Copying character to string problem

[Lainkes 0]

Hello,

 

I have this code 

procedure Tfrm_langauge.Button1Click(Sender: TObject);
var
  I: Integer;
  dirtystr, cleanstr : string;
begin
  cleanstr := '';
  dirtystr := '123456';

  for I := 0 to dirtystr.Length - 1 do
    cleanstr := cleanstr + dirtystr[I];
end;

For some reason, the cleanstr string is always empty.

Any idea what is going wrong. I'm sure it's something stupid.

 

The purpose is to remove all characters except numbers for a string. I check the character and if it is a number, I add it to my new string.

When I tested that function, it returned always an empty string.

That's why I did this little test with the above code, with the same result.

Thanks for your help

Lainkes

 

[Uwe Raabe 1898]

Run the for-loop from 1 to length.

[Lainkes 0]

Thanks. This worked.

I thought that a string was an array of characters.

And an array starts at position 0.

[FPiette 367]

24 minutes ago, Lainkes said:

I thought that a string was an array of characters.

Not at all !

[David Heffernan 2308]

56 minutes ago, FPiette said:

Not at all !

In Delphi a string is essentially a 1 based array of char

 

[FPiette 367]

9 minutes ago, David Heffernan said:

1 hour ago, FPiette said:

Not at all !

In Delphi a string is essentially a 1 based array of char

Not really and I know you know it.

It looks much like a 1 based array of char but it isn't at all compiler magic happens here.

A string is a record with data used by the compiler a pointer to the actual characters, a count of characters (length), a reference count and more.

 

[David Heffernan 2308]

7 minutes ago, FPiette said:

Not really and I know you know it.

It looks much like a 1 based array of char but it isn't at all compiler magic happens here.

A string is a record with data used by the compiler a pointer to the actual characters, a count of characters (length), a reference count and more.

 

No, viewing a string as an array of char is pretty reasonable.  I mean your argument could also be used to say that TArray<Integer> is not an array of integer because of the meta data.

 

And when you do PChar(someString) to pass a null-terminated array of char to some external library, the compiler just passes the address of the first element.  (Yes, I know about the empty string special case)

 

 

Edited October 5, 2022 by David Heffernan

[FPiette 367]

13 hours ago, David Heffernan said:

No, viewing a string as an array of char is pretty reasonable.

They key word is "Viewing". That's what I said: compiler magic makes that happens a string is not an array but sometimes looks like an array of char because of the [ ] that is usable. The two types string and array of char are not interchangeable. You will confuse the OP pretending they are the same.

[David Heffernan 2308]

46 minutes ago, FPiette said:

The two types string and array of char are not interchangeable. You will confuse the OP pretending they are the same.

I didn't say they were interchangeable. A Delphi string is an array of elements of type Char. 

 

The documentation says: "A string represents a sequence of characters."

 

Wikipedia defines an array here: https://en.m.wikipedia.org/wiki/Array_(data_structure)

 

So I guess perhaps what may be confusing here is maybe you think I am saying that a Delphi string is a Delphi dynamic array, TArray<Char>. I'm not saying that. I'm using the general computing usage of the word array. 

[FPiette 367]

7 hours ago, David Heffernan said:

I didn't say they were interchangeable. A Delphi string is an array of elements of type Char.

You said: " In Delphi a string is essentially a 1 based array of char".

No matter how you try to say it is it doesn't make it happens. A Delphi string is NOT an array of elements of type char. If the were, they would be interchangeable and you admitted they are not.

[David Heffernan 2308]

30 minutes ago, FPiette said:

No matter how you try to say it is it doesn't make it happens. A Delphi string is NOT an array of elements of type char. If the were, they would be interchangeable and you admitted they are not.

You can understand what I wrote that way if you want. But as I explained I never intended the meaning to be that a string is the same as the Delphi type array of Char.

 

As I explained I intended to use the word array in its general sense. 

 

For instance an array in C# is not assignment compatible with a Delphi array. Does that make a C# array not an array? 

[FPiette 367]

48 minutes ago, David Heffernan said:

You can understand what I wrote that way if you want.

Yes I do.