Program Design and Branching Structures

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Lecture 2
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• Program Design and Branching Structures
• Introduction to Top-Down Design Techniques
• Software Engineering
• Big Codes REQUIRE Prior Analysis
• Top-Down Design gt Standard Engineering Practice
  gt
• Start with a big problem, break it into its
  components and solve
• Keywords gt Software Engineering, Top-Down
  Design, Structured Programming, Algorithm,
  Decomposition, Step-Wise Refinement (Done with
  PseudoCode), Unit Testing, Program Testing, etc.)

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• Use of Pseudocode and Flowcharts
• Graphical Description of Algorithms

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• Logical Constants, Variables and Operators
• 3.3.1 Logical Constants and Variables
• Logical constants .TRUE. , .FALSE.
• Logical variable contains a logical value, i.e.,
  either .TRUE. or .FALSE.
• Logical variables type specified as
• LOGICAL V1, V2, V3, etc.
• Example V1 .TRUE.
• 3.3.2 Assignment Statements and Logical
  Calculations
• General Form
• Logical variable name logical expression (T or
  F)

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• 3 Relational Operators
• Operate on Numeric Values to Produce Logical
  Result
• General Form v1 .op. v2, where .op. is as
  follows
• or
  .eq. Equal to
• / or .ne. Not
  equal to
• gt or
  .gt. Greater than
• gt or
  .ge. Greater than or equal to
• lt or
  .lt. Less than
• lt or .le. Less
  than or equal to
• 
  (New) (Old)
• Examples 3lt4 is T 3gt4 is F AgtB is F.
  (Alphabetical order)
• Relational operations done after ALL arithmetic
  operations
• Example 73 lt 211 What would happen if ALL
  ops were left to right?

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Combinational Logical Operators
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• Logical Operators Operate on Logical Values to
  Produce a
• Logical
  Result.
• General Form l1 .op. l2, where .op. is as
  follows
• .AND. Logical
  AND
• A.AND.B If A and B are True, then logical
  expression is True.
• .OR. Logical
  OR
• A.OR.B If A or B is True, logical expression
  is True.
• .EQV. Logical
  Equivalence
• A.EQV.B If A is the same (T,F) as B, logical
  expression is True.
• .NEQV. Logical
  Non-Equivalence
• A.NEQV.B If A and B are different, logical
  expression is True.
• .NOT. Logical
  NOT
• .NOT.A A is changed to the reverse of A,
  i.e., TgtF, FgtT.
• Hierarchy of Computational Operations (Within
  parentheses)
• Arithmetic operations
• Relational operations (LgtR)
• .NOT. operators
• .AND. operators (LgtR)
• .OR. operators (LgtR)

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• Logical Values in I/O Statements
• Input First T or F in next input field defines
  the variable. Error otherwise.
• Output T output for a TRUE variable, F for a
  FALSE Variable.
• Significance of Logical Variables and
  Expressions
• Logical values are rarely the final result of
  Fortran programs.
• Primarily used in controlling Branching and
  Looping Structures

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• Control Constructs Branches
• IF statement and SELECT CASE statement
• 3.4.1 Block IF Construct
• Block of code executed only if logical
  expression is .T.
• IF (logical expression) THEN
• Statement 1 (Note indented code)
• Statement 2
• END IF

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• The ELSE and ELSE IF Clauses
• IF (logical expression) THEN
• Statement 1
• Statement 2
• ELSE IF (logical expression) THEN
• Statement 1
• Statement 2
• ELSE IF (logical expression) THEN
• Statement 1
• Statement 2
• ELSE
• Statement 1
• Statement 2
• END IF
• Note that ELSE is not required in above

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• Named Block IF Constructs
• Optionally name Block IF Statements Helps us
  keep code straight
• Ex L1 IF (logexp) THEN
• ..........
• L2 IF (logexp) THEN
• .
• L3 IF (Logexp) THEN
• .
• END IF L3
• .
• END IF L2
• .
• END IF L1

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• Notes Concerning Use of Block IF Constructs
• Suggestion Use Names and Indented Form for
  Nested Block IFs.
• 3.4.6 Logical IF Statement
• IF (logical expression) Statement
• Ex 1 IF (GOTO) GO TO 100
• Ex 2 IF (GOTO) IF (A-B) 100,101,102

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• The SELECT CASE Construct
• Form name SELECT CASE (case expr)
• CASE (case value)
• Statement Block Execute if case expr case
  value
• Or
• CASE (low value )
• Statement Block Execute if case expr gt low value
• Or
• CASE ( high value)
• Statement Block Execute if case expr lt high
  value
• Or
• CASE (low value high value)
• Statement Block Execute if low value lt case expr
• And high value gt case expr
• END SELECT name

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Example 1
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Example 2
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• More on DeBugging
• Use correct Relational/Logical Operators
• Testing Real Variables for Equality a NoNo!