Notes on Control Flow Categories - Programming Languages | CS 403, Exams of Programming Languages

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November 13, 2001Class 24LanguagesCS 403 - Programming

Start discussion of subprogramsstructures Finish discussion of basic control Go over exam Agenda for Today

(condition tested at end)tested at beginning) or post-test Design issue: Pre-test (condition While loops and their variantsLogically-Controlled Loops

Logically-Controlled Loops

loop statements (1.^ Pascal has separate pretest and posttest logical Language Examples:

while

- do (^) and (^) **repeat

• until )**

just like in the pretest case (expression for the posttest version is treated2.^ C and C++ also have both, but the control

while (^) - (^) do (^) and (^) do (^) -

while )

5. FORTRAN 77 and 90 have neither4. Ada has a pretest version, but no posttestentered at the beginning--Java has no goto)must be Boolean (and the body can only be3. Java is like C, except the control expression

1. Ada - conditional or unconditional; for any loop;MechanismsUser-Located Loop Control any number of levels

for ... loop

LOOP1:

...

while ... loop

exit when ...

...

...

LOOP2:

end loop

end loop LOOP1;...end loop LOOP2;...exit LOOP1 when .....for ... loop

2. C , C++, and Java -MechanismsUser-Located Loop Control

(^) break

Unconditional; for any loop or

(^) switch

one level only (Java’s can have a label)

There is also has a

(^) continue

statement for

iteration, but does not exit the looploops; it skips the remainder of this

Problem: The “Goto” statementUnconditional Branching

(^) readability

1. Unsigned int constants: Pascal (with colon) Label forms: and Java Some languages do not have them:e.g., Modula-

FORTRAN (no colon)

3. Identifiers in2. Identifiers with colons: ALGOL 60, C

(^) << ... >> : Ada

4. Variables as labels: PL/I difficult to implement^
^ Highly flexible, but make programs impossible to read andCan be assigned values and passed as parameters

• this or take another English class CS 403 is a “W” course – so it’s either Due in one more week Position paper Assignment #

^ Two general types of parameters: Parameter Matching

Positional: 1^ Two types of matching:parameters in the subprogram header.
Formal parameters: Variables that appear aspassed as parameters to a subprogram Actual parameters: Variables in the caller that are

to 1st^ , 2stnd to 2^ nd , 3to 3 (^) rd , etc.rd

(^) In this case, order doesn’t matter.
(^) foo(x => my_x, y => my_y)call^
^ Keyword: Include formal/actual mappings in the

^ Two ways to categorize: Parameter Passing Methods

Copy, referencecopied^ ^ By actual implementation of what is
IN, OUT, IN-OUT^ By intended semantics

caller^ Copy: Value copied to and/or from ImplementationsParameter Passing

parameters^
^ Not a good way to implement IN or OUT^
^ Cheap in most cases^ Reference: Address copied to callerbig arrays)^ Can be expensive for large structures (e.g.,

^ In: ImplementationCombining Semantics and

By reference: Straightforward
By copy: Straightforward (copy both directions)^ In-Out:the formal parameter prior to a local definition
By reference: Only works if you disallow uses of
By copy: Straightforward^ Out:to formal parameters within the subroutine
By reference: Only works if you disallow changes By copy: Straightforward

 by reference^ Default mode for arrays is IN-OUTcopy^ Default mode for scalars is IN by C++…

No notion of true OUT parametersbehave as IN only Use const to make array parameters

If IN-OUT implemented by reference, then output is 12. If IN-OUT implemented by copy, then output is 11.} cout << x << endl;foo(x);x = 10;void main(void){} y++;x++;void foo(IN-OUT int y){^ int x; What Happens in this Case?