CASE: what changed

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Title: Case

New		Line
1	+	The [[Reserved word\|reserved word]] `case` starts a clause where alternatives are chosen.
2	+
3	+	## structure
4	+
5	+	The general structure of `case`-clauses looks like this:
6	+
7	+	``` pascal
8	+	case selector of
9	+	caseValue0: <statement>;
10	+	caseValue1,caseValue7: <statement>;
11	+	caseValue10..caseValue20: <statement>;
12	+	else <statements>;
13	+	end;
14	+
15	+	```
16	+
17	+	The following constraints have to be met:
18	+
19	+	- The data type of `selector` has to be an ordinal type. [[FPC\|FreePascal]] additionally allows [[Character and string types\|strings]].
20	+	- All case values have to be the same data type as `selector` is.
21	+	- They have to be constant expressions, i.e. known at compile-time.
22	+	- All `case`-labels have to be mutually disjoint. For every discrete value there applies exactly one or no cases.
23	+	- A `case`-clause has to have at least one `case`-label. Imperative `case`-statements can have at most one anonymous cases (at most one `else`/`otherwise`-branches).
24	+
25	+	The actual order of `case`-labels is not relevant. They may be ascending, descending, or mixed up, it does not matter.
26	+
27	+	Originally, standard Pascal also set the constraint, that all possible values of the selector have to have a corresponding match. This is no longer the case with modern compilers.
28	+
29	+	`case` can have both, imperative and declarative meanings, depending on where it is written. The former are “statements”, whereas the latter start a “variant part” of records.
30	+
31	+	### `case`-statements
32	+
33	+	`case`-statements are a concise way of writing [[Branch\|branches]]. They suit best, where alternative paths are taken exclusively. They may contain an [[Else\|else]]-branch catching all cases that are not listed.
34	+
35	+	``` pascal
36	+	program asciiTest(input, output, stderr);
37	+
38	+	var
39	+	c: char;
40	+
41	+	begin
42	+	read(c);
43	+	case ord(c) of
44	+	// empty statement, so the control characters are not
45	+	// considered by the else-branch as non-ASCII-characters
46	+	0..$1F, $7F: ;
47	+	$20..$7E:
48	+	begin
49	+	writeLn('You entered an ASCII printable character.');
50	+	end;
51	+	else
52	+	begin
53	+	writeLn('You entered a non-ASCII character.');
54	+	end;
55	+	end;
56	+	end.
57	+	```
58	+
59	+	Note, `case`-statements accept expressions as selector.
60	+
61	+	Instead of writing `else` the word `otherwise` is allowed, too. This is an [[Extended Pascal]] extension.
62	+
63	+	While the same semantics can be achieved by consecutive [[If\|if]]-[[Then\|then]]-branches, utilizing a [[Case Compiler Optimization\|case-statement allows the code generator to optimize the branch selection]].
64	+
65	+	#### comparative remarks
66	+
67	+	There is no “fall-through” as it is the case with other languages such as shell or C. In Pascal exactly one case matches, is processed, and program flow continues after the final [[End\|end]] of the `case`-statement. The [[Break\|break-statement]] with its special meaning only appears in loops.
68	+
69	+	### variant part in records
70	+
71	+	A [[Record\|record]] may contain a variant part. The `case`-selector has to be the name of a data type, but an identifier for accessing the current variant can be provided, too.
72	+
73	+	``` pascal
74	+	program variantRecordDemo(input, output, stderr);
75	+
76	+	type
77	+	sex = (female, male);
78	+	clothingSize = record
79	+	// FIXED PART
80	+	shoulderWidth: word;
81	+	armLength: word;
82	+	bustGirth: word;
83	+	waistSize: word;
84	+	hipMeasurement: word;
85	+	// VARIABLE PART
86	+	case body: sex of
87	+	female: (
88	+	underbustMeasure: word;
89	+	);
90	+	male: (
91	+	);
92	+	end;
93	+	begin
94	+	end.
95	+	```
96	+
97	+	## see also
98	+
99	+	- [“The `case` statement” in the “FreePascal reference manual”](https://www.freepascal.org/docs-html/ref/refsu56.html)
100	+	- [[Basic Pascal Tutorial/Chapter 3/CASE\|“case” as part of the “Object Pascal introduction”]]
101	+
102	+	## external references
103	+
104	+	- [Tao Yue: “Learn Pascal!”. Chapter “case”](https://lazarus-ccr.sourceforge.io/pascal/pas3cb.html)
105	+
106	+	---
107	+	From the Free Pascal wiki, page "Case", as it stood on 2023-10-17 after a change by FPC user, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).
108	+	Category: Code

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