1. sequence of execution of high-level statements
Control Structures
sequence of execution of high-level statements

2. Control is control of sequence of execution
machine level – manipulate address register:
(un)conditional branch
high level
branch to label  goto GOTO GO TO
constrained branch: break
high level: selection, iteration

3. Control by branching - FORTRAN
IF(X .EQ. Y) X=X+1
IF(X .EQ. Y) GO TO 100
<< ‘else’ statements >>
GO TO 200
100 CONTINUE (dummy statement)
<< ‘then’ statements >>
200 CONTINUE
<< rest of program >>

4. Control by branching - FORTRAN
IF(X-Y) 10,10,20
10 CONTINUE
<< statements for X<=Y
GO TO 30
20 CONTINUE
<< statements for X>Y >>
30 CONTINUE
<< rest of program >>

5. Statement labels integers: FORTRAN, Pascal
identifiers as constants: ALGOL, C
variables! PL/1

6. The ‘goto’ debate: is branching needed?
powerful: only control needed
easy to abuse: readability, reliability suffer
‘goto’ unnecessary (Böhm and Jacopini, 1966) but most languages retain limited (unlabelled) branching – e.g., break

7. Top-down structured programming 1975
flow chart model: no goto
3 flow structures
-sequence
-if-then-else (selection)
-do-while (iteration)
boolean expr
statement

8. Selection: either / or how many alternatives:
1 do something or nothing
2 if then else
3 IF (X-Y) 10,20,30
many switch, nested IF, COND (LISP)
variations on the selection structure

9. Selection design decisions - 1
control of multiple statements
in the control statement
if (condition)
then
<statements>
else
endif
compound statement / block

10. Selection design decisions - 1
control of multiple statements
in the control statement 
compound statement / block
if (condition) if (condition)
<statement> { <statements>
else }
<statement> else
{ <statements> }

11. Selection design decisions - 2
nesting selection – syntax and logic
syntax – use block or control structure
if (sum==0)
if (count==0)
then
result = 0;
endif;
else
result = 1;
convention/rule

12. Selection design decisions - 2
nesting selection – syntax and logic
syntax – use block or control structure
convention/rule java, etc
if (sum==0) //p ?
if (count==0)
result = 0;
else
result = 1;

13. Multiple selection many variations
FORTRAN computed goto is multiple branch
GO TO (10,20,30,40,50,60), X^^2+Y
case / switch model (independent selections)
design decisions
default case allowed/ignored?
branch after section
nested ifs (dependent selections)
guarded statements

14. Multiple selection - 1 GO TO (10,20,30,40,50,60), X^^2+Y
FORTRAN computed goto is multiple branch
GO TO (10,20,30,40,50,60), X^^2+Y
label list computed index
semantics:
compute index
if index < 1 or > max(6), go to next statement
go to label at index location in label list

15. Multiple selection - 2
switch/case model

16. Multiple selection - 3 using if
Ada:
if (condition)
<stmt>
elsif (condition)
else
LISP
(COND ((condition)(value))
((condition)(value)) ((condition)(value)) ( T (value))) 1 2 1 3 4 2 3 4 1 2 3 4

17. Multiple selection - 4 guarded statements – Dijkstra
evaluate all conditions:
none true – error
one true – do statement
many true – select statement at random
if <condition> -> <statement>
[] <condition> -> <statement> fi

18. Iteration: repetition of statements
control by counter - for loop
control by data (by logic) - while loop
number of executions:
0 or more - pretest at beginning of loop body
1 or more - posttest at end of loop body
branching models of repetition

19. Counter controlled loops
developed with array indexing
counter (loop variable) parameters
start, stop, stepsize

20. Counter controlled loops
design decision
parameters computed when
once or every execution?
x = 10;
do I = 0 to x
x = x - 1
11 executions or 6 executions

21. Counter controlled loops
design decisions:
loop variable type, scope and value
x = 0;
I = 100;
do I = 1 to 10 by 3
begin
x = x + I;
I = I - 1;
print I
end;
print I, x;

22. Counter controlled loops
FORTRAN DO loop
posttest, parameters evaluated once, number of executions predetermined, loop variable retains last value
DO 100 K = 1, 10, 2
<< body >>
100 CONTINUE

23. Generalizing the counter
start, stop, step - FORTRAN
sequence, index expression & condition, “all of the above” - ALGOL
for i = 4,5,6 i*10 while i,10000, 12, 14 step 10 until 25 do
simple counting – Ada, Pascal
expressions – C,C++,java
for (<expr1>,<expr2>,<expr3>)
<statement>

24. Logical control design issue: pretest or posttest?
most languages have ‘while’ pretest loop
posttest:
Pascal C
sum := 0; sum = 0;
repeat do
sum := sum sum += 15;
until (sum > 100); while (sum <= 100);

25. Constrained branching
break
switch statement
programmer control of loop
when to exit (between start and end)
branch to end or branch out
orthogonality – separate repetition (infinite loop) from branching out