Review Questions:
1. What is a descriptor?
Descriptor is collection of the attributes of a variable.
2. What are the advantages and disadvantages of decimal data types?
The advantages of decimal data types is being able to precisely store decimal values, at least those within a restricted range, which can’t be done with floating-point. And the disadvantages of decimal data types are that the range of values is restricted because no exponents are allowed, and their representation in memory is mildly wasteful.
3. What are the design issues for character string types?
The two most important design issues that are specific to character string types are should strings be simply a special kind of character array or a primitive type and should strings have static or dynamic length.
4. Describe the three string length options.
Static length: the length can be static and set once the string is created.
Limited dynamic length : allow strings to have varying length up to a declared and fixed maximum set by the variable’s definition.
Dynamic length: allow strings to have varying length with no maximum.
5. Define ordinal, enumeration, and subrange types.
1) Ordinal type is one in which the range of possible values can be easily associated with the set of positive integers. In Java, for example, the primitive ordinal types are integer, char, and Boolean.
2) Enumeration is one in which all of the possible values, which are named constants, are provided, or enumerated, in the definition. Enumeration types provide a way of defining and grouping collections of named constants.
3) Subrange is a contiguous subsequence of an ordinal type. For example, 12..14 is a subrange of integer type.
Problem Set
1. What are the arguments for and against representing Boolean values as single bits in memory?
A Boolean value could be represented by a single bit. But, a single bit of memory cannot be accessed efficiently on many machines, so they are often stored in smallest efficiently addressable cell of memory, typically a byte.
2. How does a decimal value waste memory space?
Decimal types are stored very much like character strings, using binary codes for decimal digits. These representations are called binary coded decimal (BCD). In some cases, they are stored one digit per byte, or two digits per byte. Either way, they take more storage than binary representations. It takes at least four bits to code a decimal digit. Therefore, to store a six-digit coded decimal number requires 24 bits of memory. However, it takes only 20 bits to store the same number in binary.
3. VAX minicomputers use a format for floating-point numbers that is not the same as the IEEE standard. What is this format, and why was it chosen by the designers of the VAX computers? A reference for VAX floating –point representations is Sebesta (1991).
The existing DEC VAX formats, inherited from the PDP-11, because the PDP-11 had several uniquely innovative features, and was easier to program than its predecessors through the additional general-purpose registers.
4. Compare the tombstone and lock-and –key methods of avoiding dangling pointers, from the points of view of safety and implementation cost.
Tombstones take more memory, while lock-and-key requires additional cpu time on each pointer assignment to copy key as well as pointer. Pointer arithmetic could overwrite key in the heap.
5. What disadvantages are there in implicit dereferencing of pointers, but only in certain contexts? For example, consider the implicit dereference of a pointer to a record in Ada when it is used to reference a record field.
When implicit dereferencing of pointers occurs only in certain contexts, it makes the language slightly less orthogonal. The context of the reference to the pointer determines its meaning. This detracts from the readability of the language and makes it slightly more difficult to learn.
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