{"id":74,"date":"2023-05-04T12:46:22","date_gmt":"2023-05-04T10:46:22","guid":{"rendered":"http:\/\/oopm.org\/?page_id=74"},"modified":"2024-01-04T15:17:29","modified_gmt":"2024-01-04T14:17:29","slug":"5-data-items","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=74","title":{"rendered":"4A Data-Items and Data Types"},"content":{"rendered":"
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I have preliminary put this page around Chapter 4, since I think it is needed here, but it is up for discussion. The same for the Chapter on 4B Statements – see also the note there! <\/p>\n\n\n\n

I guess that the structure of this chapter should be like this:\n\nData items and their types   -- ikke 'and Value Types', da referencer er her\n   Reference data items\n   Value data items\n      Primitive values types\n      Composite value types\n\nOLM: Men skal man starte med references?\nOg hvor t\u00e6nker du der er et afsnit om Values and Objects<\/em><\/strong>?<\/code><\/pre>\n\n\n\n
In this Chapter, we summarize +++bmp: hvis vi f\u00f8rst indf\u00f8rer value types her, da er det ikke l\u00e6ngere bare et summary<\/mark> olm: teksten er fra den gang vi t\u00e6nkte summary, s\u00e5 den skal helt klart fornyes.<\/mark> data-items that may represent primitive values, string values and references to objects.<\/p>\n\n\n\n
As mentioned in previous chapters, objects may have attributes in the form of data-items that may represent properties characterizing the phenomena. For the Account<\/code> example, we have seen examples of data-items such as owner<\/code>, balance<\/code>, and interestRate<\/code>. <\/p>\n\n\n\n
There is a fundamental difference between the data-items owner<\/code> and balance<\/code> of Account<\/code> objects. While owner<\/code> is a reference<\/em> to the object representing the owner of the account,  balance<\/code> represents a property that is given by a value<\/em>. <\/p>\n\n\n\n
Also, some value types are denoted primitive values<\/em>. balance<\/code> and interestRate<\/code> are examples of data-items holding primitive values, in this case of type Real<\/code>. <\/p>\n\n\n\n
References<\/h2>\n\n\n\n
+++ bmp: Sidste version af Account<\/code> har owner as a ref<\/strong> Customer<\/code>. <\/mark>The data-item owner<\/code> is holding values of type String<\/code>, but is usually not considered a primitive value type. The reason is that the primitive values may be represented in a fixed number of bytes in the computer whereas a String<\/code> value may need a variable number of bytes for its representation.<\/p>\n\n\n\n
As mentioned, data-items like Account_1010, aClerk, and aCustomer are examples of data-items holding references to objects. These data-items were declared as:<\/p>\n\n\n\n
Account_1010: obj<\/strong>\n   ...\naClerk: obj<\/strong>\n   ...\naCustomer: ref<\/strong> Customer<\/code><\/pre>\n\n\n\n
The data-items Account_1010<\/code>, and aClerk<\/code>, are constant in the sense that they refer to the same object during the lifetime of the program execution. The data-time aCustomer<\/code> is variable since it may refer to different Customer<\/code> objects during the program execution.<\/p>\n\n\n\n
JohnSmith: obj<\/strong> Customer(\"John Smith\")\naCustomer: ref<\/strong> Customer\naCustomer := Customer(\"Mary Poe\")\n...\naCustomer := JohnSmith<\/code><\/pre>\n\n\n\n
In the above example, JohnSmith<\/code> is constantly referring to the object Customer(\"John Smith\")<\/code> whereas aCustomer<\/code> may refer different Customer<\/code> objects. It is first assigned a new object Customer(\"Mary Poe\")<\/code>. Then later it may be assigned JohnSmith<\/code>, which nimplies that Mary<\/code> refers to the same object as JohnSmith<\/code>.<\/p>\n\n\n\n
This page should be split into subpages.   <\/p>\n\n\n\n
Value data items<\/h2>\n\n\n\n
+++Value data items: name and value type<\/p>\n\n\n\n
