Conditional Logic in Smalltalk

1
Conditional Logic in Smalltalk

• Control Structures
• Boolean Logic
• If Statements

2
Control Structures

• Bohm and Jacopini developed the theory
• All computer languages need each structure
• All are necessary to harness the power of
  computers
• Control Structures are
• Sequence
• Selection
• Iteration

3
Sequence

• Executing instructions in sequence
• Working down the page

4
Selection

• Conditional logic
• Choosing between alternative paths based on some
  predefined test
• Commonly implemented as If statements

5
Iteration

• Executing a set of instructions more than once
• A loop
• There must be a mechanism for exiting the loop
• if not an infinite loop
• Exit mechanism always involves some test

6
Boolean Logic

• (10 gt 12) evaluates to
  false
• (3 lt 14) evaluates to
  true
• (Joe lt Sam) evaluates to
  true
• (a gt b)
• (amtDue gt amtPaid)
• (custName gt custBalance)

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true and false

• true is an instance of True
• false is an instance of False
• True and False are subclasses of the class
  Boolean

8
More Logic

• amtOwed amtPaid
• amtPaid 1000
• amtOwed 2000
• (amtPaid gt amtOwed)
• how about if amount paid is 3000

9
Look Up Boolean in the Class Browser

• The Hierarchy
• Object
• Boolean
• False
• True

10
Messages for True

• (and)
• (or)
• not
• ifFalse
• ifTrue
• ifTrueifFalse
• ifFalseifTrue

11
Messages for False

• (and)
• (or)
• not
• ifFalse
• ifTrue
• ifTrueifFalse
• ifFalseifTrue

12
Logical AND -- (and)

• true true evaluates to
  true
• true false evaluates to
  false
• false true evaluates to
  false
• false false evaluates to
  false
• and is an equivalent shortcut method

13
Logical OR -- (or)

• true true evaluates to
  true
• true false evaluates to
  true
• false true evaluates to
  true
• false false evaluates to
  false
• or is an equivalent shortcut method

14
Some Samples

• (a gt b) (7 lt 5)
• (a gt b) (7 lt 5)
• (atBats gt 100) (battingAverage gt 300)
• (amtOwed gt amtPaid) ((lastPaymentDate
  subtractDate dueDate) gt 30)
• Hint Use lots of parentheses

15
Conditional Logic

• boolean
• ifTrue block of code.
• boolean
• ifFalse block of code.
• boolean
• ifTrue block of code
• ifFalse block of code.

16
Blocks

• A Block is one or more Smalltalk statements
• a module (i.e., hunk of code)
• Contained in square brackets
• Used with conditional logic
• A block can be
• an argument
• a receiver
• A block is executed when it receives the selector
  value

17
Examples of Blocks

• 85 12 / 34
• to execute this
• 85 12 / 34 value.
• newAmt oldAmt 1.1.
• Transcript show New Amt Is , newAmt
  printString.

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Conditional Logic

• boolean
• ifTrue block of code.
• boolean
• ifFalse block of code.
• boolean
• ifTrue block of code for true
• ifFalse block of code for false.

19
IfTrue Examples

• (Janet lt Pete)
• ifTrue Janet comes first.
• .
• (Janet gt Pete)
• ifTrue Janet comes first.
• .

20
IfTrue More Complex Examples

• (amtRequestedFromATM gt maxWithdrawal)
• ifTrue window show Sorry, this
  transaction is not approved.
• customerBeingServed
  transactionReject.
• .

21
IfFalse Examples

• (Janet lt Pete)
• ifFalse Janet comes first.
• .
• (Janet gt Pete)
• ifFalse Janet comes first.
• .

22
IfTrueIfFalse Examples

• minAmt curBalance
• (minAmt lt curBalance)
• ifTrue status OK
• ifFalse status not OK.

23
IfTrueIfFalse Example Continued

• minAmt curBalance
• minAmt 100.0.
• curBalance 300.0.
• (minAmt lt curBalance)
• ifTrue status OK
• ifFalse status not OK.

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Nested Ifs

• boolean1
• ifTrue true block
• ifFalse false block.
• The true and and false blocks can themselves
  contain conditional statements
• boolean1
• ifTrue (boolean2)
• ifTrue block1
• ifFalse block2
• ifFalse (boolean3)
• ifTrue block3
• ifFalse block4
• .

25
Nested Ifs -- Social Security Example

• prompter wage yearToDateSocSec
• prompter CwTextPrompter new.
• prompter title 'Enter Wage'.
• wage prompter prompt.
• wage wage asDecimal asFloat.
• prompter CwTextPrompter new.
• prompter title 'Enter Year-To-Date Social
  Security'.
• yearToDateSocSec prompter prompt.
• yearToDateSocSec yearToDateSocSec asDecimal
  asFloat.

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If Message (example continued)

• (yearToDateSocSec lt 3757.20)
• ifTrue
• (yearToDateSocSec (wage
  0.062) lt 3757.20 )
• ifTrue wage 0.062
• ifFalse 3757.20 -
  yearToDateSocSec
• ifFalse 0.

27
Summary -- Conditional Logic

• Conditional logic is necessary for most programs
• Conditional logic allows you to take different
  paths through a program based on predefined
  criteria
• Conditional logic in Smalltalk uses message
  passing

28
Summary -- If statements

• starts with an expression in parentheses that
  evaluates to true or false
• this is followed by a keyword message with such
  selectors as ifTrue ifFalse
  ifTrueifFalse
• each of the keywords is followed by a block which
  is one or more statements in square brackets
• formatting is very important for readability
• writing If statements is easy after a little
  practice