Difference between revisions of "IF"

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3Ca - IF (author: Tao Yue, state: unchanged)
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{{IF}}
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{{TYNavigator|Boolean_Expressions|CASE}}
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Back to [[Reserved words]].
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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>
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 +
<syntaxhighlight lang=pascal>
 
if BooleanExpression then
 
if BooleanExpression then
 
   StatementIfTrue;  
 
   StatementIfTrue;  
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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> to enclose the statements:
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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>
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 +
<syntaxhighlight lang=pascal>
 
if BooleanExpression then
 
if BooleanExpression then
 
begin
 
begin
 
   Statement1;
 
   Statement1;
   Statement2
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   Statement2;
 
end;  
 
end;  
 
</syntaxhighlight>
 
</syntaxhighlight>
  
 
There is also a two-way selection:
 
There is also a two-way selection:
<syntaxhighlight>
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 +
<syntaxhighlight lang=pascal>
 
if BooleanExpression then
 
if BooleanExpression then
 
   StatementIfTrue
 
   StatementIfTrue
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</syntaxhighlight>
 
</syntaxhighlight>
  
If the Boolean expression evaluates to <tt>FALSE</tt>, the statement following the <tt>else</tt> will be performed. Note that 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.
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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.
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 +
<syntaxhighlight lang=pascal>
 +
if BooleanExpression then
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begin
 +
  Statement1;
 +
  Statement2;
 +
end
 +
else
 +
begin
 +
  Statement3;
 +
  Statement4;
 +
end;
 +
</syntaxhighlight>
 +
 
 +
 
 +
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
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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>
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 +
<syntaxhighlight lang=pascal>
 
if Condition1 then
 
if Condition1 then
 
   if Condition2 then
 
   if Condition2 then
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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
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You can get by with a null statement:
 
You can get by with a null statement:
<syntaxhighlight>
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 +
<syntaxhighlight lang=pascal>
 
if Condition1 then
 
if Condition1 then
 
   if Condition2 then
 
   if Condition2 then
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   Statement1;
 
   Statement1;
 
</syntaxhighlight>
 
</syntaxhighlight>
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Or you could use a <tt>begin-end</tt> block.
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The following proves a semicolon is ''absolutely forbidden'' before an else:
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<syntaxhighlight lang=pascal>
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// Paul Robinson 2020-12-16
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// Compiler test program  Err03.pas
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// tests the proposition that ; is
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// never legal before ELSE
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program err03;
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Var
 +
    Test,test2: Boolean;
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 +
 +
Begin
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 +
    Test := True;
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    Test2 := true;
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 +
    if test then
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      if test2 then
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          Writeln('Reached Part 1');  // semi-colon here should be illegal
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    else
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        Writeln('Reached Part 2');
 +
 +
end.
 +
</syntaxhighlight>
 +
But the best way to clean up the code would be to rewrite the condition.
  
Or you could use a <tt>begin-end</tt> block. But the best way to clean up the code would be to rewrite the condition.
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<syntaxhighlight lang=pascal>
<syntaxhighlight>
 
 
if not Condition1 then
 
if not Condition1 then
 
   Statement1
 
   Statement1
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Also notice how important indentation is to convey the logic of program code to a human, but the compiler ignores the indentation.  
 
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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{{TYNavigator|Boolean_Expressions|CASE}}
{|style=color-backgroud="white" cellspacing="20"
 
|[[Boolean_Expressions|previous]] 
 
|[[Contents|contents]]
 
|[[CASE|next]]
 
|}
 

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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