Program Control

1
Program Control

• ICS 111
• Introduction to Computer Science II
• William M. Albritton, Information and Computer
  Sciences Department at University of Hawaii at
  Manoa

2
Vocabulary

• Primitive data type
• A built-in data type
• Closely models the computers memory
• Predefined by the Java language
• Manipulated by built-in operators
• Do not have methods or instance variables (very
  different than objects)
• Java has 8 primitive data types
• byte, short, int, long, double, float, boolean,
  char

3
Integer Data Types

• Type Storage Range
• byte 1 byte (8 bits) -128 to 127
• short 2 bytes (16 bits) -32,768 to 32,767
• int 4 bytes (32 bits) -2,147,483,648 to
  2,147,483,647
• long 8 bytes (64 bits) -9,223,372,036,854,775,808
  to 9,223,372,036,854,775,807

4
Floating-point Data Types

• Type Storage Range
• float 4 bytes (32 bits) -3.4 x 1038 to 3.4 x
  1038
• (7 significant digits)
• double 8 bytes (64 bits) -1.7 x 10308 to 1.7 x
  10308
• (15 significant digits)
• Example declarations
• float f 123.456f //f for float
• double d 123.456 //double by default

5
Two Kinds of Division

• Integer division
• Cuts-off digits after the decimal point (does
  not round up)
• int x 1/2 //x0
• Floating point division
• Returns a floating point (decimal point) value
• double d 1.0/2.0 //d0.5
• d (double)1/2 // d0.5
• d 1/2 // d0.0

6
Character Data Type

• Stores a 2 byte (16 bit) Unicode character
• Have to use single quotes around the character
• char ch 'a'
• Some special characters, which are called escape
  sequences, begin with a backslash character (\)
• char newline '\n'
• char tab '\t'
• char double_quote '\"'
• char single_quote '\''
• char backslash '\\'

7
Boolean Data Type

• Models truth values
• Either true or false
• boolean variable true
• variable false

8
Vocabulary

• Wrapper class
• A class that represents a primitive data type
• Used to store the primitive data type as an
  object
• Useful when a primitive data type has to be
  manipulated by objects
• Javas wrapper classes correspond to the 8
  primitive data types
• Byte, Short, Integer, Long, Float, Double,
  Character, Boolean

9
Wrapper Class

• Example declarations
• int x 5 //primitive data type
• Integer y new Integer(5) //wrapper class
• Wrapper classes may have useful static methods
  static variables
• String five new String("25")
• int z Integer.parseInt(five) //returns integer
  value 25

10
Vocabulary

• Boolean expression
• A combination of operators operands that
  returns true or false
• Used in selection repetition statements
• Evaluated from left to right
• Operands are what the operators operate upon
• 3 Kinds of boolean operators
• Equality operators , !
• Relational operators gt, gt, lt, lt
• Logical operators , , ! (and, or, not)

11
Logical Operators

• Logical and, or, not , , !
• Evaluated left to right
• Returns true or false
• Operands have to be boolean
• Often see if(!x), which means if(xfalse)
• x can be boolean type, or boolean expression, or
  boolean (predicate) method

12
Example Code Class Exercise

• See Example.java for examples with
• Primitive data types
• Wrapper classes
• Relational operators
• Logical operators
• Class Exercise
• Tracing exercise
• See Tracing.java

13
Increment Decrement

• Preincrement operator
• Add 1, then assign/use value
• int a 3
• int b a //b4, a4
• Postincrement operator
• Assign/use old value, then add 1
• int a 3
• int b a //b3, a4
• a 3
• int c 10 a //c13, a4

14
Assignment Operators

• x x 5
• Can also be written as x 5
• a b 5
• Evaluates to a a (b 5)
• And not a a b 5

15
Conditional Operator

• if (x gt y)
• a x y
• else
• a x y
• Can also be written as (conditional operator)
• a (x gt y) ? (x y) (x y)
• (boolean expression) ? (return 1st value if true)
  (return 2nd value if false)

16
Short Circuit Evaluation

• Shortcut for evaluating logical operators
• Logical or will stop at leftmost true operand
• int a 3, b 3
• if( (a4) (b2) )
• System.out.println("a"a",b"b)
• //a4,b3

