1. Statement-Level Control Structures
Chapter 8
Statement-Level
Control Structures

2. Chapter 8 Topics Introduction Selection Statements
Iterative Statements
Unconditional Branching
Guarded Commands
Conclusions

3. Levels of Control Flow Within expressions Among program statements
Among program units

4. Control Statements Evolution
FORTRAN I control statements were based directly on IBM 704 hardware
Much research and argument in the 1960s about the issue
One important result:
It was proven that all algorithms
represented by flowcharts
can be coded with only
two-way selection and pretest logical loops

5. Introduction Control Statements
Selection among several alternative control flow paths
Repeated execution of collections of statements
Overall Design Issue
What control statements should a language have, beyond selection and pretest logical loops?

6. Categories of Control Structures
Introduction
Definition: A control structure is a control statement and the statements whose execution it controls
Categories of Control Structures
Compound statements
Selection statements
Iteration statements
Unconditional branching

7. Python uses indentation!
Compound Statements
Definition: A block is a compound statement, including data declarations, defining a new scope
Some languages have specially delimited compound statements:
begin ... end
{ ... }
Some control constructs have built-in delimiters:
repeat ... until
Python uses indentation!

8. Selection Statements
A selection statement provides the means of choosing between two or more paths of execution
Two general categories:
Two-way selectors
Multiple-way selectors

9. Two-Way Selection Statements Multiple Selection Constructs
Design Issues
The Control Expression
Clause Form
Nesting Selectors
Multiple Selection Constructs
Examples of Multiple Selectors
Multiple Selection Using if

10. Two-Way Selection General form: if <control_expression>
then <then_clause>
else <else_clause>
Design Issues:
What is the form and type of the control expression?
How are the then and else clauses specified?
How should the meaning of nested selectors be specified?

11. Two-Way Selection Design Issues
What is the form and type of the control expression?
How are the then and else clauses specified?
How should the meaning of nested selectors be specified?
Control Expressions
Syntax: Parentheses?
Data type?

12. Single statements or compound statements?
Two-Way Selection
Clause Form
Single statements or compound statements?
Single statement example: (FORTRAN)
IF (<boolean_expr>) statement
IF (.NOT.<condition>) GOTO 20
...
20 CONTINUE
Compound statement example: (ALGOL 60)
if (<boolean_expr>)
then <statement>
else <statement>

13. Two-Way Selection Nested Selectors
Example (Java)
if ...
...
else ...
Which if gets the else?
Java's static semantics rule:
else goes with the nearest if

14. Two-Way Selection Nested Selectors
Fortran 95, Ada – closing special words
Examples (Ada)
if ... then if ... then
else end if
... else
end if ...
end if end if
Advantages: flexibility and readability
Modula-2 uses the same closing special word
for all control structures: END
This results in poor readability

15. Multiple Selection Design Issues
1. What is the form and type of the control expression?
2. How are the selectable segments specified?
3. Is execution flow through the structure restricted to include just a single selectable segment?
4. How should unrepresented selector expression values be handled, if at all?
These questions are answered for each example

16. Early Multiple Selectors
Multiple Selection
Early Multiple Selectors
FORTRAN arithmetic IF (a three-way selector)
IF (<arithmetic-expression>) 10,20,30
10 ...
...
GO TO 40
20 ...
30 ...
40 ...

17. Modern Multiple Selectors
Pascal case
case <expression> of
<constant_list_1> : <statement_1>;
...
<constant_list_n> : <statement_n>
end
Design choices
1. <expression> is any ordinal type
int, boolean, char, enum
2. <statement_i> can be either single or compound
3. Only one segment can be executed per construct execution
4. Unrepresented <expression> not defined

18. Modern Multiple Selectors
The C Langauges switch
switch (<expression>) {
case <const_expr_1> : <statement_1>;
...
case <const_expr_n> : <statement_n>;
[default: <statement_n+1>] }
Design Choices
1. <expression> can be an integer type only
2. <statement_i> can be sequences
3. Many segments can be executed in one construct execution
4. The default clause is for unrepresented values
(without it, the whole statement may do nothing)

19. Modern Multiple Selectors The Ada case statement
case <expression> is
when <choice list> => <stmt_sequence>; …
when others => <stmt_sequence>;]
end case;
More reliable than C’s switch
Once a <stmt_sequence> execution is completed, control is passed to the first statement after the case statement

20. Multiple Selection Using if
Some languages specifically extend the if statement, allowing some of the special words to be left out
else-if sequences are replaced with a single special word elsif and the closing special word on the nested if is dropped
Ada
if Count < 10 then Bag1 := True;
elsif Count < 100 then Bag2 := True;
elsif Count < 1000 then Bag3 := True;
end if;
This is not easily modeled with a case statement, since each selector is a Boolean expression

21. Iterative Statements Counter-Controlled Loops Design Issues
Fortran 95 Do
Ada for
C languages for
Logically Controlled Loops
Examples
User-Located Loop Control Mechanisms
Iteration Based on Data Structures

22. for Iteration Control Statements
Iterative Statements
Repeated execution of a statement or compound statement can be accomplished either by iteration or recursion
Here we look at iteration
-- recursion is unit-level control
General Design Issues
for Iteration Control Statements
How is iteration controlled?
Where is the control mechanism in the loop?

