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					 CS 152, Programming Paradigms
 Fall 2011, SJSU
                        Jeff Smith




6/6/08
    Variables & constants


    Make sure that you understand the
      difference between
       – constants and literals
       – initialization and assignment
    For values of constants
       – may it require computation?
       – must it be computable at compile time?
         load time?


2
    Methods of assignment


    by copying
      – cf. assignment by cloning in Louden, p.
        170
      – cf. deep vs. shallow copies
    by sharing
    Note the different implications for equality.




3
    Issues re assignment & initialization


    Are multiple assignments allowed in one
      statement?
    Does assignment return a value?
    At run time, are newly allocated variables
      given default initial values?
    How can pointer variables be initialized?
      assigned to?
    How is storage allocated for the destinations
      of pointer variables?

4
    Aliases can be created


    in assignment by sharing
    in shallow copying
    indirectly
    as well as when using pointer variables and
      when passing parameters




5
    Expressions and statements


    Ideally the meaning of an expression will
      depend only on
       – the values of the subexpressions, and
       – the meaning of the operator that is used to
         combine these values.
    In this case, the semantics is said to be
      compositional.



6
    Order of evaluation

    With compositional semantics, syntax trees
      can be evaluated bottom up.
    Can subexpressions can be evaluated in any
      order without affecting the value?
       – This issue is important for compiler
         optimization.
    The answer is "yes", in the absence of side
      effects.
       – Note that side effects can include throwing
         an exception, or simply failing.

7
    Short-circuit evaluation


    With short-circuit evaluation of Boolean
     expressions, the order of evaluation of the
     subexpressions can determine whether
     the overall evaluation succeeds or fails.
    Short-circuit evaluation is also a violation of
     the commutative law that holds for AND,
     OR, etc.
    Some languages offer a choice between
     short-circuit operators and those that
     aren't short circuit.
8
    Modifying a function argument


    Another special kind of side effect arises if a
      function can modify its argument.
    In this case the order of evaluation can be
      important in an expression like
       –     f(x) + g(x)
    Functions that don't behave the way
      mathematical functions do make
      programs hard to reason about.


9
     Conditionals and ambiguity


     Conditionals can be ambiguous, e.g.
       – if X then if Y then Z else W
     Ambiguity is usually resolved here by
      associating each else with the nearest if.




10
     Disambiguating conditionals


     A different interpretation can be forced by a
       requirement of explicit delimiters, e.g.
        – if X then {if Y then Z} else W




11
     Disambiguating conditionals

     Or explicit bracketing keywords can be
      required for all conditionals, e.g.
       – if X then
       – if Y then Z endif
       – else W
       – endif
     This is a different string of tokens than
       – if X then
       – if Y then Z else W endif
       – endif
12
     Explicit bracketing keywords


     are used by Ada (cf. T&N, p 165).
     may include an elsif construction to avoid
       multiple closing endifs, e.g.
        –     if --- then ---
        –     elsif --- then ---
        –     elsif --- then ---
        –     else ---
        –     end if

13
     Tests as boolean expressions


     A language designer needs to decide
       whether the tests in conditional
       statements need to have boolean values.

     If not, then a policy is needed to determine
        which nonboolean values correspond to a
        successful test.




14
     Conditional evaluation


     Don't confuse conditional execution with
       conditional evaluation (which returns a
       value).
     In C the latter looks like
        –          A ? B : C
     When considered as ternary functions, such
      functions do not evaluate all of their
      arguments.


15
     Case and switch statements


     Many languages offer multiway conditional
      statements based on the value of a single
      expression
       – e.g., switch statements in C
       – e.g., case statements in Fortran




16
     Issues for case and switch statements


     which types are allowed for the expression?
       any discrete type?
     can cases be combined for two or more
       different values?
     is there a default case?
     what if the actual value is not among the
       specified alternatives?



17
     Indefinite iteration


     Languages may allow a block of statements
       may be executed while some condition
       remains true -- or until some condition
       becomes true.
     In the first case the condition is tested
       before the block; in the second case it's
       tested afterwards.
     C and Java have a while statement for case
       1, and a do-until statement for case 2.

18
     For loops


     Some special cases of iteration are handled
       with for loops, which use a loop control
       variable (LCV).
     Loop control variable issues:
        – what types of LCVs are ok? any ordinal
          type? any discrete type?
        – must an LCV be explicitly declared?
        – must it be local to the loop?


