Difference between revisions of "IF"

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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> [[block]] to enclose the statements:
+
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 lang=pascal>
 
<syntaxhighlight lang=pascal>
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</syntaxhighlight>
 
</syntaxhighlight>
  
Note (with one special exception explained below) there is no <tt>;</tt> following the statement before the <tt>else</tt>, even for the case with compound statements.
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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 (again, with one exception explained below) you may not 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.
+
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:
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Or you could use a <tt>begin-end</tt> block.  
 
Or you could use a <tt>begin-end</tt> block.  
  
''However,'' there is one special exception for just this case where a semicolon is not only allowed before an else statement, it is ''mandatory.'' Here is where it is required:
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The following proves a semicolon is ''absolutely forbidden'' before an else:
 
<syntaxhighlight lang=pascal>
 
<syntaxhighlight lang=pascal>
if Condition1 then
+
// Paul Robinson 2020-12-16
  if Condition2 then
+
 
    Statement2;
+
// Compiler test program  Err03.pas
else
+
// tests the proposition that ; is
  Statement1;
+
// 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>
In this particular example, the semicolon before the else warns the compiler that the else applies to the prior If statement. This particular situation - where there are two If statements, one after the other, and the else clause applies to the first statement only - is the one and only time it is permissible to use a semicolon before an else statement. Because most Pascal programmers "know" that you ''can't'' have a semicolon before an else you probably won't see it much if at all, but you should be aware of it.
 
 
 
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.
  

Latest 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.

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