Sometimes we need to repeat a set of statements.

1
Repetition

• Sometimes we need to repeat a set of statements.
  
• The body of the loop is itself a block (or set of
  blocks). It is repeated over and over until the
  test becomes false.

Test?
true
false
Loop Body
2
Designing a loop block

• Initialization
• Are there any variables to initialize for the
  test?
• Test condition
• A condition to determine whether or not to repeat
  the loop body
• Loop body
• What are the steps to repeat?
• Do something that could potentially change the
  test condition to be false sometime in the
  future.

3
Algorithm Sum from 1 to N
Count ? 1
Sum ? 0
Count N ?
true
Sum ? Sum Count
false
Count ? Count 1
4
Loops in Java
Test?
true
Body
false

• Java
• while (Test)
• Body

5
Sum from 1 to N in Java
Count ? 1

• int count
• int sum
• count 1
• sum 0
• while ( count lt n )
• sum sum count
• count count 1
• // print sum here

Sum ? 0
Count N ?
true
Sum ? Sum Count
false
Count ? Count 1
6
Infinite loops

• If the test in a loop can never become false, the
  program will run forever without stopping.
• In Dr. Java, to stop an infinite loop, click the
  Reset button.
• Example
• int count
• int sum
• count 1
• sum 0
• while ( count lt n )
• sum sum count // forgot to update
  count

7
The FOR Loop

• Java provides another format of a loop, which is
  usually used when we know how many times the loop
  body is to be executed.
• The FOR loop has the following format
• for (ltinitializationgt lttest_conditiongt
  ltincrementgt)
• // body
• In most cases, the initialization part
  initializes a counter, the test condition tests
  if the counter is within the limit, and the
  increment part modifies the counter.
• Any FOR loop can always be formed as a WHILE loop
• It does not give us any extra capability.
• However, the notation is often more convenient.

8
The FOR loop diagram
Initialization
Test condition?
true
false
Body
Increment
9
Comparison of while and for

• int count
• int sum
• count 1
• sum 0
• while ( count lt n )
• sum sum count
• count count 1
• // print sum here
• int count
• int sum
• sum 0
• for ( count 1 count lt n count count 1 )
• sum sum count
• // print sum here

10
Example The factorial function

• Definition of factorial function
• Implemented in Java
• int result
• result n
• for ( temp n-1 temp gt 1 temp temp - 1 )
• result result temp

11
Example Mean and Standard Deviation

• Arithmetic mean (average)
• Standard deviation
• As values of x are entered, keep the following
  sums
• sum sum xi
• sumSq sumSq xi xi

12
Mean and Standard Deviation in Java (1)

• // Declare variables
• int n // Number of data points
• int i // Number of current data point
• double xi // Current data point
• double sum // Sum of data points
• double sumSq // Sum of squares of data points
• double mean // Arithmetic mean
• double stDev // Standard deviation
• // Read value of n
• System.out.println("Enter number of values ")
• n Keyboard.readInt( )
• // Initialize sums
• double sum 0.0
• double sumSq 0.0

13
Mean and Standard Deviation in Java (2)

• // Calculate sums
• for ( i 1 i lt n i i 1 )
• System.out.println("Enter x sub " i )
• xi Keyboard.readDouble()
• sum sum xi // sum
• sumSq sumSq xi xi // sum of squares
• // Calculate and print results
• // Note that for both divisions, the numerator is
  a double
• mean sum / n
• stDev Math.sqrt( nsumSq sumsum ) / (
  n(n-1) )
• System.out.println("The mean is " mean )
• System.out.println("The standard deviation is "
  stDev )

14
Increment and Decrement operators

• Increment increase by 1
• Decrement decrease by 1
• Java includes special operators for increment and
  decrement
• Increment i i
• Decrement i i--
• It is recommended to use these operators ONLY in
  the following two situations
• As single statements (as above)
• In the increment area of a for loop.
• for ( temp n-1 temp gt 1 temp-- )
• for ( i 1 i lt n i )

15
Tracing

• 1) int count
• 2) int sum
• 3) count 1
• 4) sum 0
• 5) while ( count lt 4 )
• 6) sum sum count
• 7) count count 1
• 8) System.out.print(sum)

sum
count
Line