Review Questions (MIDTERM)

Question 1

what is a descriptor?

Answer 1

the collection of the attributes of a variable

Question 2

advantages and disadvantages of decimal data types?

Answer 2

advantage:
- accuracy. decimal types are able to precisely store decimal values, at leat those within a restricted range, which cannot be done by floating-point

disadvantage:
- the range of values id restricted because no exponents are allowed
- their representation in memory is mildly wasteful

Question 3

what are the design issues for character string types?

Answer 3

1. should strings be simply a special kind of character array or primitive type?
2. should strings have static or dynamic length?

Question 4

describe the three string length options

Answer 4

1. static length string: the length can be static and set when the string is created
2. limited dynamic string: allow strings to have varying length up to a declared and fixed maximum set by a variable’s definition
3. dynamic length string: allow string to have varying length with no maximum

Question 5

define ordinal, enumeration, and subrange types

Answer 5

ordinal type: range of possible values can be easily associated with the set of positive integers.

enumeration type: all of the possible values, which are named constants, are enumerated in the definition

subrange type: a contiguous subsequence of an ordinal type

Question 6

what are the advantages of user-defined enumeration types?

Answer 6

aid to readability: named values are easily recognized, whereas coded values are not

aid to reliability:
1. no arithmetic operations are legal
2. no enumeration variable can be assigned a value outside its defined range

Question 7

in what ways are the user-defined enumeration types of C# more reliable than those of c++

Answer 7

C# enumeration types are like those of C++, except that they are never coerced to integer.

So, operations on enumeration types are restricted to those that make sense.

Also, the range of values is restricted to that of the particular enumeration type.

Question 8

define static, fixed stack-dynamic, stack-dynamic, fixed heap-dynamic, and heap-dynamic. what are the advantages of each?

Answer 8

static array: the subscript ranges are statically bound and storage allocation is static (done before run-time)
advantage: efficiency. no dynamic allocation/deallocation is required.

fixed stack-dynamic array: the subscript ranges are statically bound, but the allocation is done at declaration elaboration time during execution.
advantage: space effieciency.

stack-dynamic array: both the subscript ranges and the storage allocation are dynamically bounmd at elaboration time.
advantage: flexibility: the size of an array doesn’t need to be known until array is about to be used

fixed heap-dynamic array: similar to fixed stack-dynamic array. the diference are that both the subscript ranges and storage bindings are done when the user program requests them during execution, and the storage is allocated from the heap, rather than the stack
advantage: flexibility. the array’s size always fits the problem

heap-dynamic array: the binding of subscript ranges and storage allocation is dynamic and can change any number of times during the array’s lifetime
advantage: flexibility. arrays can grow and shrink during program execution as the need for space changes.

Question 9

what happens when a nonexistent element of an array is referenced in Perl?

Answer 9

A reference to a nonexistent element in Perl yields undef, but no error is reported.

Question 10

what languages support array slices with stepsizes?

Answer 10

python, perl, and ruby

Question 11

what languages support negative subscripts?

Answer 11

Ruby and Lua

Question 12

what array initialization feature is available in ada that is not available in other common imperative languages?

Answer 12

Listing arrays in order in which they are to be stored or direcly assigning them to an index position using the => operator

Question 13

How does JavaScript support sparse arrays?

Answer 13

The value of subscripts need not to be contiguous

Question 14

what is an aggregate constant?

Answer 14

Nonscalar constant which value never change or are not changed during execution of the program

Question 15

What array operations are provided specifically for single-dimensioned arrays in Ada?

Answer 15

Ada allows array assignments, including those where the right side is an aggregate value rather than an array name.

Ada also provides catenation, which is defined between two single-dimensioned arrays and between a single-dimensioned array and a scalar.

Nearly all types in Ada have built-in relational operators for equality and inequality.

Question 16

define row major order and column major order

Answer 16

Row major order is where elements of the array that have as their first subscript the lower bound value of the subscript are stored first.

Column major order has elements of an array that have as their last subscript the lower bound value of that subscript are stored first.

Question 17

what is an access function of an array?

Answer 17

For multidimensional array : the mapping of its base address and a set of index values to the address in memory of the element specified by index values.

For two-dimensional array : the address of an element is the base address of the structure plus the element size times the number of elements that precede it in the structure.

Question 18

What are the required entries in a Java array descriptor, and when must they be stored (at compile time or run time)?

Answer 18

In Java all arrays are fixed heap-dynamic arrays. Once created, these arrays keep the same subscript ranges and storage. Secondarily, Java supports jagged arrays and not rectangular arrays. Being a fixed heap-dynamic array the entries will be established and fixed at run time.

Question 19

What is the structure of an associative array?

Answer 19

An associative array is an unordered collection of data elements that are indexed by an equal number of values called keys. User-defined keys must be stored.

