ENGINEERING COMPUTING ENG1602

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ENGINEERING COMPUTING ENG1602
BACHELOR of ENGINEERING
LECTURES Semester 1 2004
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ENGINEERING COMPUTING ENG1602
Week 6 - Lecture 2 Conditional Loops
Switch Statement
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Conditional Loops

• The For .. Loop construct considered previously
  repeats a set of instructions a number of times -
  the number of times being controlled by the index
  range of the loop variable.
• There are two other possibilities which are
  controlled by an associated condition being
  either True or False
• while loop
• do while loop

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1. do while loop

• Chapman p. 97.
• SYNTAX
• do
• while ltBoolean
  Expressiongt

ltStatement 1gt ltStatement 2gt .. .. ltStatement ngt
Loop Body
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do while Loop - Notes

• Loop Body always Executes at least once.
• ltBoolean Expressiongt true Statements
  executed again from do.
• ltBoolean Expressiongt false Program
  goes to next statement following while and
  continues execution.

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do while Flow chart
ltBody of Loopgt
true
ltBoolean Expressiongt
false
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do while The Trap!

• To STOP a do while Loop, one of the
  variables used in the controlling ltBoolean
  expressiongt must change within the loop each
  time the loop is executed.
• Otherwise the ltBoolean Expressiongt wilI never
  come false, and the loop will never terminate
  thus becoming Endless or Infi
  nite.

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do while Examples

• Example 1 N 1 do
  System.out.print( N ) N N
  1
• while (N lt 5)
• Output 1 2 3 4

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Example 2.

• N 1 do System.out.print( N
  ) while (N lt 5)
• Output 1 1 1 1 1 .....Infinite Loop

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Another do while Example

• StdIn in new StdIn()
• int Number
• do
• System.out.println(Input a Number 0-10)
• Numberin.readInt()
• while ((Number gt 10) (Number lt 0))

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2. WHILE LOOP

• while ltBoolean Expressiongt

ltStatement 1gt ltStatement 2gt .. ltStatement ngt
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WHILE - LOOP

• Loop Only Executes if ltBoolean Expressiongt is
  TRUE at the start.
• Once started something must change in the Loop
  Body to insure ultimate termination.

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WHILE Flow chart
ltBoolean Expressiongt
TRUE
ltBody of Loopgt
FALSE
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WHILE Example

• Read Numbers from the Keyboard until a negative
  number is detected.
• THEN Determine the AVERAGE of all entered numbers
  excluding the last ( negative number used to
  terminate the process ).

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WHILE Example

• double Next,Sum,Average
• int Count
• StdIn in new StdIn()
• Sum 0//Preset Variables
• Count 0
• Nextin.readDouble()//Read First Number
• while ( Next gt 0 )
• Count Count 1 Sum Sum
  Next Nextin.readDouble()
• Average Sum / Count

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Choosing Optionsswitch Statement

• IF - THEN Allows selection and execution
  of ONE Option if TRUE.
• IF - THEN - ELSE Allows selection and
  execution of one of two Alternatives depending
  upon whether TRUE or FALSE.
• NESTED IF s Allow selection and execution
  of ONE of a number of Options. However as the
  number of options increases the construction
  becomes clumsy.

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Nested IF StatementThree possible outcomes
Statement-1
True
False
IF 1
True
False
IF 2
Statement3
Statement2
Statement1
Statement1
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NESTED IF Statement
Statement-1Before IF if ltExpression 1gt
if ltExpression2 gt Statement3
else Statement2
else Statement1
Statement1 After IF
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Nested IF Statement -Example
if (Guess gt Number) System.out.println(Too
High) else if (Guess lt Number)
System.out.println(Too Low) else
System.out.println(Correct)
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Choosing Optionsswitch Statement

• Chapman p. 82
• The switch Statement Simple
  statement that allows selection and execution of
  one of many options.
• The Branching Decision is based upon the value of
  a variable of type integer or character .

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switch Statement
Statement 1
Evaluate Controlling Expression
on Label List A
on Label List B
on Label List N
Statement 2A
Statement 2B
Statement 2C
Statement 3
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switch Statement Syntax

• Statement 1 Before switch switch
  ltExpressiongt ltLabel List Agt
  ltStatement 2Agt ltLabel List Bgt ltStatement
  2Bgt Etc. ltLabel List Ngt
  ltStatement 2Ngt Stateme
  nt 2After CASE

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switch Statement - Limitations

• The Values used in the Lists must be literals or
  constants of the same type as the controlling
  expression
• As a general rule ltExpressiongt must evaluate
  to either TYPE INTEGER or TYPE CHAR.
• REAL or STRING TYPES are NOT Allowed.

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switch Example 1

• Assuming Variable "Year" in an INTEGER set to
  have the value of the current year Eg. 2001,
  then the following switch statement may be
  written
• switch ( Year 4 )
• case 0
• System.out.println(A Leap Year)
• break
• case 3
• System.out.println(Maybe Next Year)
• break
• case 1
• case 2
• System.out.println( 'Not a Leap Year' )

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switch Example2

• switch ltExpressiongt evaluates to a label not to
  be found in the label list.
• switch (N)
• case 2
• System.out.println(Value of N is 2)
• case 4
• System.out.println(Value of N is 4)
• What happens if N 0,1,3,5,6 etc. ? ( i.e. a
  label not in the list ) ?
• No branch executed if want to catch these then
  use default branch

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switch Example 3

• switch ( N )
• case 2
• System.out.println(N is 2)
• break
• case 4
• System.out.println(N is 4)
• break
• default
• System.out.println(N is not 2 or 4)
• break
• This construction gives an output for all
  possible values of N.

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Switch example 4

• Assume Mark is int and grade is char
• switch (Mark/10)
• case 8 case 9 case 10
• Grade'H'
• break
• case 7 Grade'D'
• break
• case 6 Grade'C'
• break
• case 5 Grade'P'
• break
• case 0 case 1 case 2 case 3 case 4
• Grade'N'
• break

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Switch notes

• break is placed between case branches to prevent
  fall through. Actual meaning of switch
  statement is that execution jumps to FIRST branch
  with matching label and then follows through all
  subsequent branches unless break is encountered
  to break out of switch.
• Thus a break after the last branch is superfluous
  BUT YOU ARE ADVISED PUT ONE IN BY HABIT. This
  avoids bugs when add to the cases and forget to
  go back and put in a break after what was the
  last branch.

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