Asserting Java, Ch 7: Repetition

1
Determinate Loop Pattern, Java's for Statement,
and Scanner objects 2nd part of Chapter 6
Repetition
2
Algorithmic Pattern The Determinate loop

• We often need to perform some action a specific
  number of times
• Produce 89 paychecks
• Count down to 0 (take 1 second of the clock).
• Simulate playing a game of "Let's Make a Deal"
  10,000 times
• The determinate loop pattern repeats some action
  a specific number of times

3
(No Transcript)
4
Determinate Loops

• This template repeats a process n times
• replace comments with appropriate statements
• int n / how often we must repeat the
  process /
• for( int j 1 j lt n j j 1 )
• // the process to be repeated
• determinate loops must know the number of
  repetitions before they begin
• know exactly how many employees, or students, or
  whatever that must be processed, for example

5
General FormThe Java for loop

• for ( initial statement loop-test
  update-step)
• repeated-part
• When a for loop is encountered, the
  initial-statement is executed (used here quite
  often as int j 1). The loop-test evaluates. If
  loop-test is false, the for loop terminates. If
  loop-test is true, the repeated-part executes
  followed by the update-step.

6
Flow chart view of a for loop
Initial statement
True
False
Loop test
Iterative part
update-step
7
Example for loop that produces an average

• Scanner keyboard new Scanner(System.in)
• double sum 0.0
• System.out.print("How many do you want to
  average? ")
• int n keyboard.nextInt()
• // Do something n times
• for (int j 1 j lt n j j 1)
• System.out.print("Enter number ") // lt-
  Repeat 3
• int number keyboard.nextInt() // lt-
  statements
• sum sum number // lt- n times
• double average sum / n
• System.out.print("Average " average)

8
Code Demo Use the debugger to trace this code

• int n 5
• for (int j 1 j lt n j j 1)
• System.out.println(j)
• for (int k 10 k gt 0 k k - 2)
• System.out.println(k)

9
Other Incrementing Operators

• It is common to see determinate loops of this
  form where n is the number of repetitions
• for( int j 1 j lt n j )
• // ...
• The unary and -- operators add 1 and subtract
  1 from their operands, respectively.
• int n 0
• n // n is now 1 equivalent to nn1 or
  n1
• n // n is now 2
• n-- // n is now 1 again
• The expression count is equivalent to the more
  verbose count count 1

10
Other Assignment Operators

• Java has several assignment operators in addition
  to (- and )
• j - 2 is the equivalent of j j - 2
• sum x is the equivalent of sum sum x
• What is sum when a user enters 7 and 8?
• int sum 0
• int x 0
• System.out.print("Enter a number ")
• x keyboard.nextInt() // user enters 7
• sum x
• System.out.print("Enter a number ")
• x keyboard.nextInt() // user enters 8
• sum x

11
Indeterminate loop with a Scanner

• Sometimes a stream of input from the keyboard or
  a file needs to be read until there is no more
  data in the input stream
• Consider a Scanner object constructed with a
  String argument
• The string represents an input stream
• You will need Scanner in project 2, methods 9 and
  10 sumInScanner and maximumInScanner

12
These assertions pass
_at_Test public void showScanner() Scanner
scannerWithInts new Scanner("1 2 3")
assertEquals(1, scannerWithInts.nextInt())
assertEquals(2, scannerWithInts.nextInt())
assertEquals(3, scannerWithInts.nextInt())
Scanner scanner new
Scanner("There are five words here.")
assertEquals("There", scanner.next())
assertEquals("are", scanner.next())
assertEquals("five", scanner.next())
assertEquals("words", scanner.next())
assertEquals("here.", scanner.next())
13
A test method to test num100s
_at_Test public void testNum100s()
ControlFun cf new ControlFun() Scanner
scanner0 new Scanner("1 2 3") Scanner
scanner1 new Scanner("4 100 2 5") Scanner
scanner3 new Scanner("100 100 2 -3
5 3 2 -100 100") assertEquals(0,
cf.num100s(scanner0)) assertEquals(1,
cf.num100s(scanner1)) assertEquals(3,
cf.num100s(scanner3))
14
Answer
public int num100s (Scanner scanner) int
result 0 while (scanner.hasNextInt())
int next scanner.nextInt() if (next
100) result return
result
15
Careful using next too often!

• These assertions should pass with the code that
  follows on the next slide

_at_Test public void testSumOfNegs()
ControlFun cf new ControlFun() Scanner
scanner0 new Scanner("1 2 3") Scanner
scannerA new Scanner("1 -2 3") Scanner
scannerB new Scanner("-4 1 -2 3")
assertEquals(0, cf.sumOfNegatives(scanner0))
assertEquals(-2, cf.sumOfNegatives(scannerA))
assertEquals(-6, cf.sumOfNegatives(scannerB))

16
What's wrong with this method?
public int sumOfNegatives(Scanner scanner)
int result 0 while
(scanner.hasNextInt()) if
(scanner.nextInt() lt 0) result
scanner.nextInt() return
result