• We can get the file name via a Scanner on
  System.in using the nextLine method
• We can read the file data via a Scanner on a File
  object using the nextLine method again

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CLI File Input Example

• import java.util.Scanner
• import java.io.
• public class FileDisplay
• public static void main (String args)
• throws IOException
• Scanner scan new Scanner(System.in)
• System.out.println("Enter name of file to
  display")
• File file new File(scan.nextLine())
• scan new Scanner (file) // done with
  keyboard
• while (scan.hasNext()) // ctl-D returns
  false
• System.out.println(scan.nextLine())

18
CLI File Output

• In a CLI, we want the user to create a file
  within a directory system so that its contents
  can be written (or overwritten!)
• Be careful Your code should check for a file by
  that name and ask user if OK to overwrite it.
• Again, we rely on the user typing in the file
  name
• Again, we can get the file name via a Scanner on
  System.in using the nextLine method
• We can write the file data via a PrintStream on a
  File object using the println method (System.out
  is a PrintStream object)

19
CLI File Output Example

• import java.util.Scanner
• import java.io.
• public class FileWrite
• public static void main (String args) throws
  IOException
• // Get filename and instantiate File object
  as before
• PrintStream out new PrintStream(file)
• // Use ctl-D to close System.in
• // so scan.hasNext() will return false
• while (scan.hasNext())
• String line scan.nextLine()
• out.println(line)
• out.close()

20
GUI File I/O

• In a GUI, requiring the user to enter a file name
  (including a path name or not) is considered to
  be NOT very user friendly
• We want our program to offer a choice of the
  available files so that the user can
• Move around within the available directories
• Select one of the files shown in a directory

21
File Chooser in GUIs

• Recall that a dialog box is a small window that
  "pops up" to interact with the user for a brief,
  specific purpose
• A file chooser, the JFileChooser class, supports
  a simple dialog box for this process
• See DisplayFile.java (page 521)

22
Example DisplayFile code segment

• JFileChooser chooser new JFileChooser()
• int status chooser.showOpenDialog(frame)
• // There is also a showSaveDialog(frame)
• if (status ! JFileChooser.APPROVE_OPTION)
• ta.setText ("No File Chosen")
• else
• // read file
• File file chooser.getSelectedFile()
• Scanner scan new Scanner (file)
• ...

23
File Input/Output

• Notice that the main method in all three of these
  examples indicates that the code may throw an
  IOException
• If an error such as file not found occurs
  during a file operation, an IOException is
  generated by the system
• Well study exceptions in the next lecture