programming language ch4

8/6/2019 Programming Language Ch4

1/21

CompanyLogo

@

Programming Languagechap.4 SEQUENCE CONTROL

Internet Management Technology Lab.


2/21

Sawan kumar

Contents

Introduction

Sequence Control Structures

Sequencing with Arithmetic Expressions

Sequencing with Non-Arithmetic Expressions

Sequence Control between Statements


3/21

SungKyunKwan Univ.

Introduction

Program control involves

Sequence control control of the order of execution of the operations

Data control

control of transmission of data among subprograms of aprogram


4/21

SungKyunKwan Univ.

Sequence Control Structures

Structures used in expressions precedence rules and parenthesis

Structures used between group of statements conditional and iteration statements

Structures used between subprograms

subprogram calls

Implicit control vs. explicit control Implicit control

defined by the language and not modified until user

redefine it Explicit control

programmer uses to redefine implicit control sequence(parenthesis)


5/21

SungKyunKwan Univ.

Sequencing with arithmetic expressions(1/5)

Prefix Notation

Operators come first, then the operands e.g. (a + b) * (c - a) => * + a b - c a

No ambiguity and no parenthesis needed

Number of arguments for an operator must be known a

priori Relatively easy to decode using stack mechanism

Postfix notation

An operator follows its operands

e.g. (a+b) *(c-a) => a b + c a - *

No ambiguity and no parenthesis needed

Relatively easy to decode using stack mechanism


6/21

SungKyunKwan Univ.


Infix notation

Only suitable for binary operations (Thus used with prefix)

For more than one infix operator, it is inherentlyambiguous

Parenthesis is used to explicitly annotate the order


7/21

SungKyunKwan Univ.


Implicit control rules

Hierarchy of operations (precedence) e.g. Ada **, abs, not : Exponential absolute negation

* / mod : Multiplication, division,

+ - : Unary addition and subtraction

+ - & : Binary addition and subtraction = < > : Relational

and or xor : Boolean operations

Associative left-right associativity (+, -, others)

right-left associativity (exponential)


8/21

SungKyunKwan Univ.


Issues in Evaluating Expressions

Uniform Evaluation rules (eager and lazy) Eager

evaluate the operands as soon as they appear (parallelprocessing SNOBOL)

Lazy Evaluation

Delay evaluation of operands as late as possible (LISP)

foo( x ){ ...

if ( a > 0 )a = x;

else a = a + 1;

}

foo( b/a );


9/21

SungKyunKwan Univ.


Issues in Evaluating Expressions Side effects

a * foo(x) + a; say a= 1; foo(x)generates 3

if each term is evaluated 1 * 3 + 2 =5

if a is evaluated only once 1 * 3 + 1= 4

if evaluate foo(x) first 2 * 3 + 2 = 8

Error Condition No solution other than exceptional

statements

Short-circuit expression Use the characteristics of and oror operation

a || b - a is true b is not evaluateda && b - a is false b is not evaluatede.g.b = 9;if ( a = 4 || b = 3 )printf (" a = %d b = %d\n");

(continued)


10/21

SungKyunKwan Univ.

Sequencing with Non-arithmetic expressions(1/3)

Pattern Matching

Pattern matching operation An operation matches and assigns a set of variables to a

predefined template

e.g. (palindromes)A -> 0A0 | 1A1 | 0 | 1matches 00100 -> 00A00 -> 0A0 -> Aapplied term rewriting 00A00 is a term rewrite of

00100

term rewriting

A restricted form of pattern match

e.g. Given string a1a2a3a4 and -> .if = a

3then a

1a

2a

4we say a1a2a4 is term rewrite of a1a2a3a4

e.g.) in MLfun fact(1) = 1| fact(N:int) = N * fact(N - 1);

fun length( nil ) = 0| length( a :: y) = 1 +

length( y )


11/21

SungKyunKwan Univ.


