CSCI1402: Lecture 2 Week 6

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CSCI1402 Lecture 2 Week 6
Dr. David A. Elizondo Centre for Computational
Intelligence School of Computing Office Gateway
6.61 email elizondo_at_dmu.ac.uk
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Short Circuit Logical Operators

• Normal (AND) and (OR) operands will always
  evaluate each operand.
• Short circuit (AND) (OR) will evaluate the
  second operand only when necessary (more
  efficient).
• In an AND operation, if the 1st operand is false,
  the outcome is false, no matter what value the
  2nd operand.
• In an OR operation, if the 1st operand is true,
  the outcome of the operation is true no matter
  what the value of the second operand.

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Short Circuit Logical Operators
Short circuit operator prevents division by 0

• public class SCops
• public static void main (String args)
• int n, d, q
• n 10
• d 2
• if (d ! 0 (n d) 0)
• System.out.println(d is a factor of
  n)
• d 0 // now, set d to zero
• // Since d is zero, the 2nd operand is not
  evaluated
• if (d ! 0 (n d) 0)
• System.out.println(d is a factor of n)
• // Now, try same thing without short circuit
  operator
• // This will cause a divide by zero error.
• if (d ! 0 (n d) 0)
• System.out.println(d is a factor of n)

Now both expressions are evaluated allowing a
division by 0
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Selection switch statements

• It provides a multi way branch.
• It enables a program to select among several
  alternatives.
• Switch is generally more efficient than nested
  ifs
• The value of an expression is successfully tested
  against a list of constants. When a match is
  found, the statement sequence associated with
  that match is executed.

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Selection switch statements

• General form of a switch statement
• switch(expression)
• case constant1
• statement sequence
• break
• case constant2
• statement sequence
• break
• case constant3
• statement sequence
• break
• .
• .
• .
• default
• statement sequence

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Selection switch statements

• The switch expression must be of type char, byte,
  short, or int. (floating-point expressions are
  not allowed).
• Often the expression controlling the switch is
  simply a variable.
• The case constants must be literals of a type
  compatible with the expression.
• No two case constants in the same switch can have
  identical values.

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Selection switch statements

• The default statement sequence is executed if no
  case constant matches the expression.
• The default is optional if not present, no
  action takes place if all matches fail.
• When a match is found, the statements associated
  with that case are executed until the break is
  encountered or, in the case of default or the
  last case, until the end of the switch is reached.

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Selection switch statements

• Sample program
• public class SwitchDemo
• public static void main(String args )
• int i
• for (i0ilt10i)
• switch(i)
• case 0
• System.out.println(i is zero)
• break
• case 1
• System.out.println(i is one)
• break
• case 2
• System.out.println(i is two)
• break
• case 3
• System.out.println(i is three)
• break
• case 4

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Selection switch statements

• Program output
• i is zero
• i is 0ne
• i is two
• i is three
• i is four
• i is five or more
• i is five or more
• i is five or more
• i is five or more
• i is five or more

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The break statement

• It is technically optional (most applications
  of the switch will use it).
• When encountered within a statement sequence of a
  case, the break statement causes program flow to
  exit from the entire switch.
• If a break statement does not end the statement
  sequence associated with a case, then all
  statements at and following the matching case
  will be executed until a break (or the end of the
  switch) is encountered.

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No break sample program

• class Nobreak
• public static void main(String args)
• int i
• for(i0ilt5i)
• switch(i)
• case 0
• System.out.println(i is less than one)
• case 1
• System.out.println(i is less than two)
• case 2
• System.out.println(i is less than three)
• case 3
• System.out.println(i is less than four)
• case 4
• System.out.println(i is less than five)
• System.out.println()

The case statements fall through here
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No break sample program

• Program output
• i is less than one
• i is less than two
• i is less than three
• i is less than four
• i is less than five
• i is less than two
• i is less than three
• i is less than four
• i is less than five

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No break sample program

• Program output
• i is less than three
• i is less than four
• i is less than five
• i is less than four
• i is less than five
• i is less than five
• Execution continues into the next case if no
  break statement is present

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Empty cases

• You can have empty cases
• switch(i)
• case 1
• case 2
• case 3 System.out.println(i is 1, 2, or 3)
• break
• case 4 System.out.println(i is 4)
• break
• If i has the value 1,2, or 3, the 1st println()
  statement executes. If it is 4, the 2nd println()
  statement executes.

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Nested switch statements

• It is possible to have a switch as part of the
  sequence of an outer switch.
• This is called a nested switch.
• Even if the case constants of the inner and outer
  switch contain common values, no conflicts will
  arise.

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Nested switch statements

• Program sample
• switch(ch1)
• case A System.out.println(This is part of
  an outer switch)
• switch(ch2)
• case A System.out.println(This is part of
  an inner switch)
• break
• case B //...
• // end of inner case
• break
• case B // ...