1. Loop Structures

2. Goals and Objectives while loop do-while loop for loop Infinite Loops
Nested Loops
Counter
Accumulator
Sentential Value
for each loop

3. Repetition Statements
Repetition statements allow us to execute a statement multiple times
Often they are referred to as loops
Like conditional statements, they are controlled by boolean expressions
Java has three kinds of repetition statements:
the while loop
the do loop
the for loop
The programmer should choose the right kind of loop for the situation

4. 4 Types of loops while (boolean condition)
do – while (boolean condition)
for(initialization;condition;update)
For each loop (Discussed later)

5. while Loop
Executes a set of statements over and over again based on a condition.
Pretest Loop
Variable Loop
Iteration – each execution of the loop.
Example:
while(condition) {
<statements>; }

6. 7/28/2018 8:39 PM
The while Statement
Loop structure that executes a set of statements as long as a condition is true
The condition is a Boolean expression
Will never execute if the condition is initially false
The loop below iterates once because the condition is true and then continues to iterate until response is not 1: response = 1; while (response == 1) { System.out.print("Enter 1 or 0:"); response = input.nextInt(); }
Refer to page 131 in the text.

7. 3 parts of a while loop Initialization – (Priming the loop)
Terminating condition
Updating statement
Example:
x = 0; // initialization
while(x < 10) // x < 10 is the termination condition {
x++; // increase x by 1 updating statement }

8. Logic of a while Loop
condition
evaluated
false
statement
true

9. The while Statement An example of a while statement:
int count = 1;
while (count <= 5) {
System.out.println (count);
count++; }
If the condition of a while loop is false initially, the statement is never executed
Therefore, the body of a while loop will execute zero or more times

10. The while Statement A loop can be used to maintain a running sum
A sentinel value is a special input value that represents the end of input
A loop can also be used for input validation, making a program more robust

11. Trace a while loop final int LIMIT = 5; int count = 1;
while(count <= LIMIT) {
System.out.println(count);
count = count + 1; }
System.out.println(“Done”);
Output: 1 2 3 4 5
Done

12. do – while loop Alternative form of the while loop statement.
Not evaluated until after the first execution of the loop.
Guarantee to execute once not like the other two loops.
Post-test loop
Variable loop
Example: do {
<statements>;
} while(condition);

13. 3 parts of a do while loop Initialization – (Priming the loop)
Terminating condition
Updating statement
Example:
x = 0; // initialization do {
x++; // increase x by 1 updating statement
} while(x < 10); // x < 10 is the termination condition

14. The do-while Statement
7/28/2018 8:39 PM
The do-while Statement
Alternative form of the while statement
Executes at least once
The statement do { System.out.print("Enter 1 or 0:"); response = input.nextInt(); } while (response == 1); iterates once and continues to iterate until response is not 1.
Refer to pages 131 and 132 in the text.

15. The do Statement A do statement has the following syntax: do {}
while ( condition );
The statement is executed once initially, and then the condition is evaluated
The statement is executed repeatedly until the condition becomes false

16. Logic of a do Loop
statement
true
condition
evaluated
false

17. The do Statement An example of a do loop:
int count = 0; do {
count++;
System.out.println (count);
} while (count < 5);
The body of a do loop executes at least once

18. Comparing while and do The while Loop The do Loop statement condition
true
false
condition
evaluated
The while Loop
true
condition
evaluated
statement
false
The do Loop

19. Example of a do – while loop
int count = 1;
int limit = 5; do {
System.out.println(count);
count++;
}while(count<limit);
System.out.println(“Done”)
Output: 1 2 3 4
Done

20. for loop
A counter controlled repetition loop, works well when you know exactly how many times you want to execute the loop.
Fixed Loop
Pre-Test Loop

21. 3 parts of a for loop Initialization Boolean condition Update Example:
for(initialization; boolean condition; update) {
<statements>; }

22. 7/28/2018 8:39 PM
The for Statement
Loop structure that executes a set of statements a fixed number of times
Uses a loop control variable (lcv)
The increment (++) or decrement (--) operators are used to change the value of the loop control variable
The loop below executes until i is greater than 10: for (int i = 0; i <= 10; i++) { sum += i; }
Refer to page 135 in the text.

