Basic Pascal Tutorial/Chapter 3/IF: what changed

Revision 1 to 2. Removed lines have a minus sign; added lines have a plus sign.

Old	New		Line
1	1		Back to [[Reserved words]].
2	2
3	3		3Ca - IF (author: Tao Yue, state: changed)
4	4
5	5		The `IF` statement allows you to branch based on the result of a Boolean operation.
6	6
7	7		## Format
8	8
9	9
10	10		IF expression THEN
11	11
12	12		statemrnt1
13	13
14	14		\[ ELSE
15	15
16	16		statement2\]
17	17
18	18		Where
19	19
20	20
21	21		expression is any comparison, constant or function which returns a [[boolen]] value, and
22	22
23	23		statement1 and statement2 are either a single [[statement]], a [[begin\|Begin]]-[[End\|end]] [[Block\|block]], a [[Repeat\|repeat]]-until block, or the [[;\|null statement]].
24	24
25	25		The ELSE clause is optional.
26	26
27	27		There are two ways to use an `IF` statement, a one-way branch or a two-way branch/
28	28
29	29		## One-way branch
30	30
31	31		The one-way branch format is:
32	32
33	33		``` pascal
34	34		if BooleanExpression then
35	35		StatementIfTrue;
36	36		```
37	37
38	38		If the Boolean expression evaluates to `true`, the statement executes. Otherwise, it is skipped.
39	39
40	40		The `IF` statement accepts only one statement. If you would like to use a [[compound statement]], you must use a `begin-end` [[Frame\|frame]] to enclose the statements:
41	41
42	42		``` pascal
43	43		if BooleanExpression then
44	44		begin
45	45		Statement1;
46	46		Statement2;
47	47		end;
48	48		```
49	49
50	50		## Two-way branch
51	51
52	52		There is also a two-way selection:
53	53
54	54		``` pascal
55	55		if BooleanExpression then
56	56		StatementIfTrue
57	57		else
58	58		StatementIfFalse;
59	59		```
60	60
61	61		Note there is never a semicolon `;` immediately before the `else`.
62	62
63	63		``` pascal
64	64		if BooleanExpression then
65	65		begin
66	66		Statement1; // semicolon here is mandatory
67	67		Statement2; // semicolon here is optional
68	68		end // semicolon here is forbidden
69	69		else
70	70		begin
71	71		Statement3;
72	72		Statement4;
73	73		end;
74	74		```
75	75
76		-	If the Boolean expression evaluates to `FALSE`, the statement following the `else` will be performed. Note that you may <u>never</u> use a semicolon after the statement preceding the `else`. That causes the computer to treat it as a one-way selection, leaving it to wonder where the else came from. And when a compiler wonders, it usually gets mad and throws a tantrum, or rather, it throws an error
	76	+	If the Boolean expression evaluates to `FALSE`, the statement following the `else` will be performed. Note that you may never use a semicolon after the statement preceding the `else`. That causes the computer to treat it as a one-way selection, leaving it to wonder where the else came from. And when a compiler wonders, it usually gets mad and throws a tantrum, or rather, it throws an error
77	77
78	78		If you need multi-way selection, simply nest `if` statements:
79	79
80	80		``` pascal
81	81		if Condition1 then
82	82		Statement1
83	83		else
84	84		if Condition2 then
85	85		Statement2
86	86		else
87	87		Statement3;
88	88		```
89	89
90	90		Be careful with nesting. Sometimes the computer won't do what you want it to do:
91	91
92	92		``` pascal
93	93		if Condition1 then
94	94		if Condition2 then
95	95		Statement2
96	96		else
97	97		Statement1;
98	98		```
99	99
100	100		The `else` is always matched with the most recent `if`, so the computer interprets the preceding block of code as:
101	101
102	102		``` pascal
103	103		if Condition1 then
104	104		if Condition2 then
105	105		Statement2
106	106		else
107	107		Statement1;
108	108		```
109	109
110	110		You can get by with a null statement:
111	111
112	112		``` pascal
113	113		if Condition1 then
114	114		if Condition2 then
115	115		Statement2
116	116		else
117	117		else
118	118		Statement1;
119	119		```
120	120
121	121		Or you could use a `begin-end` block.
122	122
123	123		The following proves a semicolon is absolutely forbidden before an else:
124	124
125	125		``` pascal
126	126		// Paul Robinson 2020-12-16
127	127
128	128		// Compiler test program Err03.pas
129	129		// tests the proposition that ; is
130	130		// never legal before ELSE
131	131
132	132		program err03;
133	133		Var
134	134		Test,test2: Boolean;
135	135
136	136		Begin
137	137
138	138		Test := True;
139	139		Test2 := true;
140	140
141	141		if test then
142	142		if test2 then
143	143		Writeln('Reached Part 1'); // semi-colon here should be illegal
144	144		else
145	145		Writeln('Reached Part 2');
146	146
147	147		end.
148	148		```
149	149
150	150		But the best way to clean up the code would be to rewrite the condition.
151	151
152	152		``` pascal
153	153		if not Condition1 then
154	154		Statement1
155	155		else
156	156		if Condition2 then
157	157		Statement2;
158	158		```
159	159
160	160		This example illustrates where the not operator comes in very handy. If Condition1 had been a Boolean like: `(not(a < b) or (c + 3 > 6)) and g`, reversing the expression would be more difficult than NOTting it.
161	161
162	162		Also notice how important indentation is to convey the logic of program code to a human, but the compiler ignores the indentation.
163	163
164	164		---
165	165		From the Free Pascal wiki, page "Basic Pascal Tutorial/Chapter 3/IF", 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).

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