Sequence Control

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Sequence Control

• Chapter 6

2

• Control structures the basic framework within
  which operations and data are combined into
  programs.
• Sequence control
• data control

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• Sequence control the control of the order of
  execution of the operations (Primitive, user
  defined).
• Data control the control of the transmission of
  data among the subprograms of a program.

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sequence control

• Structures used in expressions (and thus within
  statements) such as precedence rules ,
  parentheses.
• Structures used between statements or groups of
  statements, such as conditional , iteration.
• Structures used between subprograms, such as
  subprograms calls , coroutines.

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• Implicit sequence control (default) defined by
  the language .
• Explicit sequence control defined by the
  programmer.

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6.2. Sequence with arithmetic expressions

• R(-BSQRT(B2-4AC))/(2A)
• 15 separate operations
• can the two references to the value of B and A be
  combined?
• what order to evaluate the expression to minimize
  the temporary storage ?

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• Tree-structure representation (p.240, fig. 6.1.)
• syntax for expression
• prefix notation no parentheses
• postfix notation no parentheses
• infix notation
• semantics for expressions
• evaluation of expressions (p.243)

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Semantics for expressions

• Hierarchy of operations (precedence rules) p.245
• Associativity (left to right ,right to left)
• p.245,246

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Execution-Time Representation

• Machine code sequences.
• tree structures.
• prefix or postfix form.

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Evaluation of tree representation of expressions

• Problems
• uniform evaluation rules
• Side effects
• error conditions
• Short-circuit boolean expressions

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uniform evaluation rules

• Eager evaluation rule first evaluate the
  operands (order is not important). P.250
• lazy evaluation rule never evaluate the
  operands first.
• Fig. 6.4. Z(Xo?YY/X)

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• Z(X0?YY/X)
• pass the operands (or at least the last two
  operands) to the conditional operation
  unevaluated an let the operation determine the
  order of evaluation.
• Passing parameters by value or by name

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• No simple evaluation rule
• In LISP functions are split into two categories,
• receives evaluated operands
• receives unevaluated operands
• In SNOBOL4
• programmer-defined operations receives evaluated
  operands,
• language-defined operations receives
  unevaluated operands.

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Side effects

• afun(x)a
• evaluate each term in sequence
• evaluate a only once
• call fun(x) before evaluating a

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Side effects

• solutions
• side effects should be outlawed in expressions,
• language definition clears the order of
  evaluation, cause optimization impossible,
• ignore the question, decided by implementer .

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error conditions

• Overflow, divide by zero.
• Solutions varies from language to language and
  implementation to implementation.

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Short-circuit Boolean expressions

• If ((A0)(B/Agtc))
• while ((IltUB)(VIgtc))
• in many languages both operands are evaluated
  before the Boolean operation is evaluated.
• Solution in Ada, explicitly
• and then , or else

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6.3. Sequence with non-arithmetic expressions

• Pattern matching
• term rewriting
• unification
• backtracking

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Pattern matching

• Pattern matching by string replacement (in
  SNOBOL4 )
• A-gt 0A0 1A1 0 1
• 00100
• A1 matches the center 1
• A2 matches 0A10
• A3 matches 0A20

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• In Prolog, a relation as a set of n-tuples,
• specify known instances of these relations
  (called facts),
• other instances can be derived.

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• ParentOf(John,Mary).
• ParentOf(Susan,Mary).
• ParentOf(Bill,John).
• ParentOf(Ann,John).
• ParentOf(X,Mary)
• ParentOf(X,Mary), ParentOf(Y,Mary), not(XY)

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• Building relations out of other relations,
• GrandparentOf(X,Y)- ParentOf(X,Z),
  ParentOf(Z,Y).

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Term Rewriting

• A restricted form of pattern matching
• string a1a2 an
• rewrite rule xgty
• if xai , a1ai-1yan is a term rewrite of a1a2
  an.

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Unification

• Prolog uses unification, or the substitution of
  variables in relations, to pattern match.
• Determine if the query has a valid substitution
  consistent with the rules and facts in the
  database.

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• A rule
• GrandparentOf(X,Y) - ParentOf(X,Z),
  ParentOf(Z,Y)
• ParentOf(X,Mary) ParentOf(John,Y)
• ParentOf(John,Mary) unifies it.

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• In Prolog
• Queries are unified with rules or with facts in
  the database until true results.
• If false results, it means that a wrong rule or
  fact is used, then an alternative (if any) must
  be tried.

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6.4. Sequence control between statements

• Basic statements
• assignments (p. 265)
• subprogram calls
• I/O statements

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Forms of statement-level sequence control

• Composition
• Alternation
• Iteration

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• Explicit sequence control
• goto
• conditional
• unconditional
• break , continue (in C)

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Structured programming design

• Gotos advantages
• hardware support
• easy to use in small programs
• familiar to older programmers(!!)
• general purpose

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• Gotos disadvantages
• lack of hierarchical program structure
• there is no one-in, one-out control structure
• spaghetti code (program text, execution order)
• groups of statements serve multiple purposes

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Structured Programming

• hierarchical program design using only THE three
  structures.
• Hierarchical Implementation like design.
• No spaghetti code, textual sequence of statements
  like execution sequence.
• groups of statements serve single purpose.

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Structured sequence control

• Compound statements
• conditional statements
• if (single-branch,multi-branch), case (p.274) .
• Iteration statements

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Iteration statements

• Head and a body (p.276,277)
• Simple repetition
• repetition while condition holds
• repetition while incrementing counter
• infinite repetition
• When is the termination test made?
• When are the variables used in the statement
  ahead evaluated?

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• In C
• simple counter from 1 to 1o
• for(I1Ilt10I)body
• infinite loop
• for()body
• counter with exit condition
• for(I1Ilt100NotEntfileI)body

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Problems in structured sequence control

• Multiple exit loops.
• do-while-do.
• exceptional conditions.
• p. 278,279