How to remake an example of a loop from delphi to js?

Question

I am trying to redo the delphi example to js. Two cycles. The first is “repeat until”. The second is “do while”. If you specify in delphi the conditions where the comparison takes place by digit, then if it matches, the cycle will be triggered once, if not, then repeatedly. In js, I try to do the same, it happens the other way around, if there is a match, then multiple repetitions, if there is no match, then once. Why is this happening and how to fix it? Delphi:

procedure TForm1.FormCreate(Sender: TObject);
var i:integer;
begin
 repeat
          i := 1;
          ShowMessage(IntToStr(i));
 until (i = 0) or (i = 4);
end;

js:

var i;
do
{
   i = 1;
   alert(i);
}while(i == 0 || i == 4)

Your Delphi loop is a bit "strange": You will only get an infinite number of 1 messages! The JS loop will give you exactly one such message. — Andreas Rejbrand
– Andreas Rejbrand, Commented Jan 21, 2021 at 9:39
To expand on Andreas's comment, you are not changing i in either loop. You must do that (probably by incrementing) in both cases to get a sensible action. — Dsm
– Dsm, Commented Jan 21, 2021 at 12:24

idmean · Accepted Answer · 2021-01-21 09:58:24Z

1

"Until" and "while" have a very different meaning in the English language, and in programming. As a matter of fact, "until A" means "while not A".

Therefore, you need to invert the while condition in JavaScript.

let i;
do {
   i = 1;
   alert(i);
} while(!(i == 0 || i == 4));

Of course, you could simplify the while condition by applying Boolean algebra, or more specifically De Morgan's Law. Also note, that I allowed myself to replace var with more modern let.

edited Jan 21, 2021 at 9:58

answered Jan 21, 2021 at 9:36

idmean

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5 Comments

Andreas Rejbrand Over a year ago

Might be educational to mention De Morgan here.

idmean Over a year ago

@AndreasRejbrand I’ve updated my answer to include that. Applying De Morgan is left as an exercise, though ;)

Dsm Over a year ago

Also (at least in Delphi) do...until always executes at least once. This is not true of while...do, so it is not quite true to imply that they are equivalent if you invert the conditions.

idmean Over a year ago

@Dsm Of course, do-while always executes the statement at least once. Please see here developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…. Please note that JavaScript (like most C related languages) has do-while and while, which do different things.

Andreas Rejbrand Over a year ago

@Dsm: idmean is comparing Delphi repeat..until and JS do..while and they both do at least one iteration, so there is no difference.

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