C/C++ language proposal: Change the 'case expression' from "integral constant-expression" to "integral expression"

**Willem** · Oct 29 '08, 06:45 PM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

Keith Thompson wrote:
) Then programmers will inevitably write
)
) case 'A' ... 'Z':
)
) which is non-portable (under EBCDIC it matches '\' and '}').

In which case, it would be relatively easy to add syntax similar to:

case [A-Z]:

Which would, of course, be portable.

SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

**Nate Eldredge** · Oct 29 '08, 06:55 PM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

"robertwessel2@ yahoo.com" <robertwessel2@ yahoo.comwrites :

On Oct 28, 7:42 pm, JoelKatz <dav...@webmast er.comwrote:

>What will be next? "case >=7:"?

>
>
Frankly I think ranges on the case constant expressions would be a
more useful addition while staying with the basic philosophy of the C
switch statement. IOW, "case 2...5:", or something along those
lines. But still not something I'm loosing sleep over...

GCC provides this as an extension, FWIW. You can write

case 2 ... 5:

The spaces are required to keep the parser from thinking it's some
malformed floating point constant.

**Keith Thompson** · Oct 29 '08, 07:25 PM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

Willem <willem@stack.n lwrites:

Keith Thompson wrote:
) Then programmers will inevitably write
)
) case 'A' ... 'Z':
)
) which is non-portable (under EBCDIC it matches '\' and '}').
>
In which case, it would be relatively easy to add syntax similar to:
>
case [A-Z]:
>
Which would, of course, be portable.

It could be, if you defined it *very* carefully.

Lexically, you have 7 tokens:
case (a keyword)
[ (a punctuator)
A (an identifier)
- (a punctuator)
Z (an identifier)
] (a punctuator)
: (a punctuator)

Do you really intend the identifier A here to refer to the character
'A', ignoring any declared entity called A? Do you really intend to
specify a range of literal character values without using the "'"
symbol? Which characters would be allowed? Does [A-Z] refer only to
the 26 uppercase letters of the Latin alphabet? Could this notation
be used for non-Latin letters? Digits? Punctuation symbols?

Properly defined, it could make it easier to work with Latin letters
-- which is both good and bad, since it would further encourage
programmers to ignore the fact that other alphabets exist. Sometimes
you really do want to determine whether a character matches one of the
26 uppercase Latin letters, but more often what you *really* want is
the locale-sensitive behavior of isupper().

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Willem** · Oct 29 '08, 07:55 PM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

Keith Thompson wrote:
) Willem <willem@stack.n lwrites:
)In which case, it would be relatively easy to add syntax similar to:
)>
) case [A-Z]:
)>
)Which would, of course, be portable.
)
) It could be, if you defined it *very* carefully.
)
) <snip>
)
) Do you really intend the identifier A here to refer to the character
) 'A', ignoring any declared entity called A?

Well, no. That's why I stated 'similar to'. It's just an idea after all.

) programmers to ignore the fact that other alphabets exist. Sometimes
) you really do want to determine whether a character matches one of the
) 26 uppercase Latin letters, but more often what you *really* want is
) the locale-sensitive behavior of isupper().

Well, then how about

case [:upper:]:

Which, to be honest, is half-borrowed from Perl syntax.
What the exact syntax is, isn't really relevant, I guess.

SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

**Eric Sosman** · Oct 29 '08, 08:15 PM

Re: C/C++ language proposal: Change the 'case expression' from "integralc onstant-expression&quot ; to "integr al expression&quot ;

Willem wrote:

[... case ranges for characters ...]
Well, then how about
>
case [:upper:]:
>
Which, to be honest, is half-borrowed from Perl syntax.
What the exact syntax is, isn't really relevant, I guess.

The syntax is less important than getting the semantics
right. One problem is in how to define the character classes;
note that isupper() et al. are locale-dependent, so the set of
characters that satisfy them is hard to compute at compile time.
And there's still the issue of ambiguous branches:

switch ('a') {
case isalpha(ch): ...
case islower(ch): ...
case isxdigit(ch): ...
case !ispunct(ch): ...

"These arguments sound very well, but I can't help
thinking that, if they were reduced to syllogistic form,
they wouldn't hold water." -- R. Murgatroyd

--
Eric.Sosman@sun .com

**Eric Sosman** · Oct 29 '08, 08:15 PM

Re: C/C++ language proposal: Change the 'case expression' from "integralc onstant-expression&quot ; to "integr al expression&quot ;

Eric Sosman wrote:

[...]
And there's still the issue of ambiguous branches:
>
switch ('a') {
case isalpha(ch): ...
case islower(ch): ...
case isxdigit(ch): ...
case !ispunct(ch): ...

Oh, botheration: I botched that one royally, didn't I? The
fragment ought to have been something like

ch = 'a';
switch (0) {
case !isalpha(ch): ...
case !islower(ch): ...
case !isxdigit(ch): ...
case !!ispunct(ch): ...

