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
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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 arguments
Solutions:
• 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 arguments
Postfix: Operator follows its arguments
e.g. (1+3*N)/[(1+N)*(1+2*N)]
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.
Control structure between functions
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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;
Y := 2
end
{ X = 1;
Y = 2;
}
Conditionals
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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
then Statements
elseif Conditions
then Statements
else Statements
if (Conditions)
{ Statements }
else if (Conditions)
then { Statements}
else { Statements }
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
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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 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;
Definite loops
for J in 1 .. 10 loop … end loop
for (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
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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 L1
L2: Next statement
Gotos
If 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: …}
}