Question 20

What is the purpose of level numbers in COBOL records?

Answer 20

It represents relation between groups and elementary items in COBOL

Question 21

Define fully qualified and elliptical references to field in records.

Answer 21

A fully qualified reference is one in which all intermediate record names, from the largest enclosing record to the specific field are named in reference.

Elliptical reference has the field named, but any or all of the enclosing record names can be omitted, as long as the resulting reference is unambiguous in the referencing environment.

Question 22

What is the primary difference between a record and a tuple?

Answer 22

record is an aggregate of data elements in which the individual elements are identified by names and accessed through offsets from the beginning of the structure.

tuple is a data type that is similar to a record, except that the elements are not named.

Question 23

Are the tuples of Python mutable?

Answer 23

No

Question 24

What is the purpose of an F# tuple pattern?

Answer 24

A tuple pattern is simply a sequence of names, one for each element of the tuple , with or without the delimiting parentheses.

Question 25

In what primarily imperative language do lists serve as arrays?

Answer 25

python

Question 26

What is the action of the Scheme function CAR?

Answer 26

The CAR function returns the first element of its list parameter.

Question 27

What is the action of the F# fuction tl?

Answer 27

it is the methods of the List class, as in List.hd[1; 3; 5; 7], which returns 1.

Question 28

On what are Python's list comprehensions based?

Answer 28

A list comprehension is an idea derived from set notaion.

Question 29

Define union, free union, and discriminated union.

Answer 29

union: a type whose variables are allowed to store different type values at different times during execution. free union: provide union constructs in which there is no language support for type checking. Discriminated union: type checking of unions require that each union include a type indicator.

Question 30

Are the unions of Ada always type checked?

Answer 30

No. the user can tell the system when the type checking can be static.

Question 31

What are the design issues for unions?

Answer 31

1. Should type checking be required? 2. Should unions be embedded in records?

Question 32

Are the unions of F# discriminated?

Answer 32

yes

Question 33

What are the design issues for pointer types?

Answer 33

1. What are the scope of and lifetime of a pointer variable? 2. What is the lifetime of a heap-dynamic variable? 3. Are pointers restricted as to the type of value to which they can point? 4. Are pointers used for dynamic storage management, indirect addressing, or both? 5. Should the language support pointer types, reference types, or both?

Question 34

What are the two common problems with pointers?

Answer 34

1. Dangling pointers (dangerous) 2. Lost heap-dynamic variable.

Question 35

Why are the pointers of most languages restricted to pointing at a single type variable?

Answer 35

The main reason for this is pointer arithmetic. For example, if ptr is a pointer variable that is declared to point at some variable of some data type, then ptr + index is a legal expression. The semantics of such an expression is as follows. Instead of simply adding the value of index to ptr, the value of index is first scaled by the size of the memory cell (in memory units) to which ptr is pointing (its base type). For example, if ptr points to a memory cell for a type that is four memory units in size, then index is multiplied by 4, and the result is added to ptr. The primary purpose of this sort of address arithmetic is array manipulation.

Question 36

What is a C++ reference type, and what is its common use?

Answer 36

C++ includes a special kind of pointer type called a reference type that is used primarily for formal parameters.

Question 37

Why are reference variables in C++ better than pointers for formal parameters?

Answer 37

because a pointer refers to an address in memory while a reference refers to an object or a value in memory.

Question 38

What advantages do Java and C# reference type variables have over the pointers in other languages?

Answer 38

the reference Java and C# provide some of the flexibility and the capabilities of pointers, without the hazards.

Question 39

Describe the lazy and eager approaches to reclaiming garbage.

Answer 39

Reference counters (eager approach): reclamation is gradual. Mark-sweep (lazy approach): reclamation occurs when the list of variable space becomes empty.

Question 40

What is a compatible type?

Answer 40

A compatible type is one that either is legal for the operator or is allowed under language rules to be implicitly converted by compiler-generated code to a legal type.

Question 41

Define type error

Answer 41

Type error is the application of an operator to an operand of an inappropriate type.

Question 42

Define strongly typed.

Answer 42

Characteristic of programming language where the typing errors are always detected

Question 43

What is nonconverting cast?

Answer 43

when no conversion takes place. It is merely a means of extracting the value of a variable of one type and using it as if it were of a different type.

Question 44

Define strongly type

Answer 44

A programming language is strongly typed if types errors are always detected.

Question 45

Why is Java not strongly typed?

Answer 45

because there are no implicit ways type errors can go undetected.

Question 46

what is name type equivalence?

Answer 46

two variables have equivalent types if they are defined either in the same declaration or in declarations that use the same type name

Question 47

what is structure type equivalence?