23. The for Statement A for statement has the following syntax:
The initialization
is executed once
before the loop begins
The statement is
executed until the
condition becomes false
for ( initialization ; condition ; increment )
statement;
The increment portion is executed at the end of each iteration

24. Logic of a for loop initialization condition evaluated false statement
true
increment

25. The for Statement
A for loop is functionally equivalent to the following while loop structure:
initialization;
while ( condition ) {
statement;
increment; }

26. The for Statement An example of a for loop:
for (int count=1; count <= 5; count++)
System.out.println (count);
The initialization section can be used to declare a variable
Like a while loop, the condition of a for loop is tested prior to executing the loop body
Therefore, the body of a for loop will execute zero or more times

27. The for Statement The increment section can perform any calculation
for (int num=100; num > 0; num -= 5)
System.out.println (num);
A for loop is well suited for executing statements a specific number of times that can be calculated or determined in advance

28. The for Statement
Each expression in the header of a for loop is optional
If the initialization is left out, no initialization is performed
If the condition is left out, it is always considered to be true, and therefore creates an infinite loop
If the increment is left out, no increment operation is performed

29. Example of a for loop int limit = 5;
for(int count = 1; count <limit; count++) {
System.out.println(count); }
System.out.println(“Done”);

30. Missing sections of a for loop
You can have sections of a for loop missing just as long as you have that section declared and initialized somewhere else.
Semicolon still must appear between sections
Example:
for( ; x<5; x++) Or
for( ; x < 5; )

31. Trace a for loop final int LIMIT = 5;
for(int count = 1; count<= LIMIT; count++)
System.out.println(count);
System.out.println(“Done”);
OUTPUT: 1 2 3 4 5
Done
Notice the for loop has no braces. Just like the if statement, it will only repeat the first statement only

32. Nested Loops
Similar to nested if statements, loops can be nested as well
That is, the body of a loop can contain another loop
For each iteration of the outer loop, the inner loop iterates completely

33. Nested Loops How many times will the string "Here" be printed?
count1 = 1;
while (count1 <= 10) {
count2 = 1;
while (count2 <= 20)
System.out.println ("Here");
count2++; }
count1++;
10 * 20 = 200

34. Nested Loops Loop that contains another loop in its body. Example:
for(int x = 0; x< 5; x++) {
System.out.println(“Outer Loop”);
for(int y = 0; y < 5; y++)
System.out.println(“Inner Loop”); }
It is very important to indent the inner loop
Make sure you reset inner loop to reenter again.

35. Infinite Loops A loop that continues executing forever
7/28/2018 8:39 PM
Infinite Loops
A loop that continues executing forever
Can be caused by syntax or logic errors. For example: while (num < 0) //error--no braces System.out.print("Enter a vale: "); num = input.nextInt();
Some errors can result in an overflow
Refer to page 132 in the text.
An infinite loop can cause an application to stop responding or just "hang". Closing the application window (with JCreator Pro) will end the application so that the source of the infinite loop can be located and corrected.
Errors that result in an overflow may generate unexpected results rather than "hanging" the application.

36. Infinite Loops Loops where the boolean condition never becomes false.
Example
int x = 0;
while(x < = 5) {
System.out.println(“Hello”); }
x is never updated to become bigger than 5

37. Infinite Loops
The body of a while loop eventually must make the condition false
If not, it is called an infinite loop, which will execute until the user interrupts the program
This is a common logical error
You should always double check the logic of a program to ensure that your loops will terminate normally

38. Infinite Loops An example of an infinite loop:
int count = 1;
while (count <= 25) {
System.out.println (count);
count = count - 1; }
This loop will continue executing until interrupted (Control-C) or until an underflow error occurs

39. Accumulators, Counters, & Sentinel Values
Accumulator is a variable in a loop that keeps a running total or sum of a variable.
Counter – is usually a variable increase by one to keep a record of how many times a loop has executed.
Sentinel Value (Flag) – are inputted by the user to stop a loop. –999 or –1
Be careful not to add the sentinel value to the accumulator.

40. A variable that is incremented by a constant value
7/28/2018 8:39 PM
Counters
A variable that is incremented by a constant value
Often used for counting loop iterations
Should be initialized to 0 when declared
The counter in the loop counts the number of responses: do { System.out.print("Enter 1 or 0:"); response = input.nextInt(); numResponses += 1; } while (response == 1);
Refer to page 133 in the text.

