Basic Pascal Tutorial/Chapter 3/Boolean Expressions: what changed

Revision 3 to 4. Removed lines have a minus sign; added lines have a plus sign.

Old	New		Line
1	1		3B - Boolean Expressions (author: Tao Yue, state: unchanged)
2	2
3	3		[[Boolean]] expressions are used to compare two values and get a `true-or-false` answer:
4	4
5		-	~~```~~
	5	+	```text
6	6		value1 relational_operator value2
7	7		```
8	8
9	9		The following relational operators are used:
10	10
11		-	~~```~~
	11	+	```text
12	12		< less than
13	13		> greater than
14	14		= equal to
15	15		<= less than or equal to
16	16		>= greater than or equal to
17	17		<> not equal to
18	18		```
19	19
20	20		You can assign Boolean expressions to Boolean variables. Here we assign a `true` expression to some_bool:
21	21
22	22		``` pascal
23	23		some_bool := 3 < 5;
24	24		```
25	25
26	26		Complex Boolean expressions are formed by using the Boolean operators:
27	27
28		-	~~```~~
	28	+	```text
29	29		not negation (~)
30	30		and conjunction (^)
31	31		or disjunction (v)
32	32		xor exclusive-or
33	33		```
34	34
35	35		`NOT` is a unary operator — it is applied to only one value and inverts it:
36	36
37	37		- not true = false
38	38		- not false = true
39	39
40	40		`AND` yields `TRUE` only if both values are `TRUE`:
41	41
42	42		- TRUE and FALSE = FALSE
43	43		- TRUE and TRUE = TRUE
44	44
45	45		`OR` yields `TRUE` if at least one value is `TRUE`:
46	46
47	47		- TRUE or TRUE = TRUE
48	48		- TRUE or FALSE = TRUE
49	49		- FALSE or TRUE = TRUE
50	50		- FALSE or FALSE = FALSE
51	51
52	52		`XOR` yields `TRUE` if one expression is `TRUE` and the other is `FALSE`. Thus:
53	53
54	54		- TRUE xor TRUE = FALSE
55	55		- TRUE xor FALSE = TRUE
56	56		- FALSE xor TRUE = TRUE
57	57		- FALSE xor FALSE = FALSE
58	58
59	59		When combining two Boolean expressions using relational and Boolean operators, be careful to use parentheses.
60	60
61	61		``` pascal
62	62		(3>5) or (650<1)
63	63		```
64	64
65	65		This is because the Boolean operators are higher on the order of operations than the relational operators:
66	66
67	67		1. not
68	68		2. * / div mod and
69	69		3. \+ - or
70	70		4. \< \> \<= \>= = \<\>
71	71
72	72		So `3 > 5 or 650 < 1` becomes evaluated as `3 > (5 or 650) < 1`, which makes no sense, because the Boolean operator or only works on Boolean values, not on integers.
73	73
74	74		The Boolean operators (`AND, OR, NOT, XOR`) can be used on Boolean variables just as easily as they are used on Boolean expressions.
75	75
76	76		Whenever possible, don't compare two real values with the equals sign. Small round-off errors may cause two equivalent expressions to differ.
77	77
78	78		---
79	79		From the Free Pascal wiki, page "Basic Pascal Tutorial/Chapter 3/Boolean Expressions", as it stood on 2022-08-20 after a change by Kai Burghardt, 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).

Back to history