pascal - What is wrong with my if-statement?

Question

I am now trying to explore pascal. And I ran into some compiler errors. I wrote a if else if statement like this:

  if ((input = 'y') or (input = 'Y')) then
    begin
      writeln ('blah blah');
    end;
  else if ((input = 'n') or (input = 'N')) then
    begin
      writeln ('blah');
    end;
  else
    begin
      writeln ('Input invalid!');
    end;

And it gives me an error at the first else:

";" expected but "ELSE" found

I looked for a lot of tutorials about if statements and they just do it like me:

if(boolean_expression 1)then 
   S1 (* Executes when the boolean expression 1 is true *)

else if( boolean_expression 2) then 
   S2 (* Executes when the boolean expression 2 is true *)

else if( boolean_expression 3) then 
   S3 (* Executes when the boolean expression 3 is true *)

else 
   S4; ( * executes when the none of the above condition is true *)

I tried to delete the begin and end but the same error occured. Is this a compiler bug?

P.S. I am doing this in a case statement. But I don't think it matters.

Answer 1

; is not allowed before else in the majority of cases.

  if ((input = 'y') or (input = 'Y')) then
    begin
      writeln ('blah blah');
    end
  else if ((input = 'n') or (input = 'N')) then
    begin
      writeln ('blah');
    end
  else
    begin
      writeln ('Input invalid!');
    end;

will compile. But... Prefer using begin ... end brackets to avoid misunderstanding of code in complicated if then else statements. something like this will be better:

  if ((input = 'y') or (input = 'Y')) then
  begin
    writeln('blah blah');
  end
  else
  begin
    if ((input = 'n') or (input = 'N')) then
    begin
      writeln('blah');
    end
    else
    begin
      writeln('Input invalid!');
    end;
  end;

The second sample is much easier to read and understand, isn't it?

The code does not work when you remove begin and end because there is a semicolon before else. This will compile without errors:

  if ((input = 'y') or (input = 'Y')) then
    writeln('blah blah')
  else
  begin

  end;

Appended on comment of @lurker

Please, see the following example without begin ... end brackets.

  if expr1 then
    DoSmth1
  else if expr2 then
    if expr3 then
      DoSmth2
    else
     DoSmth3;//Under what conditions is it called?

It is not clearly seen here, if DoSmth3 is called on not (expr2) or (expr2) and (not (expr3)). Though we can predict the compiler behaviour in this sample, the more complicated code without begin ... end becomes subect to mistakes and is difficult to read. See the following code:

  //behaviour 1
  if expr1 then
    DoSmth
  else if expr2 then
  begin
    if expr3 then
      DoSmth
  end
  else
    DoSmth;

  //behaviour 2
  if expr1 then
    DoSmth
  else if expr2 then
  begin
    if expr3 then
      DoSmth
    else
      DoSmth;
  end;

Now the code behavior is obvious.