Control Structures

Hierarchical Statement Structure

• Standard in imperative languages since Algol60.

• Exceptions: Early FORTRAN, COBOL, early BASIC, APL. (Fortran kludged definite loops.)

Control structures within function

• Sequence: { S1, S2 … Sk }

• Conditional: if statement, case statement.

• Loop: while loop, for loop.

• Jump: goto, exception raising

Expression evaluation

Ambiguity of grouping: is 1+2*3 interpreted as (1+2)*3=9 or 1+(2*3)=7?

Solutions:

• Operator precedence. * binds more tightly than +, so 1+2*3 = 7.

• Order precedence. Always compute left to right so 1+2*3=(1+2)*3 = 9

Best to use a lot of parentheses.

Short circuit evaluation

If Y <> 0 and X/Y > 5, compute Z.• if (Y <> 0 and X/Y > 5) Z = … --- but operators evaluate their argumentsSolutions:• Lazy evaluation of Booleans: if (Y <> 0 && X/Y > 5) Z = …• Explicit conditionals if (Y <> 0) then if (X/Y > 5) Z= …

Prefix and postfix notation

Prefix: Operator precedes its argumentsPostfix: Operator follows its argumentse.g. (1+3*N)/[(1+N)*(1+2*N)]Prefix: / + 1 * 3 N *+ 1 N + 1 * 2 NPostfix: 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.

Control structure between functions

• Function call and return

• Coroutine yield

• Parallel synchronization

• Exception raising

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 (R==0) then jump to A

Possible forms of conditions and addresses vary.

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.

Semicolons

• Pascal: Semicolons are separators

• C etc.: Semicolons are terminators

begin X := 1; { X = 1;

Y := 2 Y = 2;

end }

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

if-else ambiguity

if Conditions if (Conditions)

then then {

if Conditions if (Conditions)

then Statements { Statements }

else Statements else { Statements }

end if }

end if

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

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: N=1; V: N=5; X: N=10 L: N=50 }

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.

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

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

Multiple exits

If you want to exit from imbedded loops, you need to specify which one you’re exiting.

Ada solution: Label loopsOuter: 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;

Definite loops

for J in 1 .. 10 loop … end loopfor (int I=0; I < 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

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 I=0; I < N; I++)and optimizes the code for this.

C: break and continue

break: exit innermost loop (exitloop)

continue: go to next iteration of innermost loop.

Loop counter

• Local to loop?

• If not, what is the value on exit?

• Settable in body of loop?

• Multiple loop counters?

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(); }

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 L1L2: Next statement

GotosIf you don’t have hierarchical statement structure, you need goto’s.

“Go To Statements Considered Harmful” --- Dijkstra, 1968

Many more recent languages (e.g. Java) don’t have them.

Certainly must prohibit jumping into the middle of a embedded structure; e.g.

{ … go to L for (int I=0; I<N; I++) { … L: …} }