Chapter 6 - Data Types

Review Questions

1.A descriptor is the collection of the attributes of a variable.

2.Decimal types have the advantage of being able to precisely store decimal values, at least those within a restricted range, which cannot be done with floating-point. The disadvantages of decimal types are that the range of values is restricted because no exponents are allowed, and their representation in memory is mildly wasteful.

3.- The two most important design issues that are specific to character string types are the following:

• Should strings be simply a special kind of character array or a primitive type?

• Should strings have static or dynamic length?

5.A subrange type is a contiguous subsequence of an ordinal type. An ordinal type is one in which the range of possible values can be easily associated with the set of positive integers. An enumeration type is one in which all of the possible values, which are named constants, are provided, or enumerated, in the definition. The third option is to allow strings to have varying length with no maximum, as in JavaScript, Perl, and the standard C++ library. These are called dynamic length strings. This option requires the overhead of dynamic storage allocation and deallocation but provides maximum flexibility.

6.Enumeration types can provide advantages in both readability and reliability. Readability is enhanced very directly: Named values are easily recognized, whereas coded values are not.

7.C# includes both the references of Java and the pointers of C++. However, the use of pointers is strongly discouraged. In fact, any subprogram that uses pointers must include the unsafe modifier. Note that although objects pointed to by references are implicitly deallocated, that is not true for objects pointed to by pointers. Pointers were included in C# primarily to allow C# programs to interoperate with C and C++ code.

15.An aggregate constant is a nonscalar constant which value never change or are not changed during execution of the program.

18.Arrays are part of most programming languages. The relationship between a reference to an array element and the address of that element is given in an access function, which is an implementation of a mapping.

20.An associative array is an unordered collection of data elements that are indexed by an equal number of values called keys. In the case of non-associative arrays, the indices never need to be stored (because of their regularity). In an associative array, however, the user-defined keys must be stored in the structure. So each element of an associative array is in fact a pair of entities, a key and a value. We use Perl’s design of associative arrays to illustrate this data structure.an associative array?

21.evel numbers in COBOL indicate by their relative values the hierarchical structure of the record. Any line that is followed by a line with a higher-level number is itself a record.

43.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 (or the interpreter) to a legal type.

44. A type error is the application of an operator to an operand of an inappropriate type. For example, in the original version of C, if an int value was passed to a function that expected a float value, a type error would occur (because compilers for that language did not check the types of parameters).

45.A programming language is strongly typed if type errors are always detected. This requires that the types of all operands can be determined, either at compile time or at run time. The importance of strong typing lies in its ability to detect all misuses of variables that result in type errors. A strongly typed language also allows the detection, at run time, of uses of the incorrect type values in variables that can store values of more than one type.

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

Problem Set

2.The CPU doesn't care whether a byte holds -1 or 15 when it moves it from one place to another. There's no such thing as a "signed move" (to a location of the same size - there is a signed move for larger or smaller destinations). The CPU only cares about the representation when it does arithmetic on the byte. The CPU knows whether to do signed or unsigned arithmetic according to the op-code that you (or the compiler on your behalf) chose.

3.The collection of values that can be represented by a floating-point type is defined in terms of precision and range. Precision is the accuracy of the fractional part of a value, measured as the number of bits. Range is a combination of the range of fractions and, more important, the range of exponents.

5.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.

6.The Boolean or logical data type is a data type, having two values (usually denoted true and false), intended to represent the truth values of logic and Boolean algebra. It is named after George Boole, who first defined an algebraic system of logic in the mid-19th century. The Boolean data type is the primary result of conditional statements, which allow different actions and change control flow depending on whether a programmer-specified boolean condition evaluates to true or false. C++ has a separate Boolean data type ('bool'), but with automatic conversions from scalar and pointer values that are very similar to those of C. This approach was adopted also by many later languages, especially by some scripting ones such as AWK. One problem with this approach is that the tests if(t==TRUE){...} and if(t) are not equivalent. Python has a related situation, where the Boolean type, bool is a subtype of the integer type, int, and Booleans False and True act as 0 and 1, respectively, in arithmetic contexts.

8.In their quest for increased safety over C++, the designers of Java removed C++-style pointers altogether. Unlike C++ reference variables, Java reference variables can be assigned to refer to different class instances; they are not constants. All Java class instances are referenced by reference variables. That is, in fact, the only use of reference variables in Java.

16.C and C++ are not strongly typed languages because both include union types, which are not type checked. ML is strongly typed, even though the types of some function parameters may not be known at compile time. F# is strongly typed. Java and C#, although they are based on C++, are strongly typed in the same sense as Ada. Types can be explicitly cast, which could result in a type error. However, there are no implicit ways type errors can go undetected.