CHAPTER 8 SEQUENCE CONTROL

1
CHAPTER 8 SEQUENCE CONTROL

• Hardware- Von Neumann architecture (sequence from
  incrementing program counter) loop and
  conditional from test and jump (branch)
• Control Abstraction includes intramodule,
  intermodule, and concurrent module control
• Implicit or explicit

2
Expressions

• Precedence rules (consider exponentiation)
• Tree-structure
• Execution time representation when operand values
  are determined
• prefix postfix to avoid ambiguity
• Ex Forth and Lisp
• Side effects
• X A fun (A) where fun (A) changes the value
  of A
• What is the value of the expression?
• What if A itself must be evaluated? Should A be
  fetched once or twice? (compiler optimization an
  issue)
• Adas approach allow side effects but if
  different results occur they are erroneous

3
Error conditions (exceptions)

• Divide by 0, overflow, underflow
• In Ada, orthogonality combines all types of
  errors, such as input errors in Exception
  handling

4
Short-circuit boolean operators

• In Ada, and .. then or.. else
• and / or are not short circuit
• In C, C, Java, are short circuit
• Optimization
• Program structure
• if (x ! 0 )
• if (a/x gt 5)

5
Intramodule

• Scalar (unstructured) statements
• Assignment
• Input
• Parameter passing
• Go to/ break/ (multiple) exit/multiple entries
• Composite
• Conditional
• Discuss case (switch)
• Discuss nested if
• Loop
• Perform 5 (times)
• Test at bottom of Fortran do loop
• Recursion
• Infinite (for servers, etc.)

6
Exceptions

• Program controlled termination on serious error
• COBOL on size error (compiler supplied code),
  PL/1
• Ada user defined exceptions identical to system
  defined ones
• Constraint_Error, Storage_Error, Tasking_Error
• Exceptions propagate dynamic stack
• C assert
• Java try

7
SNOBOL4 (from the textbook)

• Output longest odd length bit string palindrome
• start grammar 0 1 0 grammar 0 1
  grammar 1
• add 00 and 11 to the above
• loop newline TRIM (INPUT) f(end)
• newline (POS (0) SPAN (01) RPOS(0))
  f(bad)
• sn SIZE (newline)
• next newline POS(0) grammar . palindrome
  POS(sn) s(match) f(not)
• match OUTPUT MATCH palindrome loop
• not sn sn 1 next
• bad OUTPUT improper input newline
  loop
• end