Primitive value types<\/h3>\n\n\n\n
Data-items such as owner<\/code>, balance<\/code> and interestRate<\/code> are examples of data-items holding values of primitive value types<\/em>. These data-items are declared as follows:<\/p>\n\n\n\n
owner: var<\/strong> String\nbalance: var<\/strong> Real\ninterestRate: var<\/strong> Real<\/code><\/pre>\n\n\n\n
The specification var<\/strong> String<\/code> in the declaration of owner<\/code>, specifies that owner<\/code> may hold string values, and String<\/code> is called the type<\/em> of owner<\/code>. <\/p>\n\n\n\n
The specification var<\/strong> Real<\/code> in the declarations of balance<\/code> and interestRate<\/code> specifies that these may hold real values, and their type is thus Real<\/code>.<\/p>\n\n\n\n
Real<\/code> and String<\/code> are examples value types<\/em>, which are similar to classes. We discuss value types in Chapter Objects and Values<\/em>.<\/p>\n\n\n\n
In the following sections we describe other common primitive values.<\/p>\n\n\n\n
Real values<\/h4>\n\n\n\n
The data-items balance<\/code> and interestRate<\/code> are as mentioned examples of data-items that represent real values. Examples of real values are: 100, 3.14, 0.56, -12.11, 13.5E7<\/code>, and 1.11E-7<\/code>. The 'E'<\/code> in 13.5E7 <\/code>means that the value is 13.7 multiplied by 107<\/sup>.<\/p>\n\n\n\n
The keyword var<\/strong><\/code> used in the declaration specifies that these data-items are variable, which means that they may be assigned different values during the execution of the program:<\/p>\n\n\n\n
balance := 500\ninterestRate := 3.07\n...\nbalance := balance + amount\ninterestRate := 2.9\n...<\/code><\/pre>\n\n\n\n
It is also possible to specify that a data-item holds a value which is constant during the whole of the program execution:<\/p>\n\n\n\n
   X: val<\/strong> 3.1\n<\/code><\/code><\/pre>\n\n\n\n
The  keyword val<\/code><\/strong> specifies that the data-item X<\/code> is constant and it holds the value 3.14<\/code> during the whole of the program execution.<\/p>\n\n\n\n
The set of real numbers is infinite, and a computer can only represent a subset of the real values. This subset is often called floating point values<\/em>. Since only a subset can be represented, the evaluation of floating point expressions often imply that some values are rounded to a value that the computer can represent. For calculations of a large amount of numbers these rounding may lead to misleading and erroneous results. It is therefore recommended that people writing programs involving floating point calculations learn about how to try to avoid rounding errors.<\/p>\n\n\n\n
The above paragraph may be placed in a side box <\/p>\n\n\n\n
Integer values<\/h4>\n\n\n\n
We need an example of a integer data-item in one of the previous chapters.<\/p>\n\n\n\n
In the example in Chapter\/Section xx.yy, anInt<\/code> is an example of a data-item that may represent an integer<\/code> value. Integer is another example of a value type representing primitive values. Examples of integer values are 17, 111, -4, 0, and -1014.<\/p>\n\n\n\n
An for Real, an integer may redeclared as a variable or a constant:<\/p>\n\n\n\n
anInt: var<\/strong> Integer\naConst: val 117<\/code><\/pre>\n\n\n\n
The data-item anInt<\/code> is an example of a variable that may hold different integers during the program execution. the data-item aConst<\/code> on the other hand is constant and holding the value 117 during the whole programm execution.<\/p>\n\n\n\n
Boolean values<\/h4>\n\n\n\n
We need an example using booleans.<\/p>\n\n\n\n
A data-item may be of type Boolean<\/code> and such a data-item may have the value True<\/code> or False<\/code>.  A data-item of type Boolean<\/code> may be declared as follows:<\/p>\n\n\n\n
   B1: val<\/strong> True\n   B2: var<\/strong> Boolean<\/code><\/code><\/pre>\n\n\n\n
