Control I Expressions and Statements

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Chapter 7

• Control I - Expressions and Statements
• Part 2

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Loops and variations on while

• Guarded Do
• do b1 -gt s1
• b2 -gt s2
• ....
• bn -gt sn
• od

• This statement is repeated until all the b's are
  false
• (nondeterministically one true b is selected at
  each step)

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Example

• What are the possible results?
• Assume x 1, y 2, z 3
• do xy -gt y--
• yz -gt z--
• zx -gt x--
• od
• What if x 1, y 2, z 2?
• What if x 1, y 1, z 1?

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Standard form of loop construct

• pretesting
• while b do s od
• Algol68
• while b DO s END module-2

• posttesting
• repeat s until b
• pascal
• Syntactic sugar
• s
• while not b do s

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loop-exit in modula-2

• Sometimes it is helpful to be able to exit a loop
  from arbitrary points inside it.
• loop-exit in modula-2
• loop
• ...
• if b1 then exit end
• ...
• if b2 then exit end
• ...
• end

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break and continue

• break - exit the loop completely
• continue - skips the remainder of the body of the
  loop, resuming execution with the next evaluation
  of the control expression
• an example of the use of break
• while (1) / always secceeds /
• if () break

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For-Loop

• a common special case of a looping construct
• C/C/Java
• for ( e1 e2 e3 ) S
• Modula-2
• FOR I 0 to MAX STEP 2 DO
• END
• Fortran
• DO 20 I 0, MAX
• ...
• 20 CONTINUE
• I is the control variable, tested and incremented
  after each iteration

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Use of Index Variable

• To gain efficiency in processing for-loop,
  restrictions must be placed on I
• I can not be changed within the body of loop
• I is undefined after the loop terminates
• I must be of restricted types e.g., local
  variables

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Bound test

• Modern languages do pretesting
• Some older Fortran do post testing

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GOTO controversy

• It should stay (Fortran)
• It should be abolished (using loop-exit instead)
• It should remain, but with severe restrictions on
  its use (Pascal)

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Exception Handling

• The control of error conditions or other unusual
  events during the execution of a program.
• An exception is any unexpected or infrequent
  event.
• When an exception occurs, it is said to be raised
  or thrown.

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Exceptions

• Runtime errors out-of-range array subscripts
  division by zero
• Static errors (such as syntax and type errors) in
  interpreted languages
• Any unusual or infrequent event end of file end
  of page bad input timeout

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Exception Handler

• a procedure or code sequence that is designed to
  be executed when a particular exception is raised
  and that is supposed to make it possible for
  normal execution to resume
• An exception handler is said to handle or catch
  an exception

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history

• Pioneered by PL/I in the 1960s
• significantly advanced by CLU in the 1970s
• design questions were largely resolved in the
  1980s and early 1990s
• virtually all major current languages have
  built-in exception handling mechanisms, e.g.,
  C, Java, Ada, ML, and Common Lisp

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Robustness

• a program must be able to recover from errors and
  continue execution
• exception handling is an attempt to imitate in a
  programming language the features of a hardware
  interrupt or error trap
• the program is said to be aborted, or crashes,
  if the underlying machine or operating system
  takes control

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Asynchronous exceptions

• Asynchronous exceptions failures occur at a low
  level that lead to situations in which the
  underlying operating system to take drastic
  action to terminate a program
• e.g., failure of a hardware device, memory
  allocation problems, communication problems
• can occur at any moment
• not in response to the execution of program code
• the program cannot do anything about it

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Synchronous exceptions

• Exceptions that occur in direct response to
  actions by the program
• e.g., open a file, perform a particular
  calculation
• user-defined exceptions can only be synchronous
• predefined or library exceptions may include a
  number of asynchronous exceptions

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Deal with exceptions in a language

• Find out exception conditions before error occurs
• if (y 0)
• handleError(denominator in ratio is zero)
• else
• ratio x / y

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If error occurs

• Pass the error condition back to the caller
• e.g., in C
• enum ErrorKind OutOfInput, BadChar, Normal
• ErrorKind getNumber ( unsigned result )
• int ch fgetc(input)
• if (ch EOF) return OutOfInput
• else if (! Isdigit(ch)) return BadChar
• / continue to compute /
• result
• return Normal

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Pass an exception handling procedure into the
procedure as a parameter (1)

• enum ErrorKind OutOfInput, BadChar, Normal
• typedef void (ErrorProc)(ErrorKind)
• ...
• unsigned value
• ...
• void handler (ErrorKind error)
• ...
• ...

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Pass an exception handling procedure into the
procedure as a parameter (2)

• unsigned getNumber ( ErrorProc handle )
• unsigned result
• int ch fgetc(input)
• if (ch EOF) handle(OutOfInput)
• else if (! Isdigit(ch)) handle(BadChar)
• / continue to compute /
• result
• return result
• ...
• value getNumber (handler)

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Declaring exceptions in advance

• Declaring exceptions in advance of their
  occurrence and specifying what a program is to do
  if an exception occurs
• Factors to consider
• Exceptions predefined exceptions user defined
  exception scopes
• Exception handlers default handlers provided for
  predefined exceptions user defined exception
  handlers scopes
• Control control pass to a handler control pass
  after a handler is executed

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Exception handling mechanisms used by
block-structured languages

• Adas predefined exceptions
• CONSTRAINT_ERROR subrange constraints array
  index constraints
• NUMERIC_ERROR overflow division by zero
• PROGRAM_ERROR errors occur during the dynamic
  processing of a declaration
• STORAGE_ERROR failure of dynamic memory
  allocation
• TASKING_ERROR errors occur during concurrency
  control

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User defined exceptions

• Syntax (in Ada or ML)
• exception Trouble
• exception Big_Trouble
• The declaration of an exception follows the same
  static scope rules as other declaration

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Exception handlers

• try-catch blocks in C
• try
• // normal code goes here
• catch (Trouble t)
• // handle trouble, if possible
• displayMessage(trouble here!)
• catch (Big_Trouble b)
• // handle big trouble, if possible
• catch ()
• // actual three dots here, not an ellipsis
• // handle any remaining uncaught exceptions

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Exception handlers (in Ada syntax)

• begin
• -- normal code goes here
• exception
• when Trouble gt
• -- handle trouble, if possible
• displayMessage(trouble here!)
• ...
• When Big_Trouble gt
• -- handle big trouble, if possible
• ...
• when others gt
• -- handle any remaining uncaught exceptions
• end

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Continuation of the exception

• The environment under which execution continues
  after an exception
• Resumption model resume the execution at the
  point where the exception occurred
• Termination model cause the block in which the
  exception occurred to terminate execution

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Dynamic propagation of exceptions

• Ada (also C, ML, ...) follows termination model
  for exceptions
• Dynamic propagation of exceptions If the block
  has no exception part, or if no handler for the
  exception exists, the block is exited, and the
  exception is re-raised in the controlling block
  in the execution path (call unwinding or stack
  unwinding)
• If an exception is raised for which no handler is
  found in the dynamic chain, a PROGRAM_ERROR is
  raised by the system, and execution is aborted

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End of Chapter 7 Part 2