19
     Values of loop control variables (LCVs)


     must integer LCVs begin at 0? at 1?
     can an LCV be assigned a value inside a
       loop? if so, is the assignment effective?
     how can LCVs be incremented?
       decremented?
        – by successor or predecessor only?
     are loop bounds evaluated just once, or at
       every iteration?


20
     Control and for loops


     can control be transferred into a loop?
        – if so, what's the LCV's initial value?
     can control be transferred out of a loop?
     can control be transferred out of the current
       iteration?
        – if so, where does control go? can a
          programmer decide?
     outside of the loop, do LCVs have a value?
        – if so, what is it?
21
     Collections and iteration


     One common use of for loops is to process
       each element of a collection.
     For this sort of traversal, languages
       sometimes provide a specialized for loop
       known as a for-each loop.
     Another possibility is an iterator data
       type.




22
     Transferring control out of a loop


     The ability to transfer control out of a loop is
       useful enough that some languages
       provide special mechanisms for doing so.
     Java has a break construction (and a
       related continue construction) for this
       purpose.
     One advantage of such constructions is that
       they can replace a goto statement of the
       sort found in assembly languages.

23
     Programming using goto


     Using goto often leads to bugs and hard-to-
        read programs.
     It's common for languages not to providing
        a goto statement at all.
     Arguably, using break and continue
        avoids these problems of goto.
     So languages that do not provide a goto
        statement often provide versions of these
        constructions.

24
     Detecting and handling errors


     Many languages have a special mechanism
      for detecting and handling errors.

     The alternative to a special mechanism
       would be to test for every possible error in
       the program code.
     But some errors (out of memory, corrupt
       file) don't result from program code.


25
     Errors in functions


     could be reported by a special return type
        – but the function may already have a
          conflicting return type.
     could be reported by a special value of an
       existing return type (cf. p 181, T&N)
        – but all values of the return type might be
          needed for nonerroneous cases.



26
     Errors in functions, part 2


     could be dealt with by passing an error
       handling function as a parameter.
        – But not all languages allow function
          parameters.
     could be reported by creating an extra
       return type, or passing an extra pointer
       (or reference) parameter.
        – But not all languages allow this.


27
     Detecting and reporting errors


     All of these alternate mechanisms are
       arguably overly complicated, compared to
       an exception mechanism as in Java.

     Errors that are found can sometimes be
       handled locally.
     But the program segment that detects the
       error is not always the best segment to
       handle the error.

28
     Issues for exceptions


     Are they types, or instances of types?
     What types may be exception types?
     Which such types are predefined?
     How can users define their own exception
       types?
     Are exceptions basic, or composite?
     What is the scope of an exception (name)?



29
     Raising and handling exceptions


     How are exceptions raised (or thrown)?

     How are handlers defined?
       – i.e., where does handler code go?




30
     Issues for exception handlers

     what is the scope of a handler?
     what predefined handlers are available?
         – can they be redefined?
     are there default handlers?
     how are handlers bound to exceptions?
     can handling be disabled?
     If there's no local handler,
         – where is a nonlocal handler sought?
         – must it be sought explicitly?
         – how does the handler get control?
31
     Issues for continuation

     after handling, does the handler return to
       the invoking unit?
         – can the programmer decide?
     is the exception reraised?
     where exactly does control resume?
         – in what environment?
     can handlers raise exceptions?
     can a value be returned? if so, how?
     what if there's cleanup to do?
         – (e.g., a file to close)
32
     Issues regarding functions


     what is the syntax of a call? a definition? a
       return?
        – what if there are no arguments?
     can a function have side effects?
     can a function have side effects only
        – i.e., no return value?
     can function definitions be nested?



33
     Function overloading


     can predefined functions be overloaded?

     how can overloaded functions be
       distinguished?
        – is it enough to have distinct return types?




34
     Operator overloading


     (Infix) operators
        – can they be user-defined?
        – if so, how are they distinguished from
          ordinary (prefix) functions?
     can predefined operators be overloaded?
        – if so, must the precedence and
          associativity stay the same?



35
     Parameters

     is an evaluation order specified?
     how are parameters & arguments matched?
        – is it by position? by name?
     are default parameters possible?
        – if so, must they come last?
     are extra arguments just ignored?
        – or treated as an error?
     is a variable number of parameters
       possible?
        – if so, can their types be checked?
36
     Returns and return values

     can a return value be of any type?
        – an aggregate? a function?
        – If a function, what environment is used?
     how is the type of the return value
       specified?
     what is the syntax for a return?
        – assignment to a special variable?
        – an explicit return statement?
     can the return value be ignored by the
       caller?
37

				
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