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Sequence Control

Dr Nguyen Hua Phung

HCMC University of Technology, Viet Nam

07, 2014

Dr Nguyen Hua Phung Sequence Control 1 / 43

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An expression is a syntactic entity whose evaluationeither:

produces a valuefails to terminate → undefinedExamples

4 + 3 * 2(a + b) * (c - a)(b != 0) ? (a/b) : 0

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Expressions (cont’d)

Expression Evaluation Mechanism

Expressions have functional composition nature

*+

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Infix Notation

(a + b) * (c - d)

Good for binary operatorsUsed in most imperative programming languageMore than two operands?

(b != 0) ? (a/b) : 0Smalltalk:

myBox displayOn: myScreen at: 100@50

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3 + 4 * 5 = 23, not 35

Evaluation priorities in mathematicsProgramming languages define their ownprecedence levels based on mathematics

A bit different precedence rules among languagescan be confusing

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If operators have the same level of precedence, thenapply associativity rules

Mostly left-to-right, except exponentiation operator

An expression contains only one operatorMathematics: associative

Computer: optimization but potential problems

1020∗ 1020∗ 10−20

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average = (count == 0) ? 0 : sum / count;

C-based languages, Perl, JavaScript, Ruby

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Prefix Notation

Polish Prefix: * + a b - c dCambridge Polish Prefix: (* (+ a b) (- c d))Normal Prefix: *(+(a,b),-(c,d))

Derived from mathematical function f(x,y)Parentheses and precedence is no required,provided the -arity of operator is knownMostly see in unary operators

LISP: (append a b c my_list)

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Postfix Notation

Polish Postfix: a b + c d - *Cambridge Polish Postfix: ((a b +) (c d -) *)Normal Postfix: ((a,b)+,(c,d)-)*

Common usage: factorial operator (5!)Used in intermediate code by some compilers

PostScript: (Hello World!) show

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Operand Evaluation Order

C program int a = 5;

What is the value of a? 8 20Reason: Side-effect on the operand of the expression

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Evaluation Mechanisms

Eager evaluationFirst evaluate all operandsThen operators

How about a == 0 ? b : b / aLazy evaluation

Pass the un-evaluated operands to the operatorOperator decide which operands are requiredMuch more expensive than eager

Lazy for conditional, eager for the rest

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Ada: "and", "or" versus "and then", "or else"

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Assignment Statements

C-based languages consider assignment as anexpression

while ((ch = getchar()) != EOF) { }

Introduce compound and unary assignmentoperators (+=, -=, ++, –)

Increasing code legibilityAvoiding unforeseen side effects

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Control Structures

Control statementsSelecting among alternative control flow pathsCausing the repeated execution of sequences ofstatements

Control structure is a control statement and thecollection of its controlled statements

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Two-way Selection

if control_expression then clause

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Python: indentation matters

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Type of selector expression?

How are selectable segments specified?

Execute only one segment or multiple segments?How are case values specified?

What if values fall out of selectable segments?

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Case Study: C

switch (index) { case 1:

Multiple segments exited by break

for unrepresented values

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Case Study: Pascal

Integer or character

- Single statement

- Blockmultiple values,subrange

for unrepresented values

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Mammoth amounts of memory to storeDesign questions:

How is iteration controlled?

Logic, counting

Where should the control appear in the loop?

Pretest and posttest

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Counter-Controlled Loops

Counter-controlled loops must have:

Loop variableInitial and terminal valuesStepsize

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Case Study: Algol-based

General form

f o r i : = f i r s t to l a s t by s t e p do

l o o p body

end

Semantic

[define end_save]end_save := last

i = firstloop:

if i > end_save goto out

[loop body]

i := i + stepgoto loopout:[undefine end_save]

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if expr_2 = 0 goto out

[loop body]expr_3goto loopout:

Can be infinite loop

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Logically Controlled Loops

Repeat based on Boolean expression rather than acounter

Are more general than counter-controlledDesign issues:

Should the control be pretest or posttest?

Should the logically controlled loop be a specialform of a counting loop or a separate statement?

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Case Study: C

loop:

[loop body]goto loopout:

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User-Located Loop Control

Programmer can choose a location for loop controlrather than top or bottom

Simple design: infinite loops but include user-locatedloop exits

Languages have exit statements: break and continue

A need for restricted goto statement

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What if we replace break by continue?

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Iteration Based on Data Structures

Controlled by the number of elements in a datastructure

Iterator:

Called at the beginning of each iterationReturns an element each time it is called in somespecific order

Pre-defined or user-defined iterator

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Unconditional Branching

Unconditional branch, or goto, is the most powerfulstatement for controlling the flow of execution of aprogram?s statements

Dangerous: difficult to read, as the result, highlyunreliable and costly to maintain

Structured programming: say no to gotoJava, Python, Ruby: no goto

It still exists in form of loop exit, but they are severelyrestricted gotos

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ExpressionsOperator precedence and associativitySide effects

StatementsAssignmentSelection StatementLoop structures

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