Control statements
• Simple statements
• Basic structured statements
• Sequence
• Selection
• Iteration
• The jump statement
CSI 3125, Control, page 1
Simple statements
in imperative languages
These are atomic (all or nothing) operations:
• assignment,
• the empty statement,
• a procedure call,
• exit, next, break, continue
• go to (jump).
A block is also an all-or-nothing operation.
CSI 3125, Control, page 2
Structured statements
Three fundamental mechanisms
allow us to group simple statements
into structured statements.
• sequence, or the compound statement:
{ S1 S2 }
• selection, or the conditional statement:
if (C) S1 else S2
• iteration, or the loop statement:
while (C) S
CSI 3125, Control, page 3
Structured statements (2)
All other control structures can be easily derived
from the three basic mechanisms.
if (C) S

if (C) S else {}
do S while (C)

S
while (C) S
switch (i) {
case C1: S1 …

if (i == C1) S1
else …
and so on.
CSI 3125, Control, page 4
Sequence (1)
Languages
Algol, Pascal, Ada, ...
C, Java, Perl
Fortran IV
Prolog
Mechanisms
begin ... end
{ ... }
nothing
implicit
a :- b, c, d.
This means evaluating b, then c, then d.
Scheme
(begin ...)
CSI 3125, Control, page 5
Sequence (2)
A compound statement is treated as a simple
statement. This is based on an important
abstraction principle.
The inner structure can be “abstracted away”.
single entry, single exit
CSI 3125, Control, page 6
Selection
The if-else construct is present in almost all programming
languages (Prolog is the only major exception). Modula and
Ada were the first to have properly bracketed if-then-else,
with four keywords around three elements of the selection:
if C then S1 else S2 end if
Nested selection if-elsif-...-else was also introduced in Ada:
if C1 then S1
elsif C2 then S2
......
elsif Cn then Sn
else Sn+1 end if
CSI 3125, Control, page 7
Special forms of selection in Fortran IV
Computed GO TO.
GO TO (label1, ..., labeln), expression
Assigned GO TO.
ASSIGN labeli TO variable
GO TO variable(label1, ..., labeln)
CSI 3125, Control, page 8
Special forms of selection (2)
The switch statement in C and Java has been probably
inspired by computed GO TO.
switch(expression){
case const1: S1;
...
case constn: Sn;
default: Sn+1;}
After Si has been executed, control "falls through" to the
subsequent case: Si+1 is executed next.
Fall-through can be avoided by adding break statements.
CSI 3125, Control, page 9
Special forms of selection (3)
Case statement in Pascal, Ada and other
similar languages: each case is separate,
there is no "fall-through". In Ada:
case expression is
when constantList1 => S1;
...
when constantListn => Sn;
when others => Sn+1;
end case;
CSI 3125, Control, page 10
Special forms of selection (4)
Selection in Prolog is driven by success and
failure, not by the true-false opposition.
Selection is implicit in backtracking: if you succeed,
stop; if not, try another choice .
union( [Elem | S1], S2, S1_S2 )
member( Elem, S2 ),
union( S1, S2, S1_S2 ).
:-
union( [Elem | S1], S2, [Elem | S1_S2] )
\+ member( Elem, S2 ),
union( S1, S2, S1_S2 ).
:-
union( [], S2, S2 ).
CSI 3125, Control, page 11
Graphical representation
flowgraphs — flow diagrams — flowcharts
if–then–else
Y
S1
C
N
S2
if–then
Y
C
N
S
CSI 3125, Control, page 12
Graphical representation (2)
The abstraction principle:
if ( C ) S1 else S2
is a simple statement.
Y
S1
C
N
S2
Single entry,
single exit
CSI 3125, Control, page 13
Graphical representation (3)
if–then–elsif-…elsif-then-else
C1
Y
N
C2
Y
S1
S2
N
C3
Y
S3
…
N
Cn
Y
Sn
Sn+1
CSI 3125, Control, page 14
Graphical representation (4)
case e of v1: S1; ... else Sn+1 end
e=v1
Y
N
e=v2
Y
S1
S2
N
e=v3
Y
S3
…
N
e=vn
Y
Sn
Sn+1
CSI 3125, Control, page 15
Iteration
Variations: pretest iteration or posttest iteration.
while C do S
Pascal
repeat S until C
while (C) S
Java
do S while (C)
while C loop S end loop;
Ada
(no posttest iteration)
CSI 3125, Control, page 16
Iteration (2)
In Ada, the prefix while C is an extension
of the basic iterative statement:
loop S end loop;
Another prefix:
for i in range
The bare loop statement must be stopped
from inside the loop.
Forced exit closes the nearest iteration:
exit;
unconditional
exit when C;
CSI 3125, Control, page 17
Iteration (3)
The while prefix is an abbreviation. The
construction
while C loop S end loop;
is equivalent to
loop
exit when not C;
S
end loop;
CSI 3125, Control, page 18
Example: use of exit
SUM := 0;
SUM := 0;
get(X);
loop
while X /= 0 loop
get(X);
SUM := SUM + X;
exit when X = 0;
get(X);
SUM := SUM + X;
end loop;
end loop;
Simpler, more intuitive
Condition reversed,
get(X) appears twice
CSI 3125, Control, page 19
Graphical representation
while-do
repeat-until repeat-until
S
Y
S
C
N
Y
S
N
C
Y
¬C
N
S
CSI 3125, Control, page 20
Graphical representation (2)
loop - exit - end loop
S1
C
Y
N
S2
CSI 3125, Control, page 21
for loops
For-loops ("counter-controlled") are historically earlier
and less general than condition-controlled iterative
structures.
DO 1000 var = lo, hi
Fortran IV
...
1000 CONTINUE
DO label var = lo, hi, incr
for var := expr do S
Algol 60
for var := low step incr until high do S
for var := expr while C do S
Iterators can be combined:
for i := 0, i+1 while i ≤ n do S(i)
CSI 3125, Control, page 22
for loops (2)
for var in range
loop S end loop;
Ada
for var in reverse range
loop S end loop;
for (e1; e2; e3) S
C, Java, Perl
(enough said J)
What is this?
for (;;) S
CSI 3125, Control, page 23
for loops 32)
Iteration in Prolog and in Scheme is
expressed by recursion. The same,
of course, is possible in most other
typical languages.
CSI 3125, Control, page 24
Jump (the goto statement)
Unconstrained transfer of control is the only
mechanism available in low-level languages —
but they are very general. One-branch selection
and goto allow us to express all other control
structures.
The jump mechanism is dangerous, may hurt
readability, and should be avoided — advanced
control structures work well for all typical and for
most less typical uses.
CSI 3125, Control, page 25
Jump (the goto statemen)t (2)
Some languages restrict goto (do not allow jumping
inside an iteration or selection) and make it hard to use.
Ada makes labels visible from far away (so that your
boss can see it!).:
SUM := 0;
loop
get(X);
if X = 0 then goto DONE; end if;
SUM := SUM + X;
end loop;
<<DONE>>
put(SUM);
goto may leave “unfinished business” — active
control structures that must be "folded" at once.
CSI 3125, Control, page 26