Pascal Boolean return value? - pascal

Question

I'm currently working on an exercise of testing Boolean value from user input, as being presented below:
function ReadBoolean(prompt: String): Boolean;
var
choice: String;
exit: boolean;
begin
repeat
begin
WriteLn(prompt);
ReadLn(choice);
case choice of
'yes','y','t','true': exit := true;
'no','n','f','false': exit := false;
else
WriteLn('Not a boolean input. Enter again: ');
end;
end;
until exit=true or exit=false;
result := exit;
end;
It is expected to keep looping asking for value until it receives the input from specified string, however at my first attempt when I try to input 'fred' the boolean variable is automatically assigned as TRUE and exit the function.
Any helps would be very appreciated.

Answer 1

As my understanding, you only want the loop to end when user enters some specific strings.
It can be achieved by modifying the until condition like this:
choice='yes' or choice='y' or choice='t' or choice='true' or choice='no' or choice='n' or choice='f' or choice='false'
Or alternatively, create an indefinite loop and break it when user enters an expected string:
while true do
...
'yes','y','t','true':
begin
exit := true;
break;
end;
'no','n','f','false':
begin
exit := false;
break;
end;
...
end;

Answer 2

Your loop terminates when exit=true or exit=false. As exit can only ever be one of those two values, it will always meet that condition, so it will never run your loop.
But also, consider explicitly setting the value of exit := false before starting your loop.

Answer 3

What you're asking here is "nullable" boolean thing (value is true, value is false, value is not provided). As far as I know it's not implemented in any Pascal dialect. So that, you have to split your indication into two separate flags: a) is there any well-formed input provided by user; b) is the input is recognized as true or false
function ReadBoolean(prompt: String): Boolean;
var
choice: String;
exit: boolean;
recognized: boolean; { this is our termination flag }
begin
recognized := false; { we place it to false initially as no user input recognized yet }
repeat
begin
WriteLn(prompt);
ReadLn(choice);
case choice of
'yes','y','t','true': begin exit := true; recognized := true; end; { we mark it as recognized }
'no','n','f','false': begin exit := false; recognized := true; end; { we mark it as recognized }
else
WriteLn('Not a boolean input. Enter again: ');
end;
end;
until not recognized; { we keep asking for user input until known input provided }
result := exit;
end;