Sequence Control

Chapter 6

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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=(-B+SQRT(B**2-4*A*C))/(2*A)• 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+(X==o?Y:Y/X)

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Z+(X=0?Y:Y/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

a*fun(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 ((A==0)||(B/A>c))while ((I<=UB)&&(V[I]>c)) 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-> 0A0 | 1A1 | 0 | 1 00100A1 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(X=Y)

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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 x=>y

• if x=ai , a1…ai-1y…an 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 repetitionWhen 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(I=1;I<=10;I++){body}

• infinite loop• for(;;){body}

• counter with exit condition• for(I=1;I<=100&&NotEntfile;I++){body}

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Problems in structured sequence control

Multiple exit loops. do-while-do. exceptional conditions.

p. 278,279