Expressions and Statements

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Expressions and Statements
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Contents

• Side effects expressions and statements
• Expression notations
• Expression evaluation orders
• Conditional statements
• Loops
• Unstructured statements

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Side Effects

• Expressions and statements represent the most
  fundamental computation mechanisms
• An expression, in its pure form, returns a value
  and produces no side effect
• A statement is mainly executed for its side
  effect and returns no value

change variable values, do input or output
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Expression Notations

• Operators can be written in infix, postfix, or
  prefix notation 3 4 5 3 4 5 3 4 5
• No operator precedence and associativity are
  required for postfix and prefix notations
• Lisp uses prefix notation
• Postscript and FORTH use postfix notation

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Applicative Order Evaluation

• Applicative order (or strict) evaluation
  evaluates operands before operators (3 4) (5
  6)
• The evaluation order for operands is usually not
  specified (3 4) (5 6)

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An Example
int x 1 int f(void) x 1 return
x int p( int a, int b) return a
b main() printf("d\n",p( x, f() ))
return 0
f() produces side effect
order of evaluation matters
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Operators with side effects

• Sometimes expressions are explicitly constructed
  with side effects in mind
• C is an expression-oriented language. In C,
  assignment is an expression, not a statement x
  y z
• In C, sequence operator is used to combine
  several expressions (with side effects) into a
  single expression x (y 1, x y, x 1)

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Delayed Evaluation

• The evaluation of an expression can sometimes be
  performed without the evaluation of all its
  subexpressions. This is called delayed (or
  non-strict) evaluation
• Short-circuit evaluation of Boolean
  expressions true x x true false x x
  false if (i lt lastindex ai gt x) if
  (p ! NULL p-gtdata x) if (x ! 0 and y
  x 0)

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if and case Expressions

• An if-expression always evaluates the
  test-expression, but based on that value, only
  one of the then-expression or else-expression is
  ever evaluated e1 ? e2 e3
• ML has case-expression case e1 of a gt e2
  b gt e3 c gt e4

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Referential Transparency

• In the absence of side effects, the order of
  evaluation of subexpressions is immaterial to the
  final value of the expression
• Such expressions share an important property with
  mathematical expressions, called referential
  transparency
• Any two expressions in a program that have the
  same value may be substituted for each other
  anywhere in the program
• Variables would be unknown constants

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Normal Order Evaluation

• Referential transparency allows for a very strong
  form of delayed evaluation to be used
• Normal order evaluation evaluates its operator
  before its operands and each operand is evaluated
  only it is needed for the calculation of the
  value of the operation

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An Example
int double(int x) return x x int
square(int x) return x x

• square(double(2))
• double(2) double(2)
• (2 2) (2 2)
• 4 (2 2)
• 4 4
• 16

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Normal Order Evaluation

• Normal order evaluation might seem inefficient.
  It can be made efficient (2 2) (2 2)
• Delayed evaluation does not need special
  syntax int if_exp(int x, int y, int z)
  if (x) return y else return z

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Normal Order Evaluation

• The presence of side effects can substantially
  change the semantics of an expression int
  get_int(void) int x
  scanf(d, x) return x
  square(get_int())

get_int() get_int()
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If statements
if-statement ? if ( expression ) statement
else statement
if-statement ? if expression then statement
else statement end if
if e1 then s1elsif e2 then s2elsif e3 then
s3end if
C
Ada
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Case-Statements
switch (x - 1) case 0 y 0 z 2
break case 2 case 3 case 4 case 5 y 3
z 1 break case 7 case 9 z 10 break
default break
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Case-Statements
case x - 1 is when 0 gt y 0 z 2
when 2 .. 5 gt y 3 z 1 when 7
9 gt z 10 when others gt null end
case
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Guarded If Statements

• Guarded if statement if B1 -gt S1 B2 -gt
  S2 Bn -gt Sn fi
• If one of the Bis evaluates to true, then Si is
  executed. If more than one of the Bis is true,
  then one and only one Si is selected for
  execution. If none of the Bi is true, then an
  error occurs

Nondeterminism
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Loop-Statements
while (e) Sdo S while (e)for (e1 e2 e3) S
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Restrictions on Index Variable of For Loop

• The value of i cannot be changed within the body
  of the loop
• The value of i is undefined after the loop
  terminates
• i must be of restricted type and may not be
  declared in certain way

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Questions about For Loop

• Are the bounds evaluated only once
• If the lower bound is greater than the upper
  bound, is the loop executed at all
• Is the value of the index variable undefined even
  if an exit or break statement is used to leave
  the loop before termination
• What translator checks are performed on loop
  structure

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Guarded Loops

• Guarded do statement do B1 -gt S1 B2 -gt
  S2 Bn -gt Sn od
• This statement is repeated until all the Bis are
  false. At each step, one of the true Bis is
  selected nondeterministically, and Si is executed

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CLU Iterators
numcount proc (s string) returns (int)
count int 0 for c char in stringchars(s)
do if numeric (c) then count count
1 end end return (count) end
numcount
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CLU Iterators
stringchars iter (s string) yields (char)
index int 1 limit int stringsize(s)
while index lt limit do yield
(stringfetch(s, index)) index index 1
end end stringchars
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The goto Controversy

• Bohm and Jacopini (1966) demonstrated that every
  possible control structure could be expressed
  using structured control constructs, i.e., gotos
  are unnecessary
• Dijkstra (1968) argued that the use of gotos was
  damaging in many ways
• Rubin (1987) argued that the use of gotos was
  justified in certain cases

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Labeled Break in Java
if (ok) while (more)
while (!found) if (disaster)
goto 99 99
ok_break if (ok) while (more)
while (!found) if
(disaster) break ok_break