(8.1) COEN Control Structures  Control structure general issues  Compound statements  Selectors (conditional structures) – single – two-way – multiway  Iteration – counter controlled – logical – intest – user-defined  Guarded commands

(8.2) Control Structure General Issues  Can consider control flow at several levels – within expressions » associativity, operator precedence, order of evaluation – between program units » procedures, coroutines, concurrency – between statements » normal control flow is sequential  A control structure is a control statement and the statements whose execution it controls  To solve any programming problem, at minimum need (Bohm and Jacopini, 1966) – sequence – means of choosing between alternatives – means of repeating execution of statements

(8.3) Control Structure General Issues (cont.)  Much activity into control structure design in – single entry - exit for all structures preferable – unrestricted GOTOs are problematical  Overall design question – what control statements should a language have, beyond selection and pretest logical loops?  Control structure design trades off – expressiveness (many control structures) – readability (few control structures)

(8.4) Compound Statements  Many (older) languages specify only single statements as targets of many actions  Compound statement is way to group many statements together so they are treated as one – begin end in most languages » Algol 60 was first to use – { } in C, C++, Java  If the compound statement can also scope variables it’s referred to as a block – Most modern languages  Modern languages use syntax to avoid compound stmts – if Algol 68 – then – else – fi

(8.5) Selector Statements  Allow execution of alternative statement sequences based on a test  Design issues – what is the form and type of the control expression? – what is the selectable segment form (single statement, statement sequence, compound statement)? – how is the meaning of selectors nested in then clauses of other selectors specified?  Single selector (simple conditional) – some languages (FORTRAN, BASIC) provide explicitly » IF ( ) » bad because requires GOTOs to make a 2-way selector – for that reason most make this a special case of

(8.6) Two-Way Selector  Also called 2-alternative conditional – if then else  “Dangling else” problem – solve with semantics (Pascal) – forbid nesting unless embedded in compound statement (ALGOL 60) – solve with reserved words to mark the end of each if statement (Algol 68, Ada) » provides both flexibility and reliability – Modula-2 follows last approach with same word used to end all statements » flexibility but reduced reliability

(8.7) Multiple Selector  Also called multi-alternative conditional – choose among 3 or more alternatives  Design issues – what is the form and type of the control expression? – what segments are selectable (single, compound, sequential)? – is the construct encapsulated? – is execution flow through the structure restricted to include just a single selectable segment? – what is done about unrepresented expression values?  Examples – FORTRAN arithmetic if » IF ( ) labelneg, label0, labelpos » not encapsulated - needs GOTOs

(8.8) Multiple Selector (continued)  Examples (continued) – C switch » switch (index) { » case 1: » case 3:odd += 1; » sumodd += index; » case 2: » case 4:even += 1; » sumeven += index; » default:printf (“error in switch”); } » select statement based on value in index integer only » straight line control flow after that default always executes » unless break executed to transfer to statement after switch

(8.9) Multiple Selector (continued)  Examples (continued) – case statements (implicit break in alternatives) » case index is » when 1 | 3 =>odd := odd + 1; » sumodd := sumodd + 1; » when 2 | 4 =>even := even + 1; » sumeven := sumeven + 1; » when others =>put (“error in case”); » end case; » selector type is any ordinal » Ada requires others clause » original Pascal left undefined what happens if no case for index value » ISO Pascal says run-time error

(8.10) Multiple Selector (continued)  Examples (continued) – elsif (allows multiple tests like nested if, but easier to read) » if COUNT < 10 » then BAG1 := TRUE; » elsif COUNT < 100 » then BAG2 := TRUE; » elsif COUNT < 1000 » then BAG3 := TRUE; » else BAG4 := FALSE; » end if; – how is this different than case/switch or nested If-Then-Else? » syntactically? » logically? » with respect to implementation?

