Categories of Control Flow Constructs

1
Categories of Control Flow Constructs

• Sequencing
• order of expressions and statements
• Selection
• if, else, switch
• Iteration
• loops
• Procedural abstraction
• Recursion
• Concurrency
• Exception handling and speculation
• Nondeterminacy

2
Expressions vs. Statements

• It is useful to differentiate expressions vs.
  statements
• Statements (e.g., assignment) have side effects
  but usually do not return a value (C doesnt
  follow this strictly)
• Expressions (e.g., right hand side of a
  statement) provide a value but usually dont have
  side effects (again except C)
• Expression syntax may be prefix, infix, postfix
• Prefix and postfix dont require parentheses for
  precedence
• Comparison to procedures and functions
• Operands are viewed as arguments or actual
  parameters
• Referential transparency assumes no side effects
• Side effects may change control flow as well as
  values
• Some statements mimic complete control constructs
• E.g., conditional operator in C acts like
  if/else construct

3
Structured vs. Unstructured Control Flow

• Goto considered too low level
• Still available in some languages
• Rarely necessary, often better to use other
  features instead
• E.g., break and continue statements in C
• Many languages offer structured alternatives
• E.g., break to exit a loop or a selected branch
  in C
• E.g., continue to skip the rest of an iteration
  in C
• Return and multi-level return
• Return may set a value and also transfer control
  to caller
• Multi-level return (or exception) may unwind
  farther
• Continuations capture a context for further
  execution
• E.g., to defer part of the execution until later

4
Sequencing

• Ordering of statements
• Happens-before relationship in control flow
• Particularly important when thinking about
  concurrency
• Data dependences
• Previous write (store) to a variable (location)
  seen by subsequent read (load) instruction(s)
• Compound statements
• A block of statements is used in place of a
  single statement

5
Selection

• If statements (e.g., in C)
• If statement evaluates expression (not
  necessarily Boolean)
• If true (non-zero) executes statements in its
  first block
• Otherwise executes else block if one was provided
• Can next if statements, so else associates with
  most recent If that does not already have an else
  part
• Switch (case) statements
• Condense if/else logic into cases of an ordinal
  expression
• Default blocks (no case matches)
• Breaks, fall through can be used to emulate
  ranges of cases

6
Todays Studio Exercises

• Well code up ideas from Scott Chapter 6.1-6.4
• Looking at expressions, statements, sequencing,
  selection
• Next time well look at iteration, recursion,
  non-determinacy
• Todays exercises are again in C
• C allows us to explore control flow issues for
  statements
• Functional languages rely more on function calls
  for control
• Please take advantage of the on-line tutorial and
  reference manual pages that are linked on the
  course web site
• As always, please ask us for help as needed
• When done, email your answers with subject line
  Control Flow Studio I to cse425_at_seas.wustl.edu