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

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) –Ada’s approach – allow side effects; but if different results occur they are erroneous

Error conditions (exceptions) Divide by 0, overflow, underflow In Ada, orthogonality combines all types of errors, such as input errors in Exception handling

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 > 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.)

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

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