1. Chapter 4 – Loops Dr. Larry G. Thomas – University of Toledo / LCCC
Most of the material in this file is derived from Introduction to Java Programming, 8th Edition by Daniel Liang

2. §4.1 Introduction So far, all of our code has been sequential
The program starts at the top, and proceeds towards the bottom
if and switch statements may allow us to skip over some lines of code, but we're still constantly moving down towards the bottom of the program
Loops allow us to repeat one or more lines of code as many times as we want

3. §4.1 Introduction
Suppose we want to print a table of the square roots of the integers from 1 to 100:
System.out.println(1 + ":square root = " + Math.sqrt(1));
System.out.println(2 + ":square root = " + Math.sqrt(2));
System.out.println(3 + ":square root = " + Math.sqrt(3));
System.out.println(4 + ":square root = " + Math.sqrt(4));
System.out.println(5 + ":square root = " + Math.sqrt(5)); …
There must be a better way!
There is – use a loop

4. §4.1 Introduction
Java has three types of loop that we can use to execute code over and over:
while
do…while
for

5. Section 4.2
The while Loop

6. §4.2 The while Loop The syntax of the while loop: while (condition) {
statement(s); }
As with the if-then and if-then-else statements, if statement(s) consists of a single statement, then the braces are optional.
The statement(s) will continue to be executed as long as condition remains true

7. §4.2 The while Loop while (condition) { statement(s); }
Think of the while loop as running until (!condition)

8. Loop continuation condition
§4.2 The wihle Loop
The square root problem:
int count = 1;
while (count <= 100) {
System.out.println(
count + ":square root = " +
Math.sqrt(count));
count++; }
One pass through the body of the loop is called an iteration
Loop continuation condition
Loop Body

9. §4.2 The while Loop
In the square root problem, we knew how many times we wanted the loop to execute, so we counted each iteration
This is an example of a counter-controlled loop
In the case of a counter-controlled loop, we must increment the counter in each iteration, and our loop-continuation-condition must check the counter every iteration
Note: some loops will count from a higher starting value down towards 1 (or zero), and we decrement the counter in each iteration

10. §4.2 The wihle Loop
Section implements a hi-low number guessing game
The program picks a random number between 0 and 100 (inclusive)
The user guesses numbers until they guess the number
For each user guess, the program tells the user if they are too high or too low

11. §4.2 The while Loop
The book reminds us about good programming practice – don't start by writing code!
Start by examining the problem, and determining how we can go about solving it
We should program incrementally – when we do start writing code, we don't necessarily have to start at the top and write everything all at once, top-to-bottom, in one pass
We can write it in pieces, and then assemble the pieces

12. §4.2 The while Loop Loop design strategies:
Identify the statements that need to be repeated (i.e., the body of the loop
Wrap the statements in the condition / braces syntax
Code the loop-continuation-condition and add the appropriate statements for controlling the loop

13. §4.2 The Hi-Lo Guessing Game (1)
import java.util.Scanner; public class GuessNumberOneTime { public static void main(String[] args) // Generate a random number to be guessed int number = (int)(Math.random() * 101); Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); // Prompt the user to guess the number System.out.print("\nEnter your guess: "); int guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); }

14. §4.2 The Hi-Lo Guessing Game (2)
import java.util.Scanner; public class GuessNumberOneTime { public static void main(String[] args) // Generate a random number to be guessed int number = (int)(Math.random() * 101); Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); int guess = -1; while (guess != number) // Prompt the user to guess the number System.out.print("\nEnter your guess: "); guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); }

15. §4.2 The while Loop
Rather than counting iterations through the loop, sometimes we want the body of the loop to run some unknown number of times, until something happens
We may determine when to exit the loop based on some special value whose only significance is to end the program
Such a value is called a sentinel value, and such a loop is called a sentinel-controlled loop

16. §4.2 The wihle Loop
Be careful using float or double values as loop control variables
Recall that floating point math is not exact!!
If your loop is looking for a particular value to determine when to stop, it may never see it, because it's off a little in the 15th decimal place!
Rather than using ==, use <= or >=

17. §4.3 The do-while Loop
The do-while loop is a subtle variation on the while loop do {
Statement(s); // Loop body;
} while (loop-continuation-condition);

18. §4.3 The do-while Loop
The biggest difference between the while and do-while loops is that checking the condition in a do-while loop follows the loop body; in a while loop, checking the condition precedes the loop body
This means that the body of a while loop will execute zero or more times; the body of a do-while loop will execute one or more times
In other words, the body of…
…a do-while loop is guaranteed to run at least once
…a while loop is not guaranteed to run at all!

