Chapter 3 of Programming Languages by Ravi Sethi

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Chapter 3of Programming LanguagesbyRavi Sethi
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Chapter 3 Structured Programming

• 3.1 The Need for Structured
• programming
• Values of variables can change
• The basic units of imperative
• programming are actions, which
• can change the values of variables (assignments,
  procedure calls, read(x)

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Static Programs, Dynamic Computations

• Programs specify computations
• A sequential computation consists of a sequence
  of actions, such as
• writeln (1, 1 1)
• writeln (2, 2 2)
• writeln (3, 3 3)

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• The text is static in a program, but the
  computation is dynamic which occurs every time a
  program runs.
• The gap between the static program and dynamic
  execution has motivated the need for structured
  programming.
• So the reader of a program can understand the
  actions that occur when a program runs

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Design Principles for Imperative Languages

• Built with structure and efficiency in mind
• Structured Programming
• The structure of the program text
• should help us understand what
• the program does.
• (easier to modify and tune for efficiency)

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• Efficiency
• A language must allow an underlying
  assignment-oriented machine to be used directly
  and efficiently.
• A Running Example
• Skip duplicates - removal of adjacent
  duplicates
• Invariants Program Design
• Invariants relate programs and computations -
  tells us about the property of its computations

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• Programming language design must deal at some
  level with program design. The purpose is to make
  programming easier.
• -- An Invariant at some point in a program is an
  assertion that holds whenever that point is
  reached at run time, that is, whenever control
  reaches that point
• -- An Assertion is a true / false condition about
  the state of a computation. An example is the
  condition X gt Y, which relates the value of X and
  Y

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3.2 Syntax-Directed Control Flow

• --Structured statements are by definition
  single-entry and single-exit
• --Structured control flow A program is
  structured if the flow of control through the
  program is evident from the syntactic structure
  of the program text
• Composition of Statements
• Control flows sequentially through a sequence
  of statements, as in

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• temp x x y y temp
• Selection Conditional Statements
• A conditional statement selects one of two
  alternative substatements for execution
• Looping constructs, divided into two forms
• Definite (for)
• Indefinite (while, repeat until)
• Selection case statements - case constants,
  appear in any order

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Implementation of Case Statements

• Except for case statements, the statement
  constructs in imperative languages can be used
  without thinking about how they were implemented
• some implementations recommend only be used when
  the case constants are essentially adjacent

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3.3 Design Consideration Syntax

• Syntax affects usability
• Sequences separators versus terminators, e.g.,
  semicolons
• Fewer programming errors are believed to
  occur if semicolons terminate statements than if
  they separate statements
• Avoiding dangling else

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3.4 Handling Special Cases in loops

• Break and continue statements in loops
• -- A break statement sends control out of the
  enclosing loop to the statement following the
  loop.
• -- A continue statement repeats the enclosing
  loop by sending control to the beginning of the
  loop
• Return statements
• Go to Statements

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3.5 Programming with Invariants

• constructed program is defined as
  single-entry/single-exit.
• Precondition and postcondition
• -- attached just before and after a statement
  both are assertions.

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• x gt 0 and ygt 0 - precondition
• while x gt y do
• y gt 0 and x gt y invariant
• x x - y
• x gt 0 and y gt 0 postcondition
• Linear Search
• Linear search proceeds by examining all the
  elements until either x is found or no elements
  remain to be examined.
• Invariants can describe data structures -A Table
  Organized Around an invariant

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• An approach to implementing linear search
• while elements remain to be examined do begin
• if this element is x then
• return its position
• else
• not found, so return 0

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• Linear Search with a sentinel
• The post condition after the search
• x Ai and x is not in Ai 1 n and
• 0lt i lt n
• The condition for staying within the while loop
  is Ai ! x.

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• The final developed program fragment
• A0 x
• i n
• while x ! Ai do
• i i - 1
• return i

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Your chance at correct high-level pseudocode

• You are to determine whether the sorted array
  X1N contains the element T. Use a binary search

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A Possible Answer
first 1 last N while(first lt last)
middle (first last) /2 if( T
Xmiddle ) return middle else
if (T lt Xmiddle) last middle -1
else first middle 1
print not found
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Proof rules for partial correctness

• Proof rules -- the theory behind invariants
• -- good training for dealing with subtle code.
• Partial correctness
• -- the program is correct if it terminates
• Assertions and formulas
• -- P (pre), Q(post) ,(and), v(or) ,!(this is
  my not since PP cannot easily do the hook

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• Some examples of proof rule and axioms
• Rule for statement composition
• S1 S2
• P S1 Q, S2 R (lt- given formula)
• P S1 S2 R ( lt- conclusion)
• Rule for conditionals
• PE S1 Q, P !E S2 Q
• P if E then S1 else S2 Q
• Rule for while statement
• P E S P
• P while E do S P !E

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• Rule for Assignments
• Q E / x x E Q (assignment axiom -
  since it is an atomic statement, its proof rule
  has no formulas above the line)
• -- The relationship between the postassignment
  value and preassignment value of E is of x
• Rule for Simplification (predicates ex. Igt1)
• P implies P, P S Q ,Q implies Q
• P S Q

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• Statements in Pascal
• control flow in Pascal is purely
  syntax-directed
• the flow of control through a construct can be
  described purely in terms of components of the
  construct.

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3.7 Control Flow in C

• Comparison of C to Pascal
• C Pascal
• if ( E ) S if E then S
• if ( E ) s1 else s2 if E then s1 else s2
• while ( E ) s while E do s
• Assignment Operations
• , , !

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• Assignments within Expressions
• c getchar() can appear as a sub expression
  within a larger expression
• while ( (c getchar()) ! EOF)
• For loops in C Indefinite Iteration
• --An empty test expression is assumed to be true
• for ( ) can be read as forever (the
  expressions E1, E2, E3 are optional in a for loop)

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• Break and continue statements in loops
• example
• for( c getchar( ) )
• if ( c c \t )
• continue
• if ( c ! \n )
• break
• lineno