Outline

1
Outline

• Dangling if
• Loops
• for
• while
• do/while
• break and continue statements
• Arrays

2
Recap

• Blocks, scopes
• Conditions
• String comparisons
• Decision making
• if/else
• switch

3
Dangling if

• int a 40
• int b 2
• int c 10
• boolean isCGreatest true
• if ( c gt a )if ( c gt b )
• isCGreatest true
• else
• isCGreatest false
• isCGreatest will be true according to this code!

4
Why?

• In the absence of braces separating the if and
  the else, the else condition is associated with
  the last if preceding it
• It is a common mistake to assume it is associated
  with the first if statement

5
Equivalent Code to Dangling if

• int a 40
• int b 2
• int c 10
• boolean isCGreatest true
• if ( c gt a )
• if ( c gt b )
• isCGreatest true
• else
• isCGreatest false

6
Intended Code for Dangling if

• int a 40
• int b 2
• int c 10
• boolean isCGreatest true
• if ( c gt a )
• if ( c gt b )
• isCGreatest true
• else
• isCGreatest false
• else
• isCGreatest false

7
Loops

• Execute a statement or block repeatedly
• May have an associated
• control variable
• initialization of control variable
• condition involving the control variable
• modification to the control variable on each
  iteration

8
for loop
9
for loop format

• for (initial condition step) statement
• Statement can be a compound statement
• E.g.
• for (int i 0 i lt 10 i ) System.out.print
  ln(Value of i is i)

10
for loop example

• public class ForLoop
• public static void main(String arg)
• int sum 0
• for( int counter 0 counter lt 100 counter
  )
• sum counter
• System.out.println( The sum of first
  counter
• numbers is sum )

11
for loop, continued

• The scope of counter is the for loop, however, so
  this example would not work (counter is used
  outside the for loop).
• The loop in the above example calculates the sum
  of all numbers from 0 to 100.

12
while loop
13
while loop format

• while ( condition ) statement
• Any variables used in the condition must be
  declared beforehand
• Any update to them must be done in the statement
  or the condition itself
• Continues executing the statement or statement
  block until the condition evaluates to false

14
while loop example

• public class WhileLoop
• public static void main(String arg)
• int counter 1
• int sum 0
• while( counter lt 100 )
• sum counter
• counter
• System.out.println(sum is sum , counter
  is
• counter)
• // outputs sum is 5050, counter is 101

15
do-while loop
16
do-while loop format

• do statementwhile( condition )
• The loop is executed at least once, since the
  condition is checked after the loop, not before
• Otherwise, exactly the same as a while loop

17
do-while loop example

• import java.io.
• public class DoWhileLoop
• public static void main(String arg) throws
  Exception
• String input
• BufferedReader reader new BufferedReader(new
• InputStreamReader(Sy
  stem.in))
• do
• System.out.print(Enter no to end )
• input reader.readLine()
• while(!input.equals(no))

18
break statement

• Immediately exits the loop
• E.g.
• do
• input reader.readLine()
• if (input.equals(no))
• break
• while( true )

19
continue statement

• Skips to the next loop iteration
• E.g., to add only odd numbers
• int sum 0
• for( int counter 0 counter lt 100 counter )
• if ( counter 2 0 )
• continue
• sum counter
• System.out.println(The sum is sum)

20
Nested Loops

• import java.io.
• public class NestedLoop
• public static void main(String arg) throws
  Exception
• BufferedReader reader new BufferedReader(new
• 
  InputStreamReader(System.in))
• while( true )
• System.out.print(Enter a number, or no to
  quit )
• String input reader.readLine()
• if ( input.equals(no) )
• break
• int factorial 1
• for( int i Integer.parseInt(input) i gt0
  i-- )
• factorial i
• System.out.println(Factorial is
  factorial)

21
Summary (loops)

• Be careful when nesting if/else statements
• Use for, while, and do-while to repeatedly
  execute a block of code
• Use break to exit a loop prematurely
• Use continue to skip to the next iteration

22
Arrays

• A data structure that holds a collection of data
  all of the same type
• Conceptually, a numbered list of data elements
  size is fixed
• Items can be accessed by their position in the
  array (starting at 0)

23
Array as a Collection

• a0 0
  a
• a1 10
• a2 11
• a3 59
• a4 43
  You can only store
• a5 31
  values of the same
• a6 89
  type in an array.
• a7 104
  Array name is a
• a8 6
  reference to the array.
• a9 10

24
Declaring arrays

• An array has to be declared
• Memory for the array has to be allocated with the
  new operator
• Declaring and allocating an array lttypegt
  ltnamegt new lttypegtltsizegt
• The following code declares and allocates memory
  for an array of 10 integers int myArray new
  int10

25
Declaring vs. Allocating

• It is possible to declare an array variable, and
  use it without explicitly allocating memory for
  it
• E.g.,
• int anArray
• int anotherArray anArray new int20
• anotherArray anArray
• Both variables access the same data

26
Accessing array elements

• Need
• Array variable
• Position of the desired element in the array -
  the element index. (Indexes start from 0, not 1)
• Use ltvariable namegtltindexgt
• E.g., this is an expression evaluating to the
  value of the 6th element of array anArray
• anArray5

27
Array example

• int a new int10
• a0 10
• a1 1
• for( int i2 ilt10 i )
• ai -1
• Note that accessing a10, a11, etc. would
  cause an error to occur

28
Array size

• The size is the number of elements an array
  holds, and is fixed when the array is allocated
• The member variable length stores the arrays
  size (you cannot modify it!)
• E.g.,
• int a new int12System.out.println(a.lengt
  h) // Prints out 12

29
Initializing arrays

• Typically, loops are used
• If all values are known beforehandint a
  0, 1, 2, 3, 4, 5
• No need to specify how many elements - inferred
  from the assignment.
• E.g.,double d 0.5, 1.2, 500.201 char
  vowels a, e, i, o, u

30
Example reversing an array

• public class ArrayReverser
• public static void main(String args)
• int a 1, 2, 3, 4, 5
• int temp for( int i0 i lt (a.length / 2)
  i )
• temp ai
• ai a a.length - i - 1
• aa.length - i - 1 temp
• for( int i0 i lt a.length i
  ) System.out.println(i ai)

31
Summary (arrays)

• Use arrays to store collections of similar data
• Declaring an array variable does not allocate it
  new allocates it
• Elements are numbered 0..length-1
• Initialize using comma-separated lists in s,
  or loops

32
Quiz 1

• Will be held at start of class
• 60 minutes
• Closed book/notes, all you need is a pen/pencil
  and your student ID.
• Covers first four lectures
• Marks break-up Multiple choice (20),
  true/false(10), Programming(20)
• Review session time/place TBD

33
Quiz 1 Material

• Java basics
• variables, data types, operators, expressions,
  simple I/O
• Statements, compound statements
• Conditions
• Control flow
• if/else, switch
• Loops for, while, do-while
• Loop control break, continue
• Arrays