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Topic: undefined, null, and other signal values
topics >
computer science >
data >
boolean values, binary numbers, and bit strings
Group:
data type
Topic:
constants
Topic:
data type as a set of values
Topic:
database consistency and reliability
Topic:
database record
Topic:
database schema
Topic:
default value
Topic:
exception handling by unique value
Topic:
initialization of data
Topic:
kinds of numbers
Topic:
non-primitive values
Topic:
object slice
Topic:
safe use of pointers
Topic:
units
Topic:
value as an object
Summary
The 'undefined', 'null', 'don't care', 'missing', 'infinity', 'uninitialized' value is used: to terminate lists, to indicate the empty string or sequence, to indicate an error value, or to define partially specified functions. It is usually equated with false in control returning functions. Most machines do not directly indicate a null value so it is represented by another value. Zero is often used for empty pointers, or the length of a null string. Exceptions may return a 'null value'. Some statistical functions can process missing or undefined values. Slices across an array are indicated by an 'any index' value. (cbb 5/80)
Subtopic: null or unassigned
Quote: a signal is a structured value with no components [»hoarCA8_1978] Quote: reduce the number of orders by using tags instead of counting; e.g., use -1 to end a sequence of positive numbers [»wilkMV_1951] Quote: instead of using end-of-file, introduce end-of-day markers to initiate once a day processing [»jackMA5_1976] Quote: Galaxy exceptions are the targets of goto's; can be assigned to variables, e.g., for jumps to outer functions [»beet5_1989]
Subtopic: null terminator
Quote: a reference value refers to an object belonging to a class or its subclasses; the value 'none' is permissible for any reference [»dahlOJ_1967] Quote: can have a value 'unassigned' that makes a variable uninitialized [»winnRI10_1984] QuoteRef: backJ_1973 ;;77 empty sequence or identity function QuoteRef: storEF_1970 ;;24 x= is the null string Quote: in arithmetic can use +-infinity as NIL; so uninitialized variables produce NIL results [»wileDS11_1973, OK] QuoteRef: mckeWM_1966 ;;95 undefined types: null and inf (for a/0) can be tested and propagated Note: should access to a None object always create one? [»cbb_1990, OK] Note: auto creation of objects on Null access runs out of space like running out of time [»cbb_2000, OK] Note: null pointers mark the boundary between visited and unvisited objects; test for null; reference returns a new object [»cbb_2000, OK]
Subtopic: tags
Quote: NUL-terminated strings important for efficiency; allows fixed-length buffer that holds variable-length strings; problem of overrun [»jimT6_2002]
Subtopic: undefined value
Quote: L* items contain attributes, relations to other items or sets, and tags (e.g., male) [»grayJC_1973, OK]
Subtopic: missing value
Quote: every property value set includes two values: undefined (alpha) and missing (omega); distinguished from zero [»bosaR4_1962] Quote: a functional program for matrix multiply yields .bottom. for inappropriate arguments; it is perfectly general [»backJ8_1978a] Quote: the value of a literal is the literal itself; e.g., true, false, and .capOmega. as undefined [»wirtN1_1966] Quote: every property value set includes two values: undefined (alpha) and missing (omega); distinguished from zero [»bosaR4_1962] QuoteRef: kayAC6_1968 ;;15 includes .omega.(undefined) and .infinity.(a/0)
Subtopic: zero
Quote: every property value set includes two values: undefined (alpha) and missing (omega); distinguished from zero [»bosaR4_1962] Quote: use A-mark for missing-but-applicable values and I-mark for missing-but-inapplicable values; in place of null values Quote: represent missing values, as appropriate, by the empty set, empty string, "UNKNOWN", etc. [»dateCJ_1998] QuoteRef: kuznOP6_1972 ;;958 immaterial value y(*) and determined by others y(?)
Subtopic: tracking nulls
Quote: Panini's grammar used zero to indicate the lack of inflectional ending [»misrVN_1966] Quote: the earliest known use of the zero symbol is by Pingala (before 200 B.C.) [»dattB_1935]
Subtopic: avoiding nulls
Quote: track the origin of nulls by piggybacking an id; may help with diagnosis in one third of cases [»bondMD10_2007a]
Subtopic: problems with nulls
Quote: alias burying guarantees that aliases are never used; fields are never null unless explicitly set [»boylJ5_2001]
Quote: return a zero-length array instead of a null; nulls require special code [»blocJ_2001] Quote: nulls are not values; a database relation does not contain nulls nor need many-valued logic [»dateCJ_1998] Quote: tuples or values may be missing; either applicable, unknown, or inapplicable [»coddEF_1990] Quote: if an attribute value is not known than the tuple can not be constructed or inserted into a relvar [»dateCJ_1998] Quote: no method exists for treating null marks orthogonally w.r.t. data types [»dateCJ_1998] Quote: if nulls aren't values, then relations with null values aren't relations and the relational model is lost
[»dateCJ_1998]
Topic: boolean values, binary numbers, and bit strings (44 items)
Group: data type (34 topics, 730 quotes)
Topic: constants (21 items)
Topic: data type as a set of values (20 items)
Topic: database consistency and reliability (15 items)
Topic: database record (22 items)
Topic: database schema (29 items)
Topic: default value (8 items)
Topic: exception handling by unique value (8 items)
Topic: initialization of data (45 items)
Topic: kinds of numbers (24 items)
Topic: non-primitive values (11 items)
Topic: object slice (3 items)
Topic: safe use of pointers (102 items)
Topic: units (23 items)
Topic: value as an object
(29 items)