Selection statements Repetition statements

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Selection statements Repetition statements
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Lesson plan

• Main concepts
• Practice session
• If- then
• Switch
• Nested if

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Sequential sequence and Control Statement

• Sequential sequences are insufficient to solve
  most problems.
• Get in the Car.
• Drive to the supermarket.
• Tank nearly empty?
• Get in the Car.
• If we need gas, then fill up.
• Drive to the supermarket.

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Dependent Actions

• A question is asked.
• A condition is checked.
• The dependent actions are only performed if the
  answer is Yes (the condition is true).
• Sometimes the dependent actions are performed,
  and sometimes they are not.

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Example Logging In

• Computer access
• Request a login name from the user.
• Request a matching password.
• If the password matches the login name, then
  grant access.

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Simple Choice Statement
if (ltboolean expressiongt) ltblockgt else
ltblockgt
if (ltboolean expressiongt) single
statement else single statement
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Boolean expression

• Boolean expression is a conditional expression
  that is evaluated to either true or false.
• Conditional expression is a three part
  expression
• ltexp.gt ltrelational operatorsgt ltexp.gt
• Boolean expressions can be combined by boolean
  operators

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Relational Operators
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• Boolean operators
• means AND
• means OR
• ! means NOT

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Boolean Operators
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• Example of Boolean expression
• testScore lt 62
• age gt 35
• Height gt 6.5
• (testScore lt 62) (age gt 35)
• testScore 95 ???????

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• Block
• Represents compound statement
• ltstatement 1gt
• ltstatement 2gt
• ltstatement 3gt

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• Example
• If (age lt 35)
• coPay 5
• System.out.println(.)
• Else
• coPay 10System.out.println(.)

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• Practice
• Identify invalid boolean expression
• Identify invalid If-then-else statement

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Answers for last practice exercise

• I.
• (i) (x1gt0) (x1)
• Valid
• (ii) x22x-1
• Invalid. Arithmetic expression
• (iii) ((y2) ! 0)
• Valid

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Answers to last practice exercise

• if (xy)
• System.out.println( xy)
• Else
• System.out.println( x does not equal to y)

Should be Lowercase
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Answers to last practice exercise

• if (xy) then
• grade A
• else
• Grade B

If we actually ask (is x equal y) then it should
be (xy)
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Answers to last practice exercise

• if (xy)
• grade A
• else
• grade B

Hanging else
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Simple Choice Statement
if (ltboolean expressiongt) ltblockgt else
ltblockgt // optional
if (ltboolean expressiongt) single
statement else single statement //
optional
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Answers to some questions

• Java machine code is generated and stored in
  .class files. For example
• LoanCalculator.java
• LoanCalculator.class
• When we compile a java program outside of
  Textpad, we need to use javac.exe ltsource_namegt
• When we run a java program outside of Textpad, we
  need to use java.exe ltclass_namegt

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Answers to some questions

• How to clear screen from DOS
• for (int i0 ilt25 i)
• System.out.println()

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• Nested If Statement
• if (ltboolean expressiongt)
• if (ltboolean expressiongt)
• if (ltboolean expressiongt)
• statement 1
• else if (ltboolean expressiongt)
• statement 2
• else
• statement 3
• else .

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• Example
• if (score gt 95)
• grade A
• else if (score gt90)
• gradeA-
• else if (scoregt85)
• grade B

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• SWITCH statement
• switch (variable)
• ltcase label 1gt ltcase body 1gt
• ltcase label 2gt ltcase body 2gt
• ltcase label igt has the form
• case ltconstantgt
• or default
• ltcase body igt is a block or a single statement,
  with break
• variable of integer, or char data type

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• Example

switch (month) case 1
System.out.println("January") break
case 2 System.out.println("February") break
case 3 System.out.println("March")
break case 4 System.out.println("Ap
ril") break case 5
System.out.println("May") break
case 6 System.out.println("June")
break .. default
System.out.println("Not a month!") break

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Char data type boolean data type

• Char
• It is 2 bytes
• Char constant is denoted by single quote
• for example s, h
• Special characters
• \b backspace, \t tab
• Boolean
• - Can be true or false

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Examples

• char aLetter
• aLetter k
• aLetter k
• boolean flag
• flag true
• flag false

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• Practice
• Evaluate boolean expressions
• Identify invalid Switch statement

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Project 1 (d)

• Exit the system normally
• System.exit(0)
• Exit the system abnormally
• e.g System.exit(1)
• a nonzero status code indicates abnormal
  termination.

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Repetition statements

• Repetition of statements is a further common
  facet of everyday life and object behavior
• Repeat every day
• Get up have breakfast go off to work have
  lunch do more work have an evening meal relax
  go to bed.
• Java has three types of loop for repetition
• while, do-while, for.

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The While Loop
while(ltboolean expressiongt) // Repeat
multiple statements. statement 1
statement 2 statement 3 ...
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• Example
• // Print the numbers 1 to maximum.
• Assuming that maximum is 100
• int maximum 100
• int nextToPrint 1
• while(nextToPrint lt maximum)
• System.out.print(nextToPrint" ")
• // Get ready to print the next number.
• nextToPrint nextToPrint 1
• System.out.println()

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The Do Loop
do // Repeat multiple statements.
statement 1 statement 2 statement 3
... while(ltboolean expression)

• Note that the statements in the body of the loop
  are always executed at least one.
• Note the final semicolon, which is required.

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Printing a Sequence
int maximum 100 int nextToPrint 1 do
System.out.print(nextToPrint" ")
// Get ready for the next number.
nextToPrint nextToPrint 1
while(nextToPrint lt maximum)
System.out.println()
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The For Loop

• Used to repeat statements a fixed number of
  times
• Add three teaspoons of sugar.
• Knock twice and then give the password.
• Used very often with arrays and collections of
  objects
• Similar in effect to the while loop.

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The For-Loop Outline
// Repeat multiple statements. for(initialization
condition post-body update) // Statements
to be repeated. statement 1 statement 2
statement 3 ...

• Commonly used with increment
• and decrement operators.

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Increment and Decrement

• Used as a shorthand for add-one-to and
  subtract-one-from
• value value1
• value 1
• value
• Prefix and postfix forms
• value
• --value
• value--

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A For Loop to Print a Sequence
int maximum 100 int nextToPrint for(nextToPrin
t 1 nextToPrint lt maximum
nextToPrint) System.out.print(next
ToPrint ) System.out.println()
The variable is not available outside the
loop 1234567891011 1 2 3 4 5 6.
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• Practice
• Develop a java class called
• SumCalculator.java
• which computes a sum of all integer from 1 to 100
  and displays the result to the screen