17
Short Circuit Evaluation

• Shortcut for evaluating logical operators
• Logical and will stop at leftmost false operand
• int a 3, b 3
• if( (a3) (b2) )
• System.out.println("a"a",b"b)
• //a4,b3

18
Example Code Class Exercise

• See Example2.java for examples on
• Increment decrement
• Assignment operators
• Conditional operator
• Short circuit evaluation
• Class exercise
• See Tracing2.java

19
Vocabulary

• Flow of control
• Order of execution of statements in a program
• Usually one statement is executed after another,
  from the first statement in a program to last
  statement
• Methods alter flow of control, by having control
  (also called the program counter or
  instruction pointer) jump to the method that is
  invoked (called), and then jump back to the next
  line after the method call (return address) in
  the program
• Within a method, the execution order of
  statements is controlled by selection
  repetition statements

20
Vocabulary

• Selection statement
• Chooses which statement to be executed next
• Also called conditional statement
• Uses a boolean expression to choose which
  statement or group of statements to execute
• 3 kinds of selection statements
• if, if-else, switch

21
Selection 3 Ways

• If structure (single selection)
• If-else structure (double, multiple selection)
• if(condition1)
• statement1
• else if(condition2)
• statement2
• . . .
• . . .
• else
• statementN
• Switch structure (multiple selection)

22
If Statement

• If no parenthesis, then only the 1st statement
  following the if statement is associated with it
• if(a gt b)
• c 3
• d 4
• So d4 will always be evaluated

23
If/else Ambiguity

• If one if statement doesnt have an ending else
  another one does, may be ambiguous
• Resolved by associating the else with the closest
  if
• if(a gt b)
• if(c gt d)
• e 3
• else
• e 4
• Must use braces to associate with the outside if
• if(a gt b)
• if(c gt d)
• e 3
• else
• e 4

24
Switch Statement

• Handles a series of decisions
• Must include a break statement
• If not, each case statement will be executed
  until encounter another break statement or reach
  end
• Several cases can execute the same statements
• By listing case statements one after another
• Can only use char or int data types
• int i 5 char c 'e'
• switch(i) switch(c)
• case 99 ... case 'a' ...

25
Example Code

• See Example3.java for examples on
• Selection statements
• Class exercise
• Write a program that prompts the user to enter a
  letter outputs on the screen whether the letter
  is a vowel or consonant
• Use method char charAt(int position) to access a
  character (char) in a String
• See Coding.java

26
Vocabulary

• Repetition statement
• Used to execute statements over and over
• Also called a loop statement, or repetition
  structure
• Uses a boolean expression to repeatedly execute a
  statement or group of statements
• 3 kinds of repetition statements
• while, do-while, for

27
Repetition 3 Ways

• while (condition) statements
• while(55) System.out.println("infinite
  loop")
• dostatements while(condition)
• do System.out.println("one") while(1 lt 0)
• for(initialize condition increment/decrement)
• for(int i0 ilt50 ii10) System.out.println("
  five")

28
for while

• for(expr1expr2expr3)
• statement
• Is equivalent to
• expr1
• while(expr2)
• statement
• expr3
• This will create an infinite loop
• for() . . .

29
Comma in For Loops

• Can put commas in for loops
• Evaluated left to right
• int a0,b0
• for(a0, b10 altb a, b--)
• System.out.println("a"a
• ",b"b)

30
Nested Loops

• Can have loops within loops (or any other
  combination of nested repetition selection
  statements)
• for(a0 alt5 a2) System.out.println("a"a)
• for(b3 bgt0 b--)
• System.out.println("b"b) //inner loop
• //outer loop

31
Alter Flow of Control

• break
• Immediately exit from while, for, do/while, or
  switch statements
• continue
• Skips the remaining statements
• Performs the next iteration of while, for, or
  do/while loop

32
Example Code Class Exercise

• See Example4.java for examples on
• Repetition statements
• Class exercise
• Write a program that prompts the user for integer
  x, then uses x as input for static methods int
  Factorial(int x) and instance method int
  Fibonnaci(int x)
• Static methods of a class can be called from that
  classs main method, while instance methods need
  to be called from an object of that class
• See Coding2.java

33
Vocabulary

• The following vocabulary from these slides will
  be on the next exam
• Primitive data type
• Wrapper class
• Boolean expression
• Flow of control
• Selection statement
• Repetition statements