The data-item B1<\/code> is a constant having the value True during the whole program execution. The data-item B2<\/code> is a variable that may hold the values True or False at different point during the program execution. In the next example, B2<\/code> is assigned the value False<\/p>\n\n\n\n
   B2 := False<\/code><\/code><\/pre>\n\n\n\n
Character values<\/h4>\n\n\n\n
We need an example using char<\/p>\n\n\n\n
A data-item of type Char may hold a value representing a character from the UniCode character set. A character may be a letter, a digit, a special character, a blank\/space or an end-of-line character. A character literal is typically written within single quotes like:<\/p>\n\n\n\n
   \u2018a\u2019, \u2018b\u2019, \u2018Q\u2019, 'L\u2019  \u2014 examples of letters\n   \u20180\u2019, 7\u2032             \u2014 examples of digits\n   \u2018.\u2019, \u2018:\u2019, \u2018+\u2019,      \u2014 examples of special characters\n   \u2018 \u2018                 \u2014 the blank character<\/code><\/code><\/pre>\n\n\n\n
An end-of-line character has no graphical form and is thus denoted as \u2018\\n\u2019. The character \u2018\\\u2019 is a so-called escape character the implies that the next character defines a special character. The following are examples of characters defined using the escape character<\/p>\n\n\n\n
   \u2018\\n\u2019  \u2014 end-of-line\n   \u2018\\t\u2019  \u2014 tab\n   \u2018\\\u201d   \u2014 the quote character \u2018\n   \u2026<\/code><\/code><\/pre>\n\n\n\n
The following example shows the declaration of data-items of type char and assigment statements: <\/p>\n\n\n\n
   ch1: val<\/strong> \u2018!\u2019
   ch2: var<\/strong> Char
   ch2 := \u2018Q\u2019<\/code><\/code><\/pre>\n\n\n\n
The data-item ch1<\/code> is a a constant holding the exclamation mark character '!'<\/code>. The data-item ch2<\/code> is a variable of type char<\/code>. It may hold different character values during the program execution. Execution of the assignment statement ch2 := 'Q' <\/code>has the effect that ch2<\/code> holds the character 'Q'<\/code>. <\/p>\n\n\n\n
String values<\/h2>\n\n\n\n
As mention, owner is a data-item that may hold string values. A string value is a sequence of characters as opposed to char values, which are single characters. A string value may thus consist of zero or more characters. In the program text, a string value is enclosed in double quotes (“) and special characters are preceded by a slash (\\). Example are shown below:<\/p>\n\n\n\n
\"John Smith\"       \n\"Hello world\\n\".          -- a newline is represented by \\n\n\"a + b * (c +111)\"\n\"He said: \\\"Go away!\\\"\"   -- the character \" is represented by \\\" in the string<\/code><\/pre>\n\n\n\n
References<\/h2>\n\n\n\n
<\/p>\n
\n\t\t<\/a>\n\t\t<\/div>","protected":false},"excerpt":{"rendered":"
I have preliminary put this page around Chapter 4, since I think it is needed here, but it is up for discussion. The same for the Chapter on 4B Statements – see also the note there! In this Chapter, we summarize +++bmp: hvis vi f\u00f8rst indf\u00f8rer value types her, da er det ikke l\u00e6ngere bare […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":2825,"menu_order":4,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-74","page","type-page","status-publish","hentry"],"mb":[],"mfb_rest_fields":["title","gutenberg_elementor_mode"],"_links":{"self":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/74","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=74"}],"version-history":[{"count":41,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/74\/revisions"}],"predecessor-version":[{"id":2268,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/74\/revisions\/2268"}],"up":[{"embeddable":true,"href":"https:\/\/oopm.org\/index.php?rest_route=\/wp\/v2\/pages\/2825"}],"wp:attachment":[{"href":"https:\/\/oopm.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=74"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}