Answer 47

two variables have equivalent types if their types have identical structures

Question 48

what is the primary advantage of name type equivalence

Answer 48

easy to implement but it is more restrictive

Question 49

what is the primary disadvantage of structure type equivalence

Answer 49

more difficult to implement

Question 50

what set operation models c's struct data type

Answer 50

Cartesian (cross) product; union

Question 51

define operator precedence and operator associativity

Answer 51

Operator precedence determines which operator would be evaluated first if they were all in the same expression. Associative can be left or right and determines which operator would be evaluated first if operators with the same precedence were next to each other.

Question 52

what is a ternary operator

Answer 52

The ternary operator is a concise, inline method used to execute one of two expressions based on a condition

Question 53

what is a prefix operator

Answer 53

they precede the operands

Question 54

what oerator usually has right associativty

Answer 54

**

Question 55

what is a nonassociative operator

Answer 55

operators that have no defined behavior when used in sequence in an expression

Question 56

what associativity rules are used by APL?

Answer 56

All operators have equal precedence and all operators associate right to left

Question 57

What is the difference between the way operators are implemented in C++ and Ruby?

Answer 57

The difference is in Ruby, all the arithmetic, relational, and assignment operator, as well as array indexing, shifts, and bitwise logic operators, are implemented as methods while in C, it doesn't.

Question 58

Define functional side effect.

Answer 58

A condition where a function changes either one of its parameters or a global variable.

Question 59

What is a coercion?

Answer 59

A method of automatically converting one type of data to another.

Question 60

What is a conditional expression?

Answer 60

A compound of expression that contains a condition that is implicitly converted to type bool in C++

Question 61

What is an overloaded operator?

Answer 61

Operator that has different implementation depending on its arguments.

Question 62

Define narrowing and widening conversion.

Answer 62

A widening conversion converts a value to a type that can include at least approximations of all of the values of the original type. A narrowing conversion converts a value to a type that cannot store even approximations of all of the values of the original type.

Question 63

In JavaScript, what is the difference between == and ===?

Answer 63

It is the same but === prevent its operands from being coerced.

Question 64

What is a mixed-mode expression?

Answer 64

A mixed-mode expression is one that has operands of different types

Question 65

What is referential transparency?

Answer 65

if any two expressions in the program that have the same value can be substituted for one another anywhere in the program, without affecting the action of the program.

Question 66

What are the advantages of referential transparency?

Answer 66

The semantics of such programs is much easier to understand than the semantics of programs that are not referentially transparent.

Question 67

What is short-circuit evaluation?

Answer 67

An expression in which the result is determined without evaluating all of the operands and/or operators.

Question 68

Name a language that always does short-circuit evaluation of Boolean expressions. Name one that never does it. Name one in which the programmer is allowed to choose.

Answer 68

C, C++, and Java: use short-circuit evaluation for the usual Boolean operators (&& and ||), but also provide bitwise Boolean operators that are not short circuit (& and |) Ada: programmer can specify either (short-circuit is specified with and then and or else. Ada also has a non-short circuit operators and and or.

Question 69

How does C support relational and Boolean expressions?

Answer 69

Relational : -for example, inequality, the C-based languages use != Boolean : -Highest postfix ++, -- unary +, -, prefix ++, --, ! *, /, % binary +, - <, >, <=, >= =, != && Lowest ||

Question 70

What is the purpose of a compound assignment operator?

Answer 70

Compound assignment operator is a shorthand method of specifying a commonly needed form of assignment. The form of assignment that can be abbreviated with this technique has the destination variable also appearing as the first operand in the expression on the right side.

Question 71

What is one possible disadvantage of treating the assignment operator as if it were an arithmetic operator?

Answer 71

It provides yet another side effect

Question 72

What two languages include multiple assignments?

Answer 72

Ruby and Perl

Question 73

What mixed-mode assignments are allowed in Ada?

Answer 73

Ada doesn't allow mixed mode assignments.

Question 74

What mixed-mode assignments are allowed in Java?

Answer 74

Java allows mixed mode assignment only if the required coercion is widening. So, int value can be assigned to a float variable, but not vice versa.

Question 75

What mixed-mode assignments are allowed in ML?

Answer 75

ML doesn't allow mixed mode assignments at all!

Question 76

What is a cast?

Answer 76

Explicit type conversion.

Question 77

what is the definition of control structure

Answer 77

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

Question 78

what did bohm and jocopini prove about flowcharts

Answer 78

that all algorithms that can be expressed by flowcharts can be coded in a programming language with only two control statements: 1. one for choosing between two control flow paths 2. & one for logically controlled iterations

Question 79

What is the definition of block?

Answer 79

Control structure is the collection of statements whose execution it controls. It determines the order of execution of statements. This group of statement is known as "Block".

