Error trying to initialize a char variable

[tfurnivall]

My jaw hit the ground when I got this error message out of what I thought was pretty standard Pascal:

Code: Pascal  [Select][+][-]

1. var
2.  
3. ch : char;

and then in the actual code area:

Code: Pascal  [Select][+][-]

1. ch := '';

I get an error "Got Constant String expected Char

When I first encountered Pascal, back in the 70s, I learnt that the way to initialize a char variable to 'nothing' was to use the form I gave above, assigning it to an empty string constant.
The guidance (from Free Pascal and Lazarus {translated from the Russian}) is:

The values of character variables and constants must be enclosed in single
quotation marks, such as: 'a', 'b', '+', in source code.

So, faced with a grumpy compiler, I went back and defined a new variable:

Code: Pascal  [Select][+][-]

1. st : string;

at which point

Code: Pascal  [Select][+][-]

1. st := '';

worked just fine.

Sooooo..

if a String is really a Packed array of charcters and char is a single character, and we can index into a string and bring out a character, then why can't we just assign the '' value to the char variable?

I could construct an empty string (st := ''; ) but then it wouldn't have any characters, so I can't get at them via the indexing method. The same goes for adding any number of empty strings - they don't exist so I can't access them. I get that.

If you have a character variable that you want to initialize, how do you do it?

Confused, cranky and curmudgeonly,

Tony

[cdbc]

Hi

Code: Pascal  [Select][+][-]

1. Ch:= #0; // or
2. Ch:= ' '; {space NOT empty}

A char cannot be empty like a string, but #0 is damn close...
Regards Benny

[440bx]

It all sounds very reasonable but think of it this way...

if instead of a char variable you had an integer variable, would you expect it possible to "initialize" the integer variable with an "empty" integer ?

Probably not, because there is no such thing as an "empty" integer.  It's the same with a char variable, there is no such thing as an "empty" character.

However, there is such a thing as an empty string.  That's why assigning '' to a string works (as it should) and assigning '' to a char does _not_ work (as it should too.)

In the case of a string assigning it the value '', depending on the type of string (longstring or shortstring) causes the value assigned to be nil (in the case of longstring) or #0 in the case of shortstring.  IOW, a value is being assigned, the "empty string" isn't an "empty" variable.

HTH.

[Fibonacci]

String is not a simple type, its a structure (record). Its never "empty", you just set one of the fields (length) to 0. String is actually a pointer to this structure. Char on the other hand is a simple type and you access it directly, the value must have the size of the type, which is 1 byte.

[Khrys]

I think part of the confusion stems from the fact that both character and string literals use the same delimiters. Consider the following:

Code: Pascal  [Select][+][-]

1. Foo: Char = 'A';
2. Bar: String = 'B';
3. //            ^
4. // single quotes in Pascal

Code: C  [Select][+][-]

1. char foo = 'a';
2. char *bar = "B";
3. //          ^
4. // double quotes in C

With that being said, there are a few other ways of representing single characters. Here are all the representations of  'Hello' + LineEnding + 'World!'  I can think of (where  LineEnding  is  \r\n  aka CRLF, common on Windows):

Code: Pascal  [Select][+][-]

1. A = 'Hello'#13#10'World!';       // Decimal char codes
2. B = 'Hello'#$D#$A'World!';       // Hexadecimal char codes
3. C = 'Hello'#&15#&12'World!';     // Octal char codes
4. D = 'Hello'#%1101#%1010'World!'; // Binary char codes
5. E = 'Hello'^M^J'World!';         // Control codes (e.g. git diff uses this format)

[Thaddy]

Benny is right: a (Ansi) char can not be empty, must lie between #0 and #255.
#0 is best.

[Zvoni]

Benny is right: a (Ansi) char can not be empty, must lie between #0 and #255.
#0 is best.

Which in Layman's terms means: Char is basically a TypeDef for a Byte
https://www.freepascal.org/docs-html/rtl/system/char.html

[Thaddy]

No, not exactly, only when you mimic a C style char *. But it is a fair comparison never the less.

Pascal char and byte are distinct types as opposed to C.

[Remy Lebeau]

String is not a simple type, its a structure (record). Its never "empty", you just set one of the fields (length) to 0. String is actually a pointer to this structure.

Mostly true, however an "empty" String actually uses a nil pointer, so there is no record stored. Thus, the record's length field is never 0.  When you call Length() on a String, it returns 0 if the pointer is nil, otherwise it returns the record's length field.

Char on the other hand is a simple type and you access it directly, the value must have the size of the type, which is 1 byte.

Or 2 bytes under {$MODESWITCH UNICODESTRINGS}/{$MODE DelphiUnicode}.

[silvercoder70]

just to add to the replies...

- consider char and integer as simple types then neither of these have no value.
- a char variable always holds a value — even if you don't explicitly set it, it will still contain some default value (often whatever is in memory at that location unless it's initialized)

lastly, strings are a bit more than packed arrays (and is a  managed type). I don't know the page number(s), but if you have a look at the Object Pascal book by Marco Canto (spelling?) you can read all about it there.