(8.11) Iteration (loops)  Categorize by – how iteration is controlled » logical loop » counter-controlled loop – where control mechanism appears in loop » pretest before loop body » intest in middle of loop body » posttest after loop body – logical direction of test » termination test ( repeat - until) stop if test true » continuation test (while - do) loop if test true

(8.12) Counter-controlled loops  for I := 1 to 207 step N do – I is iteration control variable (ICV) or loop variable – 1, 207, N are loop parameters – all pretest loops except FORTRANs before 77  ICV design issues – legal types for ICV » typically discrete » FORTRAN IV integer only » FORTRAN 77, 90 also allow real and double – can ICV be modified inside the loop » usually no – scope (Ada scopes ICV to just the loop) – value at termination » undefined (Pascal, FORTRAN IV) » last value assigned (most) » unbound (Ada)

(8.13) Counter-controlled loops (continued)  Effect of loop parameter changes – none (most languages) » calculate trip count from parameter values before loop execution – affect loop (Algol 60, PL/I, COBOL) » PascalAlgol 60 » for I := 1 to N dofor I := 1 step 1 until N do » N := N + 1; N := N + 1; » { doubles N}{ infinite loop if N >1 at start}

(8.14) General Loops  Combine counting and other forms of loop control – great flexibility – can be confusing  Algol 60 – for var := do statement where is » list of expressions » expression step expression until expression » expression while boolean_expression for index := 1 step 2 until 50, 60, 70, 80, index + 1 until 100 do ( index = 1, 3, 5, 7,..., 49, 60, 70, 80, 81, 82,..., 100)

(8.15) General Loops (continued)  Algol 60 (continued) – ICV can’t be changed inside loop – paramters can and affect loop behavior » evaluated each time through  C – for ([expr_1] ; [expr_2] ; [expr_3]) statement » the expressions can be whole statements, or even statement sequences, with the statements separated by commas » the value of a multiple-statement expression is the value of the last statement in the expression » if the second expression is absent, it is an infinite loop – for (sum = 0.0, count = 0; count <= 10 && sum < ; sum = sum + count++) – everything can be changed inside the loop » and affects loop behavior – most flexible loop statement

(8.16) Intest Loops  Provide controlled goto mechanism with destination limited to statement following loop – C break, Modula-2 exit – FORTRAN 90 exit [ ] » exit multiple levels of loop – Ada exit [ ] [ when ]  Also have statements that transfer to the control mechanism at the end of the loop – C continue – FORTRAN 90 cycle [ ]  Usually used for dealing with exceptional conditions in loop (exit if encounter something which will cause errors in rest of loop)

(8.17) User-Defined Iteration Control  Use order and number of elements of some data structure to control iteration – also called iterator – control mechanism is a call to a user-defined function that returns the next element in some chosen order, if there is one; else exit loop  Examples – COMMON LISP (DOLIST L) function » executes body once for each top level element in the list – C's for can be used to build a user-defined iterator » assume p is a pointer to a linked list » for (p=hdr; p; p=next(p)) {... }

(8.18) Unconditional Branching  Problem is readability – so some languages don’t allow them – Modula-2, CLU, Euclid  Label forms – unsigned int constants » Pascal (with colon) » FORTRAN (no colon) – identifiers with colons » ALGOL 60, C, PL/I – identifiers in > » Ada – variables as labels » PL/I » can be assigned values and passed as parameters » highly flexible, but make program impossible to read and difficult to implement

(8.19) Guarded Commands  All of the previous control structures can be implemented with if-then-elses and gotos  Dijkstra proposed two non-deterministic control structures in 1975 that can’t be implemented that way  selector – if – [] – fi – semantics: » evaluate all guards; true guards are open » non-deterministically select one open guard and execute stmts » if no guards true, run time error guard ifx >= y max := x []y >= x max := y fi

(8.20) Guarded Commands (continued)  Iteration – do – [] – od – semantics: » evaluate guards » execute statements for one open guard » repeat until no guards are open doq1 > q2 swap (q1, q2) []q2 > q3 swap (q2, q3) []q3 > q4 swap (q3, q4) od

(8.21) Guarded Commands (continued)  Good models for concurrency  Connection between control statements and program verification is intimate – verification is impossible with gotos – verification is possible with only selection and logical pretest loops – verification is relatively simple with only guarded commands  Never implemented in a real language, but similar syntax in ADA concurrency statements