Question 80

What are the design issues for selection structures

Answer 80

Two way selection statement design issues Multiple selection constructs design issues

Question 81

what is unusual about python's design of compound statements

Answer 81

Python uses indentation to specify compound statements. For example, if x > y : x = y print "case 1" equally indent statements are grouped as one compound statement.

Question 82

Under what circumstances must an F# selector have an else clause?

Answer 82

If the expression returns a value, it must have an else clause

Question 83

What are the common solutions to the nesting problem for two-way selectors?

Answer 83

The common solution to the nesting problem is to use alternating means of forming a compound statements.

Question 84

What is unusual about C's multiple-selection statement?

Answer 84

The C switch statement has virtually no restrictions on the placement of the case expressions, which are treated as if they were normal statement labels. This laxness can result in highly complex structure eithin the switch body.

Question 85

On what previous language was C's switch statement based?

Answer 85

ALGOL 68

Question 86

Explain how C#'s switch statement is safer than that of C.

Answer 86

C# has a static semantics rule that disallows the implicit execution of more than one segment. Every segment must end with an explicit unconditional branch statement which transfer control out of the switch statement, or a goto, which can transfer control to one of the selectable segments

Question 87

What is a pretest loop statement? What is a posttest loop statement?

Answer 87

Pretest means that the test for loop completion occurs before the loop body is executed and posttest means that it occurs after the loop body is executed.

Question 88

What is the difference between the for statement of C++ and that of Java

Answer 88

The loop control expression in Java is restricted to boolean, unlike that of C++, even though the syntax is similar.

Question 89

What does the range function in Python do?

Answer 89

It is used to count loops in Python

Question 90

What contemporary languages do not include a goto?

Answer 90

Java language is the contemporary language that doesn't include a goto, the loop bodies cannot be entered anywhere but at their beginning.

Question 91

what advantage does java's break statement have over c's break statement

Answer 91

Java's break statement has the advantage of allowing you to exit multiple nested loops simultaneously using labeled statements, while C's break only exits the innermost loop.

Question 92

what are the differences between the break statement of c++ and that of java

Answer 92

C++'s break is unconditional unlabeled exits, meanwhile Java's one is unconditional labeled exits.

Question 93

what is user-defined iteration control

Answer 93

A user defined iteration control is a type of looping structure that is primarily used for data structures. Instead of being controlled by a counter or boolean expression, it is controlled by the number of elements in a data structure.

Question 94

what scheme function implements a multiple selection statement

Answer 94

cond

Question 95

how does a functional language implement repitition

Answer 95

recursion

Question 96

how are iterators implemented in ruby

Answer 96

Ruby predefines several iterator methods, such as times and upto for counter-controlled loops, and each for simple iterations of arrays and hashes.

Question 97

what language predefines iterators that can be explicitly called to iterate over its predefined data structures

Answer 97

Ruby: TIMES, EACH, UPTO Or: php: CURRENT, NEXT, PREV

Question 98

What common programming language borrows part of its design from Dijkstra's guarded commands?

Answer 98

Dijkstra's guarded commands are the basis of concurrence mechanism in CSP and Ada, also function definitions of Haskell.

Question 99

what are the three general characteristics of subprograms

Answer 99

1. Each subprogram has a single entry point. 2. The calling program unit is suspended during the execution of the called subprogram, which implies that there is only one subprogram in execution at any given time. 3. Control always returns to the caller when the subprogram execution terminates.

Question 100

What does it mean for a subprogram to be active?

Answer 100

The subprogram has been called and is executing, but has not yet terminated

Question 101

What is given in the header of a subprogram?

Answer 101

name, kind of subprogram, and the formal parameters

Question 102

What characteristic of Python subprograms sets them apart from those of other languages?

Answer 102

function definitions are executable; in all other languages, they are non-executable

Question 103

What languages allow for a variable number of parameters?

Answer 103

c#, ruby, python, and lua

Question 104

what is a ruby array formal parameter

Question 105

what is a parameter profile? what is a subprogram protocol?

Answer 105

parameter profile of a subprogram contains the number, order, and type of its formal parameters protocol of a subprogram is its parameter profile plues, if it is a function, its return type

Question 106

what are formal parameters? what are actual parameters?

Answer 106

formal parameters: parameters in the subprogram header. they are bound to storage only when the subprogram is called, and that binding is often through some other program variables. actual parameters: list of parameters to be bound to the formal parameters of the subprogram

Question 107

what are the advantages and disadvantages of keyword parameters?

Answer 107

The advantage of keyword parameters is that they can appear in any order in the actual parameter list. The disadvantage to keyword parameters is that the user of the subprogram must know the names of formal parameters.

Question 108

What are the differences between a function and a procedure?

Answer 108

Functions return values and procedures do not.

Question 109

What are the advantages and disadvantages of dynamic local variables?

Answer 109

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