Control Flow

Question 1

Basic Paradigms for Control Flow

(T/F)

Details in syntax and semantics do not differ between languages.

Answer 1

False

Question 2

Seven Categories for Control Flow

Answer 2

• Sequencing
• Selection
• Iteration
• Procedural Abstraction
• Recursion
• Non-Determinacy
• Concurrency

Question 3

Describe Sequencing.

Answer 3

Statements are to be executed in a particular order, usually in the order appearing in the program text.

Question 4

Define Selection.

Answer 4

• A choice is made between two or more alternatives (statements or expressions) depending on a condition determined at runtime.
• If, case (switch) statements.

Question 5

Describe Iteration.

Answer 5

• A given code fragment is executed repeatedly, either a given number of times or until a runtime condition is satisfied.
• While, do, repeat loops.

Question 6

Describe Procedural Abstraction.

Answer 6

A potentially complex collection of control constructs is encapsulated so that it can be treated as a single unit, often subject to parameterization.

Question 7

Describe Recursion.

Answer 7

• An expression is defined in terms of simpler versions of itself.
• The computational model requires a stack on which to save information about partially evaluated instances of the expression.
• Usually defined using self-referential subroutines.

Question 8

Describe Non-Determinacy.

Answer 8

• The ordering or choice among statements or expressions is deliberately left unspecified.
• Any alternative should lead to a correct result.
• Sometimes “fairness” is required.

Question 9

Define Concurrency.

Answer 9

• Two or more program fragments are to be executed “at the same time”, either in parallel on separate processors or interleaved in a single processor.

Question 10

Sequencing

Imperative VS Functional Languages

Answer 10

• Central to imperative languages.
• Minor importance in functional languages, which emphasize expression evaluation.

Question 11

Recursion

Imperative VS Functional Languages

Answer 11

• Crucial in functional languages
• Available in imperative languages

Question 12

Iteration

Imperative VS Functional Languages

Answer 12

• Emphasized in imperative languages.

Question 13

Precedence Associativity

Answer 13

Question 14

Assignments

Answer 14

Question 15

Construct Side Effects

Answer 15

Question 16

Value Model of Variables

Answer 16

Question 17

Reference Model

Answer 17

Question 18

Boxing in Java

Answer 18

Question 19

Multiway Assignment

Answer 19

Question 20

What are some advantages to Initialization?

Answer 20

• Initial value of statically allocated variables can be set by compiler.
• Helps avoid common programming error of using variable before it has a value (or at least making behavior repeatable).

Question 21

Ordering Within Expressions

(a + b) * (c + d)
f (a+b, c+d)

Which will be evaluated first? (a + b) or (c + d)
Not determined by precedence or associativity rules

Answer 21

Question 22

Order Within Expressions

Answer 22

Question 23

C, C++ define “sequence points” that constrain order of evaluation. What happens between sequence points?

Answer 23

Question 24

Evaluation Between Sequence Points in C:

Assume x has value 1 before the expression

x[i] = i++ + 1;

What happens?

Answer 24

Undefined—both outcomes below are allowed

x[i] = i++ + 1;

x[1] = 2, i==2
x[2] = 2, i==2

Instead use

x[i] = i+1;
i++;

Question 25

Evaluation Between Sequence Points in C:

f( i++, j++, i+j)

What value is passed to f in the 3rd parameter?

Answer 25

f( i++, j++, i+j)

Could be 3,4,5

Instead use

i++;
j++;
f(i,j,i+j)

Question 26

Structured VS Unstructured

Answer 26

Unstructured Control Flow

• Use goto statement
• Fortran example
  
  • if condition goto 10

Structured

• Use only sequencing, selection, and iteration.
• Use boundaries of lexically nested constructs as targets of branches instead of labels.

Question 27

(T/F)

Goto statement are not considered harmful.

Answer 27

False

Question 28

Case/Switch Statement

Rather than testing each case sequentially, the case statement may compute the address of the jump target.

What are some of the implementation techniques?

Answer 28

Question 29

Iteration

Describe the following:

• Enumeration Controlled Loop
• Logically Controlled Loop

Answer 29

Question 30

Logically Controlled Loops

Answer 30

Question 31

Enumeration Controlled Loop

Answer 31

Question 32

Parallel Loops

Answer 32

Question 33

Recursion VS Iteration

Which is more efficient?

Answer 33

• A naive implementation of iteration usually more efficient than naive implementation of recursion since recursion makes actual subroutine calls.
• However, high quality compilers (especially those for a functional language) can generate efficient recursive code.