23. These questions are answered for each example
Counter-Controlled Loops
Design Issues
1. What are the type and scope of the loop variable?
2. What is the value of the loop variable at loop termination?
3. Should it be legal for the loop variable or loop parameters to be changed in the loop body, and if so, does the change affect loop control?
4. Should the loop parameters be evaluated only once, or once for every iteration?
These questions are answered for each example

24. Counter-Controlled Loops
Fortran 95
Do <label> <var>=<start>,<finish>[,<stepsize>]
[<name>:] Do <var>=<start>,<finish>[,<stepsize>] ...
End Do [<name>]
Design Choices
1. <var> must be Integer
2. After execution, <var> has its last value
3. <var> cannot be changed in the loop
4. Loop parameters can be changed in the loop
Notes: - Cannot branch into either Do
- <stepsize> can be any value but zero
- Loop parameters can be expressions

25. Counter-Controlled Loops
Ada
for <var> in [reverse] <discrete_range> loop
...
end loop
Design Choices
1. Type of <var> is that of <discrete_range>; its scope is the loop body
2. <var> does not exist outside the loop
3. <var> cannot be changed in the loop,
<discrete_range> can be changed in the loop
4. <discrete_range> is evaluated only once, so changing it does not affect loop control
Note: Cannot branch into the loop body

26. Counter-Controlled Loops
C Languages
for ([<exp1>];[<exp2>];[<exp3>])<statement>
Design Choices
1. There is no explicit loop variable
2. The loop variable (if there is one) has its latest value
3. Everything can be changed in the loop
4. <exp1> is evaluated once, but the other two are evaluated with each iteration
Notes: - The expressions can be statements
or statement sequences
for (i=0,j=10;j==i;i++)...
- If the second expression is absent,
it is an infinite loop

27. Counter-Controlled Loops
The loop statement of the C languages is the most flexible, but there are differences mostly based on the while loop
C99,C++ for:
The control expression is coercised Boolean
The initial expression can include variable definitions (scope is from the definition to the end of the loop body)
Can branch into them
Java,C# for:
The control expression must be Boolean
Branching into the loop? Java no
C# yes

28. Logically-Controlled Loops
Design Issues
Should the language have a pretest loop or a posttest loop or both?
Should this be a special case of the counter-controlled loop or a completely separate statement?

29. Logically-Controlled Loops
Language Examples
C Languages have both while-do and do-while
Branching into either loop construct?
C,C++,C# yes
Java no
Ada has a pretest version, but no posttest
Fortran 95 has neither
Perl has two pretest logical loops,
while and until,
but no posttest logical loop

30. User-Located Loop Controls
Design Issues
Should the conditional be part of the exit?
Should control be transferable, conditionally or unconditionally, out of more than one nested loop?

31. User-Located Loop Controls
Language Examples
Ada conditional or unconditional,
for any loop, any number of levels
for ... loop
...
exit [when ...]
end loop
LOOP1: while ... loop
...
LOOP2: for ... loop
exit LOOP1 [when ...]
end loop LOOP2;
end loop LOOP1;

32. User-Located Loop Controls
Language Examples
C Languages
break
Unconditional, for any loop or switch
C,C++ one level only
Java,Perl,C# multiple levels, by labels
continue
Skips the remainder of this iteration,
but does not exit the loop
Fortran 95
Exit, Cycle
Unconditional
for any loop, any number of levels

33. Data Structure Iteration
Concept
Use the order and number of elements of some data structure to control iteration
Control Mechanism
A function call (the iterator) that returns either the next element in a chosen order, if there is one, or else exits the loop
Languages
Specific structures: Perl, PHP, JavaScript, C#
Can be modeled in most languages by functions,
easily so in object-oriented languages

34. Data Structure Iteration
Examples
C,C++ traversing a linked list using pointers
for (p=hdr; p; p=next(p)){...} C#
String[] strList = {″Bob″,″Carol″, ...};
...
foreach (String name in strList)
System.Console.WriteLine(″Name: {0}″,name);
The foreach statement iterates the elements of any collection that implements IEnumerable

