1. Chapter 6 Loops and Files

2. Knowledge Goals Understand the semantics of while loop
Understand when a count-controlled loop is appropriate
Understand when an event-controlled loop is appropriate
Know the difference between an iteration counter and an event counter
Know where nested loops are needed in a problem solution

3. Knowledge Goals
Understand the principles of testing programs that contain loops
Recognize when file input/output is appropriate and how it differs from interactive input/output

4. Skill Goals Construct syntactically correct while loops
Construct count-controlled loops with a while statement
Construct event-controlled loops with a while statement
Use the end-of-file condition to control the input of data
Use flags to control the execution of a while statement

5. Skill Goals Construct counting loops with a while statement
Construct summing loops with a while statement
Write statements to read from a text file
Write statements to write to a text file
Write applications that use data files for input and output

6. Looping Control Flow
Around
and
around …

7. Looping Control Flow Loop
A control structure that causes a statement or group of statements to be executed repeatedly
Count-controlled loop
A loop that executes a specified number of times
Event-controlled loop
A loop that terminates when something happens inside the loop body to signal that the loop should be exited

8. Looping Control Flow

9. Looping Control Flow
See
the
difference ?

10. Looping Control Flow Loop entry
The point at which the flow of control reaches the first statement inside the loop
Iteration
An individual repetition of the body of the loop
Loop test
The point at which the while expression is evaluated and the decision is made to either repeat or exit
Loop exit
The point at which the repetition ends
Termination condition
The condition that causes the loop to be exited

11. Phases of loop execution
Looping Control Flow
loop entry
loop test
each time executed
is an iteration
exited the loop
Expression is false
Phases of loop execution

12. Looping Control Flow Count-controlled loops
A variable keeps track of the number of times the loop is executed
The variable must be initialized outside the loop
It must be tested at the beginning of the loop
It must be incremented within the loop
It is a counter (a variable that is incremented repeatedly)

13. How many times does this loop execute?
Looping Control Flow
int loopCount; // Declare loop variable
loopCount = 1; // Initialize loop variable
while (loopCount <= 10) // Test expression {
. // Repeated actions .
loopCount++; // Update loop variable }
How many times does this loop execute?

14. How many times does this loop execute?
Looping Control Flow
int count; // Declare loop variable
count = 0; // Initialize loop variable
while (count <= 4) // Test expression
{ // Repeated action
System.out.println(“count is “ + count); count ++; // Update loop variable }
System.out.println(“Done”);
How many times does this loop execute?

15. Looping Control Flow Event-controlled loops
An event within the loop controls the repetition
The event must be initialized outside the loop
The event must be tested at the beginning of the loop
The event must be re-evaluated within the loop

16. Looping Control Flow Calculating square root
Eevent: guess is good enough
Initialize: set goodEnough to false
Test: while (!goodEnough)
Re-evaluate set goodEnough to (square - guess*guess) < some threshold

17. Looping Control Flow public class NewMath {
static double squareRoot(double square)
double guess = square/4.0;
boolean goodEnough = false;
while (!goodEnough)
guess = ((square / guess) + guess)/2.0;
goodEnough = Math.abs(square - guess*guess)
< 0.001; }
return guess;

18. Devices used for file storage
File Input/Output
Devices used for file storage

19. File Input/Output

20. advantages of using files?
File Input/Output
Character stream file
A file that is stored as a sequence of characters
Java provides classes that allow us to read from a file just as we read from the keyboard and write to a file just as we wrote to the screen
import java.io.*;
What are the
advantages of using files?

21. File Input/Output To use a file, we must Import package java.io.*
Choose valid identifiers and types for the file variables and declare them
Instantiate a file object for each file variable
Use the file identifiers in I/O statements (using available methods such as nextLine, nextInt, print, println)
Close the files when through

22. File Input/Output What does instantiating a file do?
Associates the Java identifier for your file with the physical (disk) name for the file
Places a file pointer at the very beginning of the file, pointing to the first character in it
If the output file does not exist on disk, an empty file with that name is created
If the output file already exists, it is erased

23. File Input/Output import.java.io.*; input data output data disk file
“myInfile”
disk file
“myOutfile”
executing
program
your variable
(of type Scanner)
your variable
(of type PrintWriter)

24. File Input/Output All the Scanner methods can be applied to inFile
Scanner in = new Scanner(System.in);
FileReader inReader = new FileReader("myInfile");
Scanner inFile = new Scanner(inReader);
Sets up for keyboard
Set up to read "myInfile"
All the Scanner methods
can be applied to inFile

25. File Input/Output Recall that
An exception is an unusual situation detected while a program is running
Java recognizes two types of exceptions, checked and unchecked
Unchecked exceptions can be ignored, but checked exceptions must be explicitly recognized by the program

26. File Input/Output
Scanner inFile =
new Scanner(new FileReader("inData"));
If file "inData" cannot be found on the disk, an IOException is thrown, and IOExceptions are checked exceptions
public static void main(String args[])
throws IOException
Passes the exception
to the next level
See Chapter 7 for how to
handle it ourselves

27. File Input/Output What about file output?
Remember our old friends print() and println()?
PrintWriter outFile = new PrintWriter
(new FileWriter("outFile.dat"));
outFile.println("Hello world!");
writes the string argument on file outFile, which can be found on the disk under "outFile.dat"
println() and print() behave the same for a file as they do for System.out

