Basic Pascal Tutorial/Chapter 3/IF: what changed

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

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1	1		[[Basic Pascal Tutorial/Chapter 3/Boolean Expressions\|◄]] \| [[Basic Pascal Tutorial/Contents\|▲]] \| [[Basic Pascal Tutorial/Chapter 3/CASE\|►]]
2	2		---\|---\|---
3	3
4	4
5	5		Back to [[Reserved words]].
6	6
7	7
8	8		3Ca - IF (author: Tao Yue, state: changed)
9	9
10	10		The `IF` statement allows you to branch based on the result of a Boolean operation.
11	11
12	12		## Format
13	13
14		-	IF _expression_ THEN
15		-	~~_statement1_~~
16		-	~~[ ELSE~~
17		-	~~_statement2_]~~
	14	+	IF _expression_ THEN
18	15
	16	+	_statement1_
	17	+
	18	+	[ ELSE
	19	+
	20	+	_statement2_]
	21	+
19	22		Where
20	23
21		-	~~_expression_ is any comparison, constant or function which returns a [boolean](</index.php?title=boolean&action=edit&redlink=1> "boolean \(page does not exist\)") value, and~~
22		-	_statement1_ and _statement2_ are either a single [[statement]], a [Begin](</index.php?title=begin&action=edit&redlink=1> "begin \(page does not exist\)")-[[End\|end]] [[Block\|block]], a [[Repeat\|repeat]]-until block, or the [[;\|null statement]].
	24	+	_expression_ is any comparison, constant or function which returns a [[boolean]] value, and
23	25
	26	+	_statement1_ and _statement2_ are either a single [[statement]], a [[begin\|Begin]]-[[End\|end]] [[Block\|block]], a [[Repeat\|repeat]]-until block, or the [[;\|null statement]].
	27	+
24	28		The ELSE clause is optional.
25	29
26	30		There are two ways to use an `IF` statement, a one-way branch or a two-way branch/
27	31
28	32		## One-way branch
29	33
30	34		The one-way branch format is:
31	35		```
32	36		if BooleanExpression then
33	37		StatementIfTrue;
34	38		```
35	39
36	40		If the Boolean expression evaluates to `true`, the statement executes. Otherwise, it is skipped.
37	41
38		-	The `IF` statement accepts only one statement. If you would like to use a [compound statement](</index.php?title=compound_statement&action=edit&redlink=1> "compound statement \(page does not exist\)"), you must use a `begin-end` [[Frame\|frame]] to enclose the statements:
	42	+	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:
39	43		```
40	44		if BooleanExpression then
41	45		begin
42	46		Statement1;
43	47		Statement2;
44	48		end;
45	49		```
46	50
47	51		## Two-way branch
48	52
49	53		There is also a two-way selection:
50	54		```
51	55		if BooleanExpression then
52	56		StatementIfTrue
53	57		else
54	58		StatementIfFalse;
55	59		```
56	60
57	61		Note there is never a semicolon `;` immediately before the `else`.
58	62		```
59	63		if BooleanExpression then
60	64		begin
61	65		Statement1; // semicolon here is mandatory
62	66		Statement2; // semicolon here is optional
63	67		end // semicolon here is forbidden
64	68		else
65	69		begin
66	70		Statement3;
67	71		Statement4;
68	72		end;
69	73		```
70	74
71	75		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
72	76
73	77		If you need multi-way selection, simply nest `if` statements:
74	78		```
75	79		if Condition1 then
76	80		Statement1
77	81		else
78	82		if Condition2 then
79	83		Statement2
80	84		else
81	85		Statement3;
82	86		```
83	87
84	88		Be careful with nesting. Sometimes the computer won't do what you want it to do:
85	89		```
86	90		if Condition1 then
87	91		if Condition2 then
88	92		Statement2
89	93		else
90	94		Statement1;
91	95		```
92	96
93	97		The `else` is always matched with the most recent `if`, so the computer interprets the preceding block of code as:
94	98		```
95	99		if Condition1 then
96	100		if Condition2 then
97	101		Statement2
98	102		else
99	103		Statement1;
100	104		```
101	105
102	106		You can get by with a null statement:
103	107		```
104	108		if Condition1 then
105	109		if Condition2 then
106	110		Statement2
107	111		else
108	112		else
109	113		Statement1;
110	114		```
111	115
112	116		Or you could use a `begin-end` block.
113	117
114	118		The following proves a semicolon is _absolutely forbidden_ before an else:
115	119		```
116	120		// Paul Robinson 2020-12-16
117	121
118	122		// Compiler test program Err03.pas
119	123		// tests the proposition that ; is
120	124		// never legal before ELSE
121	125
122	126
123	127		program Err03;
124	128		Var
125	129		Test,Test2: Boolean;
126	130
127	131
128	132		Begin
129	133
130	134		Test := True;
131	135		Test2 := True;
132	136
133	137		if Test then
134	138		if Test2 then
135	139		Writeln('Reached Part 1'); // semi-colon here should be illegal
136	140		else
137	141		Writeln('Reached Part 2');
138	142
139	143		end.
140	144		```
141	145
142	146		But the best way to clean up the code would be to rewrite the condition.
143	147		```
144	148		if not Condition1 then
145	149		Statement1
146	150		else
147	151		if Condition2 then
148	152		Statement2;
149	153		```
150	154
151	155		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.
152	156
153	157		Also notice how important indentation is to convey the logic of program code to a human, but the compiler ignores the indentation.
154	158
155	159		[[Basic Pascal Tutorial/Chapter 3/Boolean Expressions\|◄]] \| [[Basic Pascal Tutorial/Contents\|▲]] \| [[Basic Pascal Tutorial/Chapter 3/CASE\|►]]
156	160		---\|---\|---
157	161
158	162		---
159	163		From the Free Pascal wiki, page "Basic Pascal Tutorial/Chapter 3/IF", as the Internet Archive kept it on 2025-04-01, last changed on 2025-03-01. 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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