Lecture Java methods: Object-oriented programming and data structures (2nd AP edition): Chapter 8 - Maria Litvin, Gary Litvin

Chapter 8 - Iterative statements: while, for, do–while. In this chapter, the learning objectives are: Understand the semantics and learn the Java syntax for while, for, and do-while loops; learn how to use nested loops.

Iterative Statements: while, for,

and Data Structures

Maria Litvin ● Gary Litvin

2nd AP edition with GridWorld

Iterations

• It is essential that a program be able to

execute the same set of instructions many

times: otherwise a computer would do only as much work as a programmer!

• Repeating the same code fragment several

times is called iterating.

• Java provides three control statements for

iterations (a.k.a loops): for, while, and

do-while

If the body has only

one statement, the

braces are optional

condition is any

logical expression,

as in if

The body of the loop

The while Loop (cont’d)

• Initialization: The variables tested in the

condition must be initialized to some values

If the condition is false at the outset, the loop

is never entered

• Testing: The condition is tested before each iteration If false, the program continues with the first statement after the loop

• Change: At least one of the variables tested

in the condition must change within the body

of the loop

The while Loop (cont’d)

• Sometimes change is implicit in the changed state of a variable:

Scanner input = new Scanner(inputFile);

while (input.hasNext())

System.out.println ( input.next( ) );

Changes the

state of input

• for is a shorthand that combines in one statement initialization, condition, and change:

for ( initialization; condition; change )

• Java allows you to declare the loop control variable in the for statement itself For

The scope of k is the

body of the loop, and

k is undefined outside

the loop

• “Repeat n times” idiom:

Always use braces

for readability (even

if the body has only

one statement)

if condition is false,

the next iteration is not executed

The do-while Loop (cont’d)

• do-while is convenient when the variables

tested in the condition are calculated or read

in the body of the loop:

The do-while Loop (cont’d)

• do-while can be easily avoided: we can

usually replace it with a while loop initialized

so that it goes through the first iteration:

break and return in Loops

• break in a loop instructs the program to

immediately quit the current iteration and go

to the first statement following the loop

• return in a loop instructs the program to

immediately quit the current method and

return to the calling method

• A break or return must be inside an if or an

else, otherwise the code after it in the body of the loop will be unreachable

return in Loops

• Sequential Search method:

public int search(String[ ] list, String word)

Nested Loops (cont’d)

• Braces are optional when the body of the

loop(s) is one statement:

for’s body

• Many programmers prefer to always use

braces in loops, especially in nested loops

Nested Loops (cont’d)

• Be careful with break:

for (int r = 0; r < m.length; r++)

continues with the outer loop

“Triangular” Nested Loops

• “Find duplicates” idiom:

for (int i = 0; i < a.length; i++)

}

The inner lcv starts at the outer lcv’s next value

Lab: Perfect Numbers

• A perfect number is a positive integer equal to the sum of all its divisors (including 1 but

excluding the number itself) For example:

28 = 1 + 2 + 4 + 7 + 14

Write a program to find the first four perfect

numbers (Java Methods pp 210-211).

Lab: Perfect Numbers (cont’d)

• Euclid showed that if 2n-1 is

Lab: Perfect Numbers (cont’d)

• A prime that has a form

Write a program to find the first six Mersenne

primes and the corresponding perfect numbers.

• Euler proved that any even perfect number must have that form

Lab: Perfect Numbers (cont’d)

• The largest known Mersenne Prime is

243,112,609 – 1 It has 12,978,189 digits It

was discovered by GIMPS (Great Internet

Mersenne Prime Search) particiants at

UCLA in 2008

• They also claimed the Electronic Frontier

Foundation’s $100,000 prize for finding a

prime number with more than 10 million

digits

http://www.mersenne.org

• Can any code with a for loop be

rewritten with a while loop?

• Does short-circuit evaluation apply to conditions in loops?

• Which operators can be used in the

“change” part of a for loop?

Review (cont’d):

• Are method calls allowed in a condition

in a while loop?

• Is return allowed in a loop?

• What is a nested loop?

• Name a situation where nested loops are used

• Can you have nested while loops?