Difference between revisions of "Basic Pascal Tutorial/Chapter 3/IF"
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</syntaxhighlight> | </syntaxhighlight> | ||
− | Note | + | Note there is no <tt>;</tt> following the statement before the <tt>else</tt>, even for the case with compound statements. |
<syntaxhighlight lang=pascal> | <syntaxhighlight lang=pascal> | ||
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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 | + | 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: | ||
Line 98: | Line 98: | ||
Or you could use a <tt>begin-end</tt> block. | Or you could use a <tt>begin-end</tt> block. | ||
− | '' | + | The following proves a semicolon is ''absolutely forbidden'' before an else: |
<syntaxhighlight lang=pascal> | <syntaxhighlight lang=pascal> | ||
− | if | + | // Paul Robinson 2020-12-16 |
− | + | ||
− | + | // Compiler test program Err03.pas | |
− | else | + | // 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> | </syntaxhighlight> | ||
− | |||
− | |||
But the best way to clean up the code would be to rewrite the condition. | But the best way to clean up the code would be to rewrite the condition. | ||
Revision as of 01:24, 18 December 2020
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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 block 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.