Basic Pascal Tutorial/Chapter 3/IF: what changed

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Title: Basic Pascal Tutorial/Chapter 3/IF

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