Pattern Matching (continued)

Unification and substitution In Prolog, Database consists of facts and rules

Substitution

replacement of a string to another one

Unification

A pattern match to determine if the query has a validsubstitution consistent with the rules and facts in the database

Fact : ParentOf( Ann, John )

Fact : ParentOf( Sam, Ann )

Fact : ParentOf( Tom, John )

Query : ParentOf( x, John )

X= Ann, x = TomRule : GrandOarentOf( X,Y ) :- ParentOf(X,Z), ParentOf(Z,Y)then query : GrandParentOf (x,John) => x = sam


12/21

SungKyunKwan Univ.


Backtracking

Backup to the previous subgoal that matched and tryanother possible goal for it to match when currentsubgoal is failed


13/21

SungKyunKwan Univ.

Sequence Control Between Statements (1/9)

Basic statements

Statements that apply operations to data objects Considered as a unit of step

e.g. : assignment operations, subprogram calls,input/output statements

Statement Level Sequence Control Composition

Statements may be placed in a textual sequence and beexecuted in order

Alternation Two Sequences may form alternatives and either one is

executed Iteration

A sequence of statements are executed repeatedly, zero ormore times


14/21

SungKyunKwan Univ.


Explicit Sequence Control (1/3)

Goto statement (1/2) Unconditional goto

Transfers control to the labeled statement

Ex) goto NEXT

Conditional goto

Transfers control to the labeled statement only if the specifiedcondition holds

Ex) if A=0 then goto NEXT


15/21

SungKyunKwan Univ.



Goto statement (2/2) Advantages

Direct hardware support for efficient execution

Simple and easy use and easy understanding for low levelprogrammers

May simulate any control structure

Problems

Leading unstructured programming (no goto in ML)

Superfluous any program with goto can be translated intoanother one w/o goto

Undefined control with nested structure


16/21

SungKyunKwan Univ.



Break Statement Control to move forward to an explicit point at the end of

given control structure

That is, exit the immediately enclosing while, for, or switchstatement

While (a > b){

A;

if ( k > 0 ) break;B;

}


17/21

SungKyunKwan Univ.


Structured Programming Design

Structured program is A program that contains no GOTOs

Structured programming emphasizes

Hierarchical design of program structure with thecomposition, alternation, and iteration

Use structured sequence control that represents hierarchicaldesign

Textual sequences corresponds to the execution sequence

Unique purpose group of statements then it is copied


18/21

SungKyunKwan Univ.


Structured Sequence Control (1/4)

Control statements that only contains one-in one-outcontrol sequences (no goto in ML)

Compound statement A sequence of statements that may be treated as a single

unit in construction of large one

Conditional statement Expresses alternation of two or more statements, or

optional execution of a single statement Ex) IF statement, CASE statement

If statements are implemented using the usual hardwaresupported branch and jump instruction

Case statements are implemented using ajump table toavoid the testing of the same variable

Jump table : A vector whose components are unconditionaljump instructions


19/21

SungKyunKwan Univ.


Structure Sequence Control (2/4)

Iteration Statement (1/2) Simple repetition

Repeat a fixed number of times

Ex) perform BODY Ktimes (COBOL)

Repetition while condition holds

Reevaluate the condition each time after BODY

Ex) While testdo BODY

Repetition while incrementing a counter

Initial value, final value, and the increment

Ex) for l := 1 step 2 until 30 do BODY

Indefinite repetition

Used when the exit condition is complex and hard to express

In C, all 4 forms can be written by for statement


20/21

SungKyunKwan Univ.



Iteration Statement (2/2) Advantages

Easy to debug, understand, and verify

Disadvantages

Multiple exit loops

Can be replaced with exit in ADA or break in C., butPASCALgoto must be used in the for loop

Do-while-do

The condition is checked in the middle of the loop

Exceptional conditions

Unexpected end-of-file, subscript range error, bad data Ex) raise statement in ADA raise BAD DATA


21/21

SungKyunKwan Univ.



Proper program Formal model of control structure, as a flow chart which;

Has a single entry arc

Has a single exit arc,

Has a path from the entry arc to each node and from each

node to the exit arc Prime program

A proper program that cannot be subdivided into smallerproper programs

Composite program

A program that is not a prime The Structure theorem

Bohm and Jacobini (1966)

Any prime program could be converted into tone using only whileand ifstatements called a well-structured program

programming language ch4

Documents