19. §4.2 The Hi-Lo Guessing Game (2)
import java.util.Scanner; public class GuessNumberOneTime { public static void main(String[] args) // Generate a random number to be guessed int number = (int)(Math.random() * 101); Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); int guess = -1; while (guess != number) // Prompt the user to guess the number System.out.print("\nEnter your guess: "); guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); }
This is a repeat of the slide we saw earlier

20. §4.2 The Hi-Lo Guessing Game (3)
import java.util.Scanner; public class GuessNumberOneTime { public static void main(String[] args) // Generate a random number to be guessed int number = (int)(Math.random() * 101); Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); do // Prompt the user to guess the number System.out.print("\nEnter your guess: "); int guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); } while (guess != number); }

21. Section 4.4
The for Loop

22. §4.4 The for Loop
A counter-controlled loop is usually written in an alternate form – a for loop – rather than as a while loop
Functionally, they're the same, but for some operations, the for loop is much clearer to read
A counter-based while loop:
Initializes the counter,
Has a condition it tests before each iteration, and
Modifies the counter each iteration
So does a for loop!

23. §4.4 The for Loop Let's revisit our square roots of 1-100 program:
int count = 1;
while (count <= 100) {
//compute & output sqrt
count++; }
We can do the same thing with a for loop:
for (int count = 1; count <= 100; count++)
// compute & output sqrt

24. §4.4 The for Loop The syntax of the for loop is:
for (initialize step(s);
loop-continuation-condition;
update step(s)) {
statement(s); }
The initialize, condition, and update parts of the for loop are separated by semicolons, and the whole thing must be enclosed in parentheses
Typically, they're all coded on the same line

25. §4.4 The for Loop
for (initial-action;
loop-continuation-condition;
action-after-each-iteration) {
// loop body;
Statement(s); }
As with the while loop, it's possible that the body of a for loop is never executed

26. §4.4 The for Loop
Because the for loop is almost always used to implement a counter-controlled loop:
The initial-action will be to assign the starting value to the loop control variable (i.e., the counter)
The loop-continuation-condition will be that the loop counter is less than its final value
The action-after-each-iteration will be to increment the loop counter
for (i = 1; i <= 100; i++)
As we have seen before, if the body of the loop is a single statement, the braces are optional

27. §4.4 The for Loop
If the loop counter is only needed within the loop (i.e., it's not some variable that the program needs outside the loop), it's good programming practice to declare the loop counter in the for statement:
for (int i = 0; i<100; i++)…
Declaring the loop counter in the for statement means that after the loop, the loop variable no longer exists
Restricting the visibility of the loop variable, such that it only exists where it is needed, is good practice
This is known as the scope of a variable (Chapter 5)

28. §4.4 The for Loop
Sometimes, we'll want to do more than one thing to set up the loop.
Perhaps we have one counter (i) that needs to count up from 0, and another counter (j) that needs to count down from 100 at the same time
We can separate multiple initialization and/or update steps with commas:
for (i=1, j=100; i<=100; i++, j--)

29. §4.4 The for Loop These two are equivalent:
for (i=1, j=100; i<=100; i++, j--) {
// body statement(s); }
i = 1;
j = 100;
while (i <= 100)
i++;
j--;

30. §4.4 The for Loop
Finally, the three parts of the for statement are all optional, but they're rarely omitted
If the initialization or update is omitted, then no initialization or update is performed
If the loop-continuation-condition is omitted, it is assume to be true
All of the following set up infinite loops:
for (;;) { }
for (;true;) { }
while (true) { }

31. Which Kind of Loop to Use?
Section 4.5
Which Kind of Loop to Use?

32. §4.5 Which Loop to Use?
The while and for loops are known as pre-test loops – they test the condition before each iteration of the loop
This means the body of these loops will execute zero or more times
The do…while loop is a post-test loop – it tests the condition after each iteration of the loop
This means the body of a do…while loop will execute one or more times

33. §4.5 Which Loop to Use? Recommendations:
If you need to make sure the body of your loop runs at least once, use do…while
If you know ahead of time how many times the body of the loop needs to run
– i.e., a count-controlled loop, use for
If you need to have the loop run until some event occurs (but there's no way of telling how many times the loop might need to run to get that even to occur), use while or do…while
When there's no compelling reason to use a particular kind, use whatever is most intuitive and comfortable

