Lecture 9

Question 1

What are the three levels of control flow?

Answer 1

1. Within expressions
2. Among program units
3. Among program statements

Question 2

How did control statements evolve in the 1960s?

Answer 2

FORTRAN I’s control statements were hardware-based.

1960s research led to the realization that all flowcharts can be coded with two-way selection and pretest logical loops.

Question 3

What is a control structure?

Answer 3

A control structure includes a control statement and the statements it controls.

Question 4

What are the two general categories of selection statements?

Answer 4

1. Two-way selectors
2. Multiple-way selectors

Question 5

What are the design issues for two-way selection statements?

Answer 5

1. Form and type of the control expression.
2. Specification of then and else clauses.
3. Handling of nested selectors.

Question 6

What is a drawback of FORTRAN’s IF statement?

Answer 6

It uses negative logic, which is bad for readability.

Question 7

How does ALGOL 60 specify then and else clauses?

Answer 7

if (boolean_expr)
then statement
else statement

Question 8

How do Java and ALGOL 60 handle nested selectors?

Answer 8

• Java: Else associates with the nearest if.

ALGOL 60: Disallows direct nesting.

Question 9

How do FORTRAN 90 and Ada enhance readability in nested selectors?

Answer 9

By using closing special words like end if.

Question 10

What are the design issues for multiple-way selection statements?

Answer 10

1. Form and type of the control expression.
2. Specification of selectable segments.
3. Execution flow restrictions.
4. Handling of unrepresented expression values.

Question 11

What is a three-way selector in FORTRAN, and what is its use?

Answer 11

The Arithmetic IF, used for problems with three evaluative values (less than, equal to, greater than zero).

Question 12

Describe the Pascal case structure.

Answer 12

case expression of
constant_list_1: statement_1;
…
constant_list_n: statement_n
end

Question 13

What are the key design choices for the switch statement in C/C++/Java?

Answer 13

Control expression must be an integer.

Selectable segments can be statements, blocks, or compounds.
No implicit branch, must use break.
Default clause for unrepresented values.

Question 14

How does Ada’s case statement enhance reliability?

Answer 14

By requiring exhaustive constant lists, often using an others clause.

Question 15

What are the general design issues for iterative statements?

Answer 15

1. How is iteration controlled?
2. Where is the control mechanism in the loop?

Question 16

Compare FORTRAN 90 and Pascal counter-controlled loops.

Answer 16

• FORTRAN 90: Integer loop variable, parameters evaluated once.

Pascal: Ordinal type loop variable, parameters evaluated once, loop variable undefined after termination.

Question 17

What are the key design issues for logically-controlled loops?

Answer 17

1. Pretest or posttest?
2. Special case of counting loop statement or separate statement?

Question 18

What are the design issues for user-located loop control mechanisms?

Answer 18

1. Should the conditional be part of the exit?
2. Transfer control out of more than one loop?

Question 19

How does Perl handle iteration based on data structures?

Answer 19

With built-in iterators for arrays and hashes, using the foreach construct.