Adapted from Scott, 2006 1

Chapter 6:: Control Flow

Programming Language PragmaticsMichael L. Scott

Adapted from Scott, 2006 2

Control Flow

• Basic paradigms for control flow:– Sequencing– Selection– Iteration– Subroutines, recursion (and related control

abstractions, e.g. iterators)– Nondeterminacy – ordering unspecified– Concurrency – parallel or interleaved execution

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Expression Evaluation • Infix, prefix operators• Precedence, associativity (see Figure 6.1)

– C has 15 levels - too many to remember– Pascal has 3 levels - too few for good semantics

• if A < B and C < D then… A < (B and C) < D

– Fortran has 8– Ada has 6– Lesson: The programmer working in several languages

will use parentheses!

• Usually: mult/div>add/sub>relational>logical

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Expression Evaluation

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Expression Evaluation

• Short-circuiting– Consider (a < b) && (b < c)

• If a >= b there is no point evaluating whether b < c because (a < b) && (b < c) is automatically false

– Other similar situationsif( b != 0 && a/b == c ) ...if( *p && p->foo ) ...if( f || messy() ) ...

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Expression Evaluation • Variables as values vs. variables as references

– value-oriented languages• A variable is a named container for a value (r-value)• C, Pascal, Ada

– reference-oriented languages• Every variable is an l-value (denotes location)• most functional languages (Lisp, Scheme, ML)• Clu, Smalltalk

– Java deliberately in-between• built-in types are values• user-defined types are objects - references

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Expression Evaluation

• Expression-oriented vs. statement-oriented languages– expression-oriented:

• functional languages (Lisp, Scheme, ML)• Algol-68

– statement-oriented:• most imperative languages

– C kinda halfway in-between (distinguishes)• allows expression to appear instead of statement

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Expression Evaluation • Orthogonality

– Features that can be used in any combination– All combinations of features make sense– Meaning of a feature is consistent, regardless of

the context in which it is used

How about this?if (if b != 0 then a/b == c else false) then ...if (if f then true else messy()) then ...

Adapted from Scott, 2006 9

Expression Evaluation • Assignment

– statement (or expression) executed for its side effect– assignment operators (+=, -=, etc)

• handy• avoid redundant work (or need for optimization)• perform side effects exactly onceA[index(f-4*j)] = A[index(f-4*j)] + 1 A[index(f-4*j)] += 1

– C --, ++• Prefix vs. postfix form • Array[k++] vs. Array[++k]

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Expression Evaluation

• Assignment (cont.)– Multiway assignment (e.g., Perl, Python, Ruby)

• a, b = c, d• tuple = tuple

– Multiple function return values (e.g., Matlab, G2)• a, b, c = funct( x, y );

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Expression Evaluation

• Side Effects– often discussed in the context of functions– a side effect is some permanent state change

caused by execution of function• some noticeable effect of call other than return value• in a more general sense, assignment statements

provide the ultimate example of side effects– they change the value of a variable

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Expression Evaluation

• Side effects are fundamental to the whole VonNeuman computing approach

• In (pure) functional and logic languages, there are no such changes– These languages are called SINGLE-

ASSIGNMENT languages

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• Sequencing– specifies a linear ordering on statements

• one statement follows another

– very imperative, Von-Neuman

Sequencing

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• Selection– sequential if statementsif ... then ... elseif ... then ... elsif ... else(cond(C1) (E1)(C2) (E2)...(Cn) (En) (T) (Et))

Selection

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– Fortran computed gotos• goto( 15, 20, 30, 100 ) n

– Case/switch statementsSwitch( expression ) {

Case 1: 1st arm of codeCase 2: 2nd arm of codeDefault: default arm of code}

• Default clause?• List or range of values for each case?• Integral values or general values for expression?• "Fall through" semantics or not?• What if expression evaluates to missing value?

Selection

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• jump code generation• for selection and logically-controlled loops• no point in computing a Boolean value into a register, then testing

it• instead of:

– passing register containing Boolean out – pass inherited attributes INTO expression indicating where to jump to

if true, and where to jump to if false

• Example (follows):if ((A > B) and (C > D)) or (E <> F)) thenthen_clauseelseelse_clause

Selection

17

• Code generated w/o short-circuiting (Pascal)if ((A > B) and (C > D)) or (E <> F))

r1 := A -- loadr2 := B

r1 := r1 > r2 r2 := C

r3 := Dr2 := r2 > r3

r1 := r1 & r2 r2 := E r3 := F

r2 := r2 $<>$ r3 r1 := r1 $|$ r2 if r1 = 0 goto L2

L1: then_clause -- label not actually usedgoto L3

L2: else_clause L3:

Selection

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• Code generated w/ short-circuiting (C)if ((A > B) and (C > D)) or (E <> F))

r1 := A r2 := B if r1 <= r2 goto L4 r1 := Cr2 := Dif r1 > r2 goto L1L4: r1 := Er2 := F if r1 = r2 goto L2L1: then_clause goto L3 L2: else_clauseL3:

Selection

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• Enumeration-controlled do/for loops– Pascal or Fortran-style do/for loops

do 20 i = 1 10 . . . 20 continue

• scope of control variable?– Local to for loop in C, C++, Java; global in Fortran

• changes to loop variable or bounds within loop?– Prohibited by Pascal, Fortran 77; permitted by Fortran IV

• value of i after the loop?– Undefined by Pascal, Fortran IV; most recent for Fortran 77

• always executes at least once?– Yes in Fortran, Pascal; no in C, C++, Java

Iteration

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Iteration • The goto controversy

– assertion: gotos are needed almost exclusively to cope with 1. lack of one-and-a-half loops

– mid-loop exit– multi-level return from loop

2. early return from procedure3. error returns

– Scott: "In many years of programming, I can't remember using one for any other purpose."• Usually there is a structured alternative in modern languages

1.break/continue2. multiple returns3. exception handling mechanisms

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Recursion

• Recursion– equally powerful to iteration– mechanical transformations back and forth– often more intuitive (sometimes less)– naïve implementation less efficient

• no special syntax required• fundamental to functional languages like Scheme

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Recursion

• Tail recursion– No computation follows recursive call /* assume a, b > 0 */

int gcd (int a, int b) {if (a == b) return a;else if (a > b) return gcd (a - b, b);else return gcd (a, b – a);

}