41. Accumulators A variable that is incremented by a varying amount
7/28/2018 8:39 PM
Accumulators
A variable that is incremented by a varying amount
Often used for summing
Should be initialized to 0 when declared
The accumulator in the loop sums values: do { System.out.print("Enter grade:"); grade = input.nextInt(); sumOfGrades += grade; } while (grade != 999);
Refer to page 133 in the text.

42. 7/28/2018 8:39 PM
Using Flags
A flag, or sentinel, indicates when a loop should stop iterating
Often a constant
Code is easier to modify when sentinels are constants declared at the beginning of an application
A flag is used in the condition of the loop: final int STOP = 999; do { System.out.print("Enter grade:"); grade = input.nextInt(); sumOfGrades += grade; } while (grade != STOP);
Refer to page 133 in the text.

43. Breaking out of a loop
An if statement with the keyword break as a statement. When the if statement becomes true it will execute the break statement and take you out of that loop.
If you have multiple nested loops the break will only break you out of the loop that it is located in.

44. Debugging Techniques The debugger included with many compilers
7/28/2018 8:39 PM
Debugging Techniques
The debugger included with many compilers
Variable trace, which is a manual technique of list values of variables at the points of assignment
Additional println() statements for displaying variable values at points of assignment
"Commenting out" code to detect bugs through a process of elimination
Refer to pages 136 and 137 in the text.

45. Using println() to Debug
7/28/2018 8:39 PM
Using println() to Debug
int num1 = 0; int num2 = 0; System.out.println("num1 before while: " + num1); //debug while (num1 < 10) { System.out.println("num1 in while: " + num1); //debug if (num1 % 3 == 0) { num2 += num1; System.out.println("num2: " + num2); //debug System.out.print(num2 + " "); } num1 += 1; }
Refer to page 137 in the text.
When using println() statements to debug a segment of code, it is usually most effective to place statements just after assignment changes the value of a variable.

46. Using Comments to Debug
7/28/2018 8:39 PM
Using Comments to Debug
int num1 = 0; int num2 = 0; while (num1 < 10) { //if (num1 % 3 == 0) { // num2 += num1; // System.out.print(num2 + " "); //} num1 += 1; }
Refer to page 137 in the text.
Comments can be used to comment out segments of code to determine through a process of elimination the location of any bugs.

47. 7/28/2018 8:39 PM
Variable Trace
int num1 = 0; int num2 = 0; while (num1 < 10) { if (num1 % 3 == 0) { num2 += num1; System.out.print(num2 + " "); } num1 += 1; }
Refer to page 136 in the text.

48. Iterators
An iterator is an object that allows you to process a collection of items one at a time
It lets you step through each item in turn and process it as needed
An iterator object has a hasNext method that returns true if there is at least one more item to process
The next method returns the next item
Iterator objects are defined using the Iterator interface.

49. Iterators
Several classes in the Java standard class library are iterators
The Scanner class is an iterator
the hasNext method returns true if there is more data to be scanned
the next method returns the next scanned token as a string
The Scanner class also has variations on the hasNext method for specific data types (such as hasNextInt)

50. Iterators
The fact that a Scanner is an iterator is particularly helpful when reading input from a file
Suppose we wanted to read and process a list of URLs stored in a file
One scanner can be set up to read each line of the input until the end of the file is encountered
Another scanner can be set up for each URL to process each part of the path

51. Iterators and for Loops
Recall that an iterator is an object that allows you to process each item in a collection
A variant of the for loop simplifies the repetitive processing the items
For example, if BookList is an iterator that manages Book objects, the following loop will print each book:
for (Book myBook : BookList)
System.out.println (myBook);

52. Iterators and for Loops
This style of for loop can be read "for each Book in BookList, …"
Therefore the iterator version of the for loop is sometimes referred to as the for each loop
It eliminates the need to call the hasNext and next methods explicitly
It also will be helpful when processing arrays.

53. Common Errors
Boolean condition never becoming false causing an infinite loop.
Semicolon after a for or while loop
Adding a sentinel value to an accumulator
Missing semicolon after the while in a do – while loop.
Forgetting to reset the inner loop to go back in.