Control Structures

1
Control Structures
2
Hierarchical Statement Structure

• Standard in imperative languages since Algol60.
• Exceptions Early FORTRAN, COBOL, early BASIC,
  APL. (Fortran kludged definite loops.)

3
Control structures within function

• Sequence S1, S2 Sk
• Conditional if statement, case statement.
• Loop while loop, for loop.
• Jump goto, exception raising

4
Expression evaluation

• Ambiguity of grouping is 123 interpreted as
  (12)39 or 1(23)7?
• Solutions
• Operator precedence. binds more tightly than
  , so 123 7.
• Order precedence. Always compute left to right
  so 123(12)3 9
• Best to use a lot of parentheses.

5
Short circuit evaluation

• If Y ltgt 0 and X/Y gt 5, compute Z.
• if (Y ltgt 0 and X/Y gt 5) Z
• --- but operators evaluate their arguments
• Solutions
• Lazy evaluation of Booleans
• if (Y ltgt 0 X/Y gt 5) Z
• Explicit conditionals
• if (Y ltgt 0) then if (X/Y gt 5) Z

6
Prefix and postfix notation

• Prefix Operator precedes its arguments
• Postfix Operator follows its arguments
• e.g. (13N)/(1N)(12N)
• Prefix / 1 3 N 1 N 1 2 N
• Postfix 1 3 N 1 N 1 2 N /
• If each operator has a fixed number of arguments,
  then prefix and postfix are unambiguous with no
  need for brackets, precedence, or associativity
  conventions.

7
Control structure between functions

• Function call and return
• Coroutine yield
• Parallel synchronization
• Exception raising

8
Assembly language

• In assembly language, (essentially) the only
  control structures are
• Progression Move to the next statement
  (increment the program counter).
• Unconditional jump
• JMP A Jump to address A
• Conditional jump
• JMZ R,A If (R0) then jump to A
• Possible forms of conditions and addresses vary.

9
Sequence

• Pascal begin end
• C, C, Java
• Ada Brackets for sequence are unnecessary.
  Keywords for control structures suffice.
• for J in 1 .. N loop end loop
• ABC, Python Indicate structure by indentation.

10
Semicolons

• Pascal Semicolons are separators
• C etc. Semicolons are terminators
• begin X 1 X 1
• Y 2 Y 2
• end

11
Conditionals

• if Condition then Statement -- Pascal, Ada
• if (Condition) Statement -- C, C, Java
• To avoid ambiguities, use end marker end if,
  
• To deal with alternatives, use keyword or
  bracketing

• if Conditions if (Conditions)
• then Statements Statements
• elseif Conditions else if
  (Conditions)
• then Statements then
  Statements
• else Statements else
  Statements
• end if

12
if-else ambiguity

• if Conditions if (Conditions)
• then then
• if Conditions if (Conditions)
• then Statements Statements
• else Statements else Statements
• end if
• end if

13
Compiling conditionals

• if (Cond) then Statement1 else Statement2
• Assembly
• . Compute Cond in register R
• JMZ R,L
• Code for Statement1
• JMP END
• L Code for Statement2
• END Next statement

14
Multi-way selection

• The case statement is the most useful control
  structure because most programs are
  interpreters. (Ed Schonberg)
• Can achieve same with conditionals, but case
  statements are clearer.
• case (NextChar)
• I N1
• V N5
• X N10
• L N50

15
The well-structured case statement

• Discrete type.
• No flow-through (hideous C misdesign).
• Every value is in exactly one option.
• Default option for values not covered.
• Labels are computable at compile time.

16
Implementation

• Finite set of possibilities can build a table of
  addresses, and
• convert expression into table index
• compute value
• transform into index
• retrieve address of corresponding code fragment
• branch to code fragment and execute
• branch to end of case statement

17
Loops

• while loop test at beginning
• while (Condition) Statement
• repeat loop test at end
• repeat Statement until Condition
• do Statement while (Condition)
• breaking out test in middle
• loop Statement
• if (condition) then exitloop
• Statement
• end loop

18
Multiple exits

• If you want to exit from imbedded loops, you need
  to specify which one youre exiting.
• Ada solution Label loops
• Outer while C1 loop
• Inner while C2 loop
• Innermost while C3 loop
• exit Outer when MajorFailure
• exit Inner when SmallAnnoyance
• end loop Innermost
• end loop Inner
• end loop Outer

19
Definite loops

• for J in 1 .. 10 loop end loop
• for (int I0 I lt N I)
• Design issues
• Evaluation of bounds (only at start, since
  Algol60)
• Scope of loop variables
• Empty bodies
• Increments other than 1
• Backward iteration
• Non-numeric domains

20
Definite loops

• Since the C for loop
• for (start test increment) body
• allows arbitrary test conditions and arbitrary
  increment actions, this is not really a definite
  loop at all, just an distinctively formatted
  while loop.
• However, the compiler is sensitive to the
  particular construction
• for (int I0 I lt N I)
• and optimizes the code for this.

21
C break and continue

• break exit innermost loop (exitloop)
• continue go to next iteration of innermost loop.

22
Loop counter

• Local to loop?
• If not, what is the value on exit?
• Settable in body of loop?
• Multiple loop counters?

23
Non-numeric domains

• Ada for M in months loop end loop
• Other data types
• iterator Collection.iterator()
• element thing iterator.first
• while (iterator.moreElements())
• Body
• thing interator.next()

24
Implementation of loops

• loop Statement1
• if (condition) exit loop
• Statement2
• end loop
• L1 Code for Statement1
• Compute not(condition) and put in R
• JMZ R,L2
• Code for Statement2
• JMP L1
• L2 Next statement

25
Gotos

• If you dont have hierarchical statement
  structure, you need gotos.
• Go To Statements Considered Harmful ---
  Dijkstra, 1968
• Many more recent languages (e.g. Java) dont have
  them.
• Certainly must prohibit jumping into the middle
  of a embedded structure e.g.
• go to L
• for (int I0 IltN I)
• L