Chapter 8 - Control Structures

Question 1

What are control statements?

Answer 1

means of producing non-sequential execution of program statements through selection of alternative paths or repeating statements

Question 2

definition of a control structure?

Answer 2

a control statement and the collection of statements whose execution it controls

Question 3

definition of a selection statement

Answer 3

provides a means of choosing between two or more execution paths in a program

Question 4

What are the 2 categories of selection statements?

Answer 4

2-way selection 
multiple selection (n-way)

Question 5

definition of compound statement

Answer 5

a group of statements controlled as a unit

Question 6

The 4 design issues with selection

Answer 6

1) what is the form of the control expression
2) what is the selectable segment form (single statement, compound state, etc.)
3) how should the meaning of nested selectors be specified
4) how are the ‘then’ and ‘else’ clauses specified

Question 7

Statement sequence vs a compound statement

Answer 7

statement sequence - a series of statements that are terminated with some reserved word

compound statement - see above. Surrounded by braces

Question 8

The 4 PLs that have statement sequences in their then/else clauses instead compound statements

Answer 8

FORTRAN 95
Ada
Python
Ruby

Question 9

The main issue that arises with nested selectors?

Answer 9

When a nested ‘if’ is present, there’s not a straightforward way to determine which a corresponding ‘else’ should be associated with

• C based languages resolve this by using {}
• using statement sequences resolves this ambiguity much more cleanly

Question 10

5 design issues with multiple selection statements

Answer 10

1 & 2 the same as with a two-way selection

3) is execution flow through the structure restricted to include just a single selectable statement
4) how are case values specified
5) how should unrepresented selector expression values be handled

Question 11

3 early multiple selector schemes in FORTRAN

Answer 11

arithmetic if
[if (arithmetic expression) value1, value2, valueN]
-does a ‘go to’ to the label corresponding to value_i

computed goto
[GOTO (value1, value2, valueN) ]
-does the same thing as arithmetic if

assigned GOTO
[Assign value to N GOTO N (val1, val2, valN)]
-does the same thing as above

Question 12

4 key design choices of Pascal case statements

Answer 12

1) case expressions are of any ordinal type
2) segments (the code controlled by the case expression) can be a single or compound statement
3) construct is encapsulated
4) only one segment can be executed per execution of the case structure

Question 13

5 key design choices in C switch statements

Answer 13

1) control expression can only be of integer type
2) selectable segments can be statement sequences, blocks, or compound statements
3) construct is encapsulated
4) any number of segments can be executed in one execution of the structure (if no ‘break’ keyword is provided then the code simply continues to execute sequentially)
5) a ‘default’ clause handles unrepresented values

Question 14

definition of an iterative statement

Answer 14

a statement that causes another statement of collection of statements to be executed zero to many times

Question 15

2 main design issues with loops

Answer 15

1) how is iteration controlled

2) where is the control mechanism in the loop

Question 16

2 main type of loops

Answer 16

1) count controlled loop- execute loop body a specified number of times
2) event controlled loop-execute loop body until some event occurs

Question 17

2 types of iteration control

Answer 17

pretest (before the loop body)

posttest (after the loop body)

Question 18

4 design issues with count-controlled loops

Answer 18

We know count-controlled loops use a counter variable to track the number loop body executions

1) what is the type and scope of the loop (counter) variable
2) what is the value of the loop (counter) variable at loop termination
3) should it be legal to modify the loop (counter) variable or loop parameters from within the body of the loop and does this affect loop control
4) should loop parameters be evaluated only once, or once for every iteration

Question 19

4 count-controlled loop design choices in FORTRAN

Answer 19

1) loop variable can be an integer, real, or double
2) loop variable has its last value upon loop termination
3) loop parameters are evaluated only once
4) loop variable CANNOT be changed. Loop parameters (ie. stepsize) CAN be changed but because of 3) they DO NOT affect loop control

Question 20

4 count-controlled loop design choices in Pascal

Answer 20

1) loop variable must be an ordinal type of usual scope
2) loop variable becomes undefined upon loop termination
3) loop parameters evaluated just once
4) loop variable CANNOT be changed. Loop parameters CAN be changed but because of 3) they DO NOT affect loop control

Question 21

4 count-controlled loop design choices in Ada

Answer 21

1) the loop variable is implicitly declared by specifying the range and its type is that of the range
2) loop variable does not exist outside of loop (completely destroyed on loop termination)
3) the range is evaluated just once
4) loop variable CANNOT be changed. Loop range CAN be changed but because of 3) it DOES NOT affect loop control

Question 22

4 count-controlled loop design choices in C (pre C99)

Answer 22

AKA a for-loop

1) no explicit loop variable
2) everything can be changed in the loop
3) pretested
4) the first initialization expression is evaluated just once but the other two expressions are evaluated with each iteration

Question 23

2 key differences between C++and C count-controlled loops

Answer 23

In C++…

1) control expressions can be boolean
2) initial expression scope can include variable definitions and its scope is to the end of the function where defined

Question 24

2 key differences between Java and C++ count-controlled loops

Answer 24

in Java…

1) control expressions must be boolean
2) the scope of variables defined in initial expression is only the loop body

Question 25

2 design issues with event controlled loops

Answer 25

1) should it be pretest or posttest
2) should this be a special case of the count-controlled loop or should it be its own statement (presumably with its own unique/independent syntax)

Question 26

1 design choice for event-controlled loop in Pascal

Answer 26

has separate pretest and posttest statements (can perform either)

Question 27

1 design choice for event-controlled loops in C/C++

Answer 27

has pretest and posttest loops but they are syntactically analogous

Question 28

1 design choice for event-controlled lops in Ada

Answer 28

has pretest BUT NO posttest

Question 29

1 design choice for event-controlled loops in FORTRAN

Answer 29

does not have either pretest or posttest loop but uses IF/GOTO instead

Question 30

What is a user-located loop control mechanism

Answer 30

a location for loop control chosen by the programmer that is at neither the top nor the bottom of the loop body

They use user-located /defined loop exits

Question 31

3 design issues with user-located loop control mechanisms

Answer 31

1) should the conditional (the original loop condition) be part of the exit
2) should the mechanism be allowed in an already controlled loop (as opposed to an infinite loop)
3) should control be transferable out of more than one loop

Question 32

User-located loop control mechanism in C, C++, Python, Ruby, and C#

Answer 32

an unconditional unlabeled exit (ie a ‘break’ statement)

Question 33

1 design choice for user-located loop control in Ada

Answer 33

has both conditional and unconditional exit from any number of levels (can exit any loop)

Question 34

What is meant by iteration based on data structures

Answer 34

using the order and number of elements of some data structure to control iteration

ie. think iterator while(p.next()) {…}

Question 35

definition of unconditional branching

Answer 35

a statement that transfers execution control to a specified location in the program

Question 36

3 PLs WITHOUT GoTo

Answer 36

Java
Python
Ruby

Question 37

7 PLs WITH GoTo

Answer 37

C
C++
FORTRAN
Pascal
Ada
C#
assembler

Question 38

1 main design issue with GoTo

Answer 38

what is the syntax for labels