Difference between revisions of "Basic Pascal Tutorial/Chapter 3/IF"
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{{TYNavigator|Boolean_Expressions|CASE}} | {{TYNavigator|Boolean_Expressions|CASE}} | ||
− | 3Ca - IF (author: Tao Yue, state: | + | |
+ | Back to [[Reserved words]]. | ||
+ | |||
+ | |||
+ | 3Ca - IF (author: Tao Yue, state: changed) | ||
The <tt>IF</tt> statement allows you to branch based on the result of a Boolean operation. The one-way branch format is: | The <tt>IF</tt> statement allows you to branch based on the result of a Boolean operation. The one-way branch format is: | ||
− | <syntaxhighlight> | + | |
+ | <syntaxhighlight lang=pascal> | ||
if BooleanExpression then | if BooleanExpression then | ||
StatementIfTrue; | StatementIfTrue; | ||
Line 12: | Line 17: | ||
If the Boolean expression evaluates to <tt>true</tt>, the statement executes. Otherwise, it is skipped. | If the Boolean expression evaluates to <tt>true</tt>, the statement executes. Otherwise, it is skipped. | ||
− | The <tt>IF</tt> statement accepts only one statement. If you would like to branch to a compound statement, you must use a <tt>begin-end</tt> [[ | + | The <tt>IF</tt> statement accepts only one statement. If you would like to branch to a compound statement, you must use a <tt>begin-end</tt> [[Frame|frame]] to enclose the statements: |
− | <syntaxhighlight> | + | |
+ | <syntaxhighlight lang=pascal> | ||
if BooleanExpression then | if BooleanExpression then | ||
begin | begin | ||
Line 22: | Line 28: | ||
There is also a two-way selection: | There is also a two-way selection: | ||
− | <syntaxhighlight> | + | |
+ | <syntaxhighlight lang=pascal> | ||
if BooleanExpression then | if BooleanExpression then | ||
StatementIfTrue | StatementIfTrue | ||
Line 30: | Line 37: | ||
Note there is no <tt>;</tt> following the statement before the <tt>else</tt>, even for the case with compound statements. | Note there is no <tt>;</tt> following the statement before the <tt>else</tt>, even for the case with compound statements. | ||
− | <syntaxhighlight> | + | |
+ | <syntaxhighlight lang=pascal> | ||
if BooleanExpression then | if BooleanExpression then | ||
begin | begin | ||
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− | If the Boolean expression evaluates to <tt>FALSE</tt>, the statement following the <tt>else</tt> will be performed. Note that you may | + | If the Boolean expression evaluates to <tt>FALSE</tt>, the statement following the <tt>else</tt> will be performed. Note that you may <u>never</u> use a semicolon after the statement preceding the <tt>else</tt>. 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 |
If you need multi-way selection, simply nest <tt>if</tt> statements: | If you need multi-way selection, simply nest <tt>if</tt> statements: | ||
− | <syntaxhighlight> | + | |
+ | <syntaxhighlight lang=pascal> | ||
if Condition1 then | if Condition1 then | ||
Statement1 | Statement1 | ||
Line 58: | Line 67: | ||
Be careful with nesting. Sometimes the computer won't do what you want it to do: | Be careful with nesting. Sometimes the computer won't do what you want it to do: | ||
− | <syntaxhighlight> | + | |
+ | <syntaxhighlight lang=pascal> | ||
if Condition1 then | if Condition1 then | ||
if Condition2 then | if Condition2 then | ||
Line 67: | Line 77: | ||
The <tt>else</tt> is always matched with the most recent <tt>if</tt>, so the computer interprets the preceding block of code as: | The <tt>else</tt> is always matched with the most recent <tt>if</tt>, so the computer interprets the preceding block of code as: | ||
− | <syntaxhighlight> | + | |
+ | <syntaxhighlight lang=pascal> | ||
if Condition1 then | if Condition1 then | ||
if Condition2 then | if Condition2 then | ||
Line 76: | Line 87: | ||
You can get by with a null statement: | You can get by with a null statement: | ||
− | <syntaxhighlight> | + | |
+ | <syntaxhighlight lang=pascal> | ||
if Condition1 then | if Condition1 then | ||
if Condition2 then | if Condition2 then | ||
Line 84: | Line 96: | ||
Statement1; | Statement1; | ||
</syntaxhighlight> | </syntaxhighlight> | ||
+ | Or you could use a <tt>begin-end</tt> block. | ||
+ | |||
+ | The following proves a semicolon is ''absolutely forbidden'' before an else: | ||
+ | <syntaxhighlight lang=pascal> | ||
+ | // Paul Robinson 2020-12-16 | ||
+ | |||
+ | // Compiler test program Err03.pas | ||
+ | // tests the proposition that ; is | ||
+ | // never legal before ELSE | ||
+ | |||
+ | |||
+ | program err03; | ||
+ | Var | ||
+ | Test,test2: Boolean; | ||
+ | |||
+ | |||
+ | Begin | ||
+ | |||
+ | Test := True; | ||
+ | Test2 := true; | ||
+ | |||
+ | if test then | ||
+ | if test2 then | ||
+ | Writeln('Reached Part 1'); // semi-colon here should be illegal | ||
+ | else | ||
+ | Writeln('Reached Part 2'); | ||
+ | |||
+ | end. | ||
+ | </syntaxhighlight> | ||
+ | But the best way to clean up the code would be to rewrite the condition. | ||
− | + | <syntaxhighlight lang=pascal> | |
− | <syntaxhighlight> | ||
if not Condition1 then | if not Condition1 then | ||
Statement1 | Statement1 |
Revision as of 19:50, 24 January 2021
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Back to Reserved words.
3Ca - IF (author: Tao Yue, state: changed)
The IF statement allows you to branch based on the result of a Boolean operation. The one-way branch format is:
if BooleanExpression then
StatementIfTrue;
If the Boolean expression evaluates to true, the statement executes. Otherwise, it is skipped.
The IF statement accepts only one statement. If you would like to branch to a compound statement, you must use a begin-end frame to enclose the statements:
if BooleanExpression then
begin
Statement1;
Statement2;
end;
There is also a two-way selection:
if BooleanExpression then
StatementIfTrue
else
StatementIfFalse;
Note there is no ; following the statement before the else, even for the case with compound statements.
if BooleanExpression then
begin
Statement1;
Statement2;
end
else
begin
Statement3;
Statement4;
end;
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
If you need multi-way selection, simply nest if statements:
if Condition1 then
Statement1
else
if Condition2 then
Statement2
else
Statement3;
Be careful with nesting. Sometimes the computer won't do what you want it to do:
if Condition1 then
if Condition2 then
Statement2
else
Statement1;
The else is always matched with the most recent if, so the computer interprets the preceding block of code as:
if Condition1 then
if Condition2 then
Statement2
else
Statement1;
You can get by with a null statement:
if Condition1 then
if Condition2 then
Statement2
else
else
Statement1;
Or you could use a begin-end block.
The following proves a semicolon is absolutely forbidden before an else:
// Paul Robinson 2020-12-16
// Compiler test program Err03.pas
// tests the proposition that ; is
// never legal before ELSE
program err03;
Var
Test,test2: Boolean;
Begin
Test := True;
Test2 := true;
if test then
if test2 then
Writeln('Reached Part 1'); // semi-colon here should be illegal
else
Writeln('Reached Part 2');
end.
But the best way to clean up the code would be to rewrite the condition.
if not Condition1 then
Statement1
else
if Condition2 then
Statement2;
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.
Also notice how important indentation is to convey the logic of program code to a human, but the compiler ignores the indentation.