35. Data Structure Iteration
Examples
Perl has a built-in iterator for arrays and hashes
foreach $name { print $name }
PHP has predefined iterators to access its arrays
current points at one element of the array
next moves current to the next element
prev moves current to the previous element
reset moves current to the first element

36. Data Structure Iteration User-defined Iteration
C++ iterators are often implemented as friend functions or as separate iterator classes
Java objects of a class implementing the Collection interface can be iterated with an implementation of the Iterator interface
- next() moves the pointer into the collection
- hasNext() is a predicate
- remove() deletes an element
C# can iterate any collection implementing the
IEnumerate interface
- MoveNext() moves the pointer in the collection
- Reset() moves the pointer to the beginning
- Current is the current element
- Can also use foreach to iterate

37. Unconditional Branching
General Syntax
goto <Label>
Disadvantage readability
Advantage flexibility
Notes: All control structures can be
built with goto and a selector
Some languages do not have them
Modula-2, Java
Loop exit statements are
camouflaged goto’s
- they are severely restricted
- not harmful to readability

38. “Go To Statement Considered Harmful”
...the quality of programmers is a decreasing function of the density of go to statements in the programs they produce....
...the go to statement should be abolished from all "higher level" programming languages...
The go to statement as it stands is just too primitive; it is too much an invitation to make a mess of one's program....
Dijkstra, Edsger W., Go To Statement Considered Harmful. Communications of the ACM, Vol. 11, No. 3, March 1968, pp

39. “Go To Statement Considered Harmful”
Dijkstra’s remark about the undesirability of the goto statement was not new, and was not final !
1959 Heinz Zemanek expressed doubts
1966 Wirth and Hoare “[multiple selection] mirrors the dynamic structure of a program more clearly than goto statements"
1966 Guiseppe Jacopini proved the (logical) superfluousness of the goto statement
1968 Dijkstra Communications of the ACM
1974 Knuth argued that there were occasions when the efficiency of the goto outweighed its harm to readability
1978 Kernighan and Ritchie called the goto “inifinitely abusable,” but included it in C anyway

40. 'GOTO Considered Harmful' Considered Harmful
In computing and related disciplines, considered harmful is a phrase used in the titles of diatribes and other critical essays (there are more than 65 such works).
The March 1987 CACM contained a criticism of Dijkstra's letter under the title
'GOTO Considered Harmful' Considered Harmful.
The May 1987 CACM printed further replies, both for and against, under the title
'"GOTO Considered Harmful" Considered Harmful' Considered Harmful?.
Dijkstra's own response to this controversy was titled
On a somewhat disappointing correspondence.

42. Guarded Commands Edsger Dijkstra (1975, 1976) Purpose
to support a new programming methodology
(verification during program development)
Idea
if the order of evaluation is not important,
the program should not specify one
Used in concurrent programming: CSP, Ada
Used in function definitions: Haskell

43. Guarded Commands Selection
Syntax: if <boolean> -> <statement>
[] <boolean> -> <statement>
... fi
[] is called a fatbar
Semantics: When this construct is reached:
Evaluate all boolean expressions
If more than one are true,
choose one nondeterministically
If none are true, runtime error

44. Selection Guarded Command Flow Chart

45. Guarded Commands Selection
examples (text pp )
if i = 0 -> sum := sum + i
[] i > j -> sum := sum + j
[] j > i -> sum := sum + i fi
if x >= y -> max := x
[] y >= x -> max := y

46. Guarded Commands
Loop
Syntax: do <boolean> -> <statement>
[ ] <boolean> -> <statement>
... od
Semantics: For each iteration:
Evaluate all boolean expressions
If more than one are true,
choose one nondeterministically;
then start loop again
If none are true, exit loop

47. Loop Guarded Command Flow Chart

48. arrange the values of q1, q2, q3, and q4 so that
Guarded Commands
Loop
example (text p. 369)
arrange the values of q1, q2, q3, and q4 so that
q1 <= q2 <= q3 <= q4
do q1 > q2 -> temp := q1; q1 := q2; q2 := temp
[] q2 > q3 -> temp := q2; q2 := q3; q3 := temp
[] q3 > q4 -> temp := q3; q3 := q4; q3 := temp od

49. Guarded Commands Rationale
Connection between control statements and program verification is intimate
Verification is impossible with goto statements
Verification is possible with only selection and logical pretest loops
Verification is relatively simple with only guarded commands

50. Conclusions
Choice of control statements beyond selection and logical pretest loops is a trade-off between language size and writability