28. File Input/Output See the difference between constructors for input
and
output
files ?

29. Loops and Files Read exactly 100 blood pressures from a file
Count-controlled loop
Read all the blood pressures from a file no matter how many are there
End-of-file controlled
loop
Read blood pressures until a dangerously high BP (200 or more) is read
Flag-controlled loop
Sentinel-controlled loop
Read blood pressures until a negative value is found

30. Count-Controlled loop
public class ReadFile {
public static void main(String args[]) throws IOException
int count = 0;
Scanner in = new Scanner(new FileReader("BP"));
int thisBP;
System.out.println("Blood pressures on file BP: ");
// Assumption: There are at least 100 values
while (count <= 100)
thisBP = in.nextInt();
System.out.print(thisBP + " ");
count++; }
System.out.println();
in.close();
Count-Controlled loop

31. End-of-file controlled loop
public class ReadFile {
public static void main(String args[]) throws IOException
Scanner in = new Scanner(new FileReader("BP"));
int thisBP;
System.out.println("Blood pressures on file BP: ");
while (in.hasNextInt())
thisBP = in.nextInt();
System.out.print(thisBP + " "); }
System.out.println();
in.close();
End-of-file controlled loop

32. Loops and Files Use hasNextInt… rather than hasNextLine for loop test
in.nextInt(); 42<EOLN>
in.hasNextInt(); <EOF> (returns false)
in.hasNextLine(); <EOF> (returns true)

33. Flag-controlled loop public class ReadFile {
public static void main(String args[]) throws IOException
Scanner in = new Scanner(new FileReader("BP"));
int thisBP;
boolean flag = in.hasNextInt();
System.out.println("Blood pressures on file BP: ");
while (flag)
thisBP = in.nextInt();
flag = thisBP < 200 && in.hasNextInt();
if (flag)
System.out.print(thisBP + " "); }
System.out.println();
in.close();
Flag-controlled loop

34. Loops and Files Sentinel-Controlled Loops Sentinel
A special data value that is used to indicate the end of the data; requires a “priming read”
read one data value (or set of data values) before entering the while loop
process data value(s) and then read next value(s) at end of loop

35. Sentinel-controlled loop
public class ReadFile {
public static void main(String args[]) throws IOException
Scanner in = new Scanner(new FileReader("BP"));
int thisBP = in.nextInt();
System.out.println("Blood pressures on file BP: ");
while (thisBP > 0)
System.out.print(thisBP + " ");
thisBP = in.nextInt(); }
System.out.println();
in.close();
Sentinel-controlled loop

36. Loops and Files

37. Loops and Files Common processes within a loop Counting Summing
End-of-file loop that counts the number of values
Sentinel-controlled loop that counts the number of values
Any loop that reads and counts the number of positive values
Any loop that reads and counts the number of values over a certain threshold
Summing
Any loop that reads and sums all data values
Any loop that reads and sums certain values

38. Loops and Files Iteration counter
A counter variable that is incremented in each iteration of a loop
Event counter
A variable that is incremented each time a particular event occurs
Are loop control variables
always iteration counters?

39. How to Design Loops Seven points to consider when designing loops
What condition ends the loop?
How should the condition be initialized?
How should the condition be updated?
What is the process being repeated?
How should the process be initialized?
How should the process be updated?
What is the state of the code on exiting the loop?

40. How to Design Loops Key Phrase Termination condition
How many values in file? ?
How many upper case letters? ?
Are there any negative values? ?
How many negative values
and how many positive values? ?
What is the sum of the positive values? ?
Are all the values less than 300? ?
What is the largest value before a
negative value is encountered? ?

41. How to Design Loops What is the process within Loop?
How many values in file? ?
How many upper case letters? ?
Are there any negative values? ?
How many negative values
and how many positive values? ?
What is the sum of the positive values? ?
Are all the values less than 300? ?
What is the largest value before a
negative value is encountered? ?

42. How to Design Loops What is the state on exit?
How many values in file? ?
How many upper case letters? ?
Are there any negative values? ?
How many negative values
and how many positive values? ?
What is the sum of the positive values? ?
Are all the values less than 300? ?
What is the largest value before a
negative value is encountered? ?

43. Nested Loops

44. Nested Loop Algorithm for designing nested loops Design outer loop
process is task name for inner loop
Design inner loop
It can't be that easy!

45. Nested Loops
Print number of uppercase characters in each line of a file
Termination condition: EOF is true
Process: Count uppercase characters
State at end:
file has been read
line contains the last line read
count contains the number of uppercase
values in the last line

46. Nested Loops is in class Character isUpperCase Open file for reading
while NOT EOF
Read line
CountUpperCase
Set count to 0
Set index to 0
while index < line.length()
letter = line.charAt();
if isUpperCase(letter)
Increment count
increment index
isUpperCase
is in
class
Character

47. public static void main (String[] args) throws IOException{
String line;
int count;
int index;
char letter;
Scanner inFile = new Scanner(new FileReader("text"));
while (inFile.hasNextLine())
line = inFile.nextLine();
count = 0;
index = 0;
while (index < line.length())
letter = line.charAt(index);
if (Character.isUpperCase(letter))
count++;
index++; }
System.out.println("Uppercase characters: "+ count);
inFile.close();

48. CountUCL was tested with 3. Can you
Testing Loops
Should test a loop for four special cases:
Loop is skipped entirely
Loop body executes just one
Loop executes some normal number of times
Loop fails to exit
CountUCL was tested with 3. Can you
design cases for 1, 2, and 4?

49. Output from JOptionPane.showConfirmDialog
Extras - GUI Tack
Output from JOptionPane.showConfirmDialog

50. Extras - GUI Track
More
output