How to switch condition position?

[Mahdi Safsafi 225]

19 minutes ago, Mike Torrettinni said:

Thank you for the example! I'm not really new to Delphi, but never grew out of the beginners's level of thinking, I guess... you can find a thread here me questioning about why should I use classes at all 🙂

Don't worry everything is gonna be alright by the time. When I was beginner I asked a similar question : what's the purpose of interface. I got many answers but it took me a lot of time to understand what they were saying.

 

Here is a free advice : first get familiar about the basic of program's life from the loader-time to the cpu-time (just get the basic things) and then master the fundamental stuff of Delphi (also just learn the basic things : statements, functions, array, records, RTL, ...). Now join the open source community and share your first library/app, fork other project and analyse people coding styles (it will be a great experience to you as was mine). after that you will find yourself understanding things quickly even you can master new language with just few days.

Good Luck.  

[Rudy Velthuis 91]

13 hours ago, David Heffernan said:

That's certainly more concise but it has a completely different meaning. Think about it Rudy. 

In the case given, it has the same meaning.

I often see people write something like

if condition then 
  Result := True 
else 
  Result := False;

while a reduction like

Result := condition;

is often not considered. 

[Rudy Velthuis 91]

On 2/9/2019 at 3:24 PM, Leif Uneus said:

Result := Result or (condition);

 

That is not the same.

 

 

 

[Rudy Velthuis 91]

On 2/9/2019 at 4:54 PM, Mike Torrettinni said:

 

I didn't provide good example, I'm trying to work on statement like this, working on strings:

 

if Property = 'A' then Result := 'N';

 

so looking for in idea how to have something like this:

 

Result := 'N' if/when (Property = 'A');

 

AFAIK, in Python such a construct is used as some kind of ternary operator:

  result = 'N' if (property == 'A') else 'Y'

I guess they didn't put the condition first because then the interpreter will have a harder time infering the type of result (pseudo-Python):

result = if (property == 'A') 'N' else 'Y';

or because there is no nice separation between the condition and the true result.

 

 

[Rudy Velthuis 91]

14 hours ago, Uwe Raabe said:

if Condition then Result := True;

Result := Condition;

These are not the same! What if Result is true before that line in the first and in the second case?

Ok, I get it now, thanks David and Uwe. You guys mean that if Condition is False, Result will not change.

 

Still, then one can do:

Result := Condition or Result;

But this assumes that Condition will be checked first, otherwise a possible function call you expect to be made is not made. I think this order is guaranteed, but I would not really count on it.

 

Anyway, the form with the if clause after the assignment doesn't make sense to me.

Edited February 10, 2019 by Rudy Velthuis
Typo: üpossible

[Rudy Velthuis 91]

15 hours ago, KeithLatham said:

I am glad too. But to address the part about you not understanding the construct, I would suggest that it resembles 'reverse Polish notation', and that is notoriously easy to implement in a stack environment. I have an book on interpreters somewhere in the bowels of my archeology site that some would laughingly call a 'library' that discusses exactly that method for implementing a Fortran interpreter.

Does Python run on a stack machine? Anyway, if they can manage to have proper if conditions too (and they do) then I don't see why they have this alternative syntax.

 

I looked and saw that CPython emulates a virtual stack machine.

[Rudy Velthuis 91]

14 hours ago, David Heffernan said:

That's certainly more concise but it has a completely different meaning. Think about it Rudy. 

Yeah, got it now. It is different if Result was already set before.

[David Heffernan 2345]

7 hours ago, Rudy Velthuis said:

Does Python run on a stack machine? Anyway, if they can manage to have proper if conditions too (and they do) then I don't see why they have this alternative syntax.

 

I looked and saw that CPython emulates a virtual stack machine.

 Nothing to do with implementation details. The Python syntax is simply the Python equivalent of the C conditional operator. You know, the operator that is missing from Delphi. 

[David Heffernan 2345]

7 hours ago, Rudy Velthuis said:

I think this order is guaranteed, but I would not really count on it.

You can count on it. http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Expressions_(Delphi)#Complete_Versus_Short-Circuit_Boolean_Evaluation

 

Very surprised you don't know this. 

[David Schwartz 426]

Rudy said: I think this order is guaranteed, but I would not really count on it.

1 hour ago, David Heffernan said:

You can count on it. http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Expressions_(Delphi)#Complete_Versus_Short-Circuit_Boolean_Evaluation

 

There's a compiler switch that overrides this, so it's not really "guaranteed".

 

[Uwe Raabe 2057]

1 minute ago, David Schwartz said:

There's a compiler switch that overrides this, so it's not really "guaranteed".

The compiler switch makes it evaluate either all parts, which is perfectly valid in the first place, or evaluates from left to right, which is exactly what we want when writing

Result := Condition or Result;

So whatever that switch is, Condition is evaluated first.

[Mahdi Safsafi 225]

36 minutes ago, David Schwartz said:

Rudy said: I think this order is guaranteed, but I would not really count on it.

There's a compiler switch that overrides this, so it's not really "guaranteed".

 

No one will switch to complete circuit evaluation. There is a rare case when it makes sense to use it.

function DoSomething(): Boolean;
begin
  // ...
  Result := (not HasError) and DumpStatus();
end;

DumpStatus is a function that will dump the status whether there was an error or not. 

If error exists, DumpStatus will dump the error status. If not it will dump a success status. 

DoSomething's result depends on HasError. So it's important to do a full evaluation of the expression  "(not HasError) and DumpStatus()".

[David Heffernan 2345]

1 hour ago, David Schwartz said:

Rudy said: I think this order is guaranteed, but I would not really count on it.

There's a compiler switch that overrides this, so it's not really "guaranteed".

 

The point about the order was to be sure that the condition was evaluated. Consider the two settings for the switch:

 

1. Left to right, condition is first sub expression, therefore it is evaluated.

2. Complete eval, all sub expressions are evaluated, therefore it is evaluated.

 

QED

[Rudy Velthuis 91]

5 hours ago, David Heffernan said:

You can count on it. http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Expressions_(Delphi)#Complete_Versus_Short-Circuit_Boolean_Evaluation

 

Very surprised you don't know this. 

I wrote"I think" but was not 100% sure anymore and hadn't looked it up. Your link confirms this:

Quote

Short-circuit evaluation means strict left-to-right evaluation that stops as soon as the result of the entire expression is determined

That is indeed what I thought to remember. It was pretty late though...

Edited February 11, 2019 by Rudy Velthuis