34. §4.5 Common Loop Errors (1)
Don't put a semicolon after the parenthesis:
for (i = 1; i <= 100; i++); {
// loop body }
This will execute what we labeled as being the loop body (i.e., what's between the braces) once
The loop's REAL body is the null statement created by the semicolon
This loop, as written, will do nothing, 100 times

35. §4.5 Common Errors (2)
A do…while loop DOES have to have a semicolon after the condition:
for (init;cond;update) stmt;
for (init;cond;update) { }
while(condition) stmt;
while(condition) { }
do stmt while (cond);
do { } while (cond);

36. §4.5 Common Errors (3) Watch the boundary conditions:
for (i = 1; i <= 100; i++)
This will execute the body for i = 1 through 100
for (i = 1; i < 100; i++)
This will execute the body for i = 1 through 99
for (i = 1; i < 100; i+=2)
This will execute the body for i = 1, 3, 5, …, 99

37. Section 4.6
Nested Loops

38. §4.6 Nested Loops
When we looked at if-then-else statements in Chapter 3, we said that the statement(s) in the then and else clauses could be any legal Java statement, including another if statement
We called this a nested if statement
When it comes to loops, the same thing applies; the body of a loop can contain another loop
We call this a nested loop
When loops are nested, the inner loop runs for each iteration of the outer loop

39. §4.6 Nested Loops for (int i = 1; i <= 10; i++) {
for (int j = 1; j <= 10; j++)
System.out.println("hello"); }
How many times will this print "hello"?

40. §4.6 Nested Loops for (int i = 1; i <= 10; i++) {
for (int j = 1; j <= 10; j++)
System.out.println(i + "," + j); }
What will this print?

41. §4.6 Nested Loops Nested loops are often used for tables
The outer loop iterates through the rows
The inner loop iterates through the columns of the current row

42. §4.6 Nested Loops – Example (p.129)
System.out.print(" | "); for (int i = 1; i <= 9; i++) System.out.print(" " + i); System.out.println("\n "); for (int row = 1; row <= 9; row++) { System.out.printf("%3d|", row); for (int col = 1; col <= 9; col++) System.out.printf("%4d", row*col); System.out.println(); } | | | | |

43. Keywords break & continue
Section 4.9
Keywords break & continue

44. §4.9 The Keywords break & continue
Normally, you should let the loop control variable handle controlling the execution of a loop
There are special occasions during which it makes sense to override the standard loop execution
break and continue can be used to alter the flow of code through a loop
Using them indiscriminately or improperly can make your code harder to read and debug

45. §4.9 The Keywords break & continue
break aborts execution of a loop
The break statement passes control to the first statement after the end of the loop
In the case of nested loops, break terminates only the loop in which it appears (unless you use a labeled block)
continue can be used to terminate the current loop iteration – control goes to the end of the loop body (directly to checking the loop control condition)
continue aborts the current iteration; break aborts the entire loop

46. §4.9 The Keywords break & continue
while loop
do…while loop
for loop
break aborts the whole loop
the first statement after the loop is executed next
continue aborts (skips the remaining statements in) the current iteration
For do…while and while loops, the condition is evaluated;
For for loops, the update action is performed and then the condition is checked

47. §4.2 The Hi-Lo Guessing Game (2)
import java.util.Scanner; public class GuessNumberOneTime { public static void main(String[] args) // Generate a random number to be guessed int number = (int)(Math.random() * 101); Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); int guess = -1; while (guess != number) // Prompt the user to guess the number System.out.print("\nEnter your guess: "); guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low"); }
This is a repeat of the slide we saw earlier

48. §4.2 The Hi-Lo Guessing Game (4)
import java.util.Scanner; public class GuessNumberOneTime { public static void main(String[] args) // Generate a random number to be guessed int number = (int)(Math.random() * 101); Scanner input = new Scanner(System.in); System.out.println("Guess a magic number between 0 and 100"); while (true) // we'll exit when the user guesses correctly // Prompt the user to guess the number System.out.print("\nEnter your guess: "); int guess = input.nextInt(); if (guess == number) System.out.println("Yes, the number is " + number); break; } else if (guess > number) System.out.println("Your guess is too high"); else System.out.println("Your guess is too low");

49. End of Chapter 4
Any Questions?