--
Eric.Sosman@sun .com

**Hallvard B Furuseth** · Oct 29 '08, 08:45 PM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

Keith Thompson writes:

>Frankly I think ranges on the case constant expressions would be a
>more useful addition while staying with the basic philosophy of the C
>switch statement. IOW, "case 2...5:", or something along those
>lines. But still not something I'm loosing sleep over...

>
Then programmers will inevitably write
>
case 'A' ... 'Z':

They do anyway. #define ISUPPER(c) ('A' <= (c) && (c) <= 'Z'). E.g. to
check for the ASCII (or 7-bit Unicode) letters regardless of locale.

which is non-portable (under EBCDIC it matches '\' and '}').

--
Hallvard

**Keith Thompson** · Oct 29 '08, 08:55 PM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

Hallvard B Furuseth <h.b.furuseth@u sit.uio.nowrite s:

Keith Thompson writes:

>>Frankly I think ranges on the case constant expressions would be a
>>more useful addition while staying with the basic philosophy of the C
>>switch statement. IOW, "case 2...5:", or something along those
>>lines. But still not something I'm loosing sleep over...

>>
>Then programmers will inevitably write
>>
> case 'A' ... 'Z':

>
They do anyway. #define ISUPPER(c) ('A' <= (c) && (c) <= 'Z'). E.g. to
check for the ASCII (or 7-bit Unicode) letters regardless of locale.
>

>which is non-portable (under EBCDIC it matches '\' and '}').

Sure, but there's not much that can be done to discourage dumb macros.
Actually, there is: <ctype.halrea dy has the locale-sensitive
isupper() function.

The problem with the proposed "..." notation is that it makes it easy
to do the wrong thing (assuming that the uppercase letters have
contiguous codes and run from 'A' to 'Z') *without* making it any
easier to do the right thing (using isupper() to determine whether a
character is an uppercase letter).

On the other hand, there are certainly times when case ranges would be
handy for numeric ranges, as opposed to character ranges, and using
them wouldn't cause any problems. Other languages do provide similar
constructs. And I suppose compilers could warn about 'A' ... 'Z'.

Caveat: Sometimes ('A' <= c && c <= 'Z') *is* exactly what you want,
if you're writing deliberately non-portable code.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Sjouke Burry** · Oct 29 '08, 09:55 PM

Re: C/C++ language proposal: Change the 'case expression' from "integralc onstant-expression&quot ; to "integr al expression&quot ;

robertwessel2@y ahoo.com wrote:

On Oct 28, 7:42 pm, JoelKatz <dav...@webmast er.comwrote:

>What will be next? "case >=7:"?

>
>
Frankly I think ranges on the case constant expressions would be a
more useful addition while staying with the basic philosophy of the C
switch statement. IOW, "case 2...5:", or something along those
lines. But still not something I'm loosing sleep over...

That was already included in microsoft fortran 5.1 (extension)
in 1990 :) :)

**Keith Thompson** · Oct 29 '08, 10:25 PM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

Sjouke Burry <burrynulnulfou r@ppllaanneett. nnlllwrites:

robertwessel2@y ahoo.com wrote:

>On Oct 28, 7:42 pm, JoelKatz <dav...@webmast er.comwrote:

>>What will be next? "case >=7:"?

>Frankly I think ranges on the case constant expressions would be a
>more useful addition while staying with the basic philosophy of the C
>switch statement. IOW, "case 2...5:", or something along those
>lines. But still not something I'm loosing sleep over...

>
That was already included in microsoft fortran 5.1 (extension)
in 1990 :) :)

Pascal had it long before that, and it's likely that other languages
had it even earlier. There's certainly ample precedent. The question
is whether the benefits would outweigh the disadvantages, especially
considering the ambiguity of ranges like 'A' ... 'Z'.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Hendrik Schober** · Oct 30 '08, 11:15 AM

Re: C/C++ language proposal: Change the 'case expression' from "integralc onstant-expression&quot ; to "integr al expression&quot ;

Keith Thompson wrote:

"robertwessel2@ yahoo.com" <robertwessel2@ yahoo.comwrites :

>On Oct 28, 7:42 pm, JoelKatz <dav...@webmast er.comwrote:

>>What will be next? "case >=7:"?

>Frankly I think ranges on the case constant expressions would be a
>more useful addition while staying with the basic philosophy of the C
>switch statement. IOW, "case 2...5:", or something along those
>lines. But still not something I'm loosing sleep over...

>
Then programmers will inevitably write
>
case 'A' ... 'Z':
>
which is non-portable (under EBCDIC it matches '\' and '}').

And why exactly would that be worse than an 'if'-'else' chain
relying on ASCII?

Schobi

**Pete Becker** · Oct 30 '08, 11:35 AM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

On 2008-10-30 07:10:13 -0400, Hendrik Schober <spamtrap@gmx.d esaid:

Keith Thompson wrote:

>"robertwessel2 @yahoo.com" <robertwessel2@ yahoo.comwrites :

>>On Oct 28, 7:42 pm, JoelKatz <dav...@webmast er.comwrote:
>>>What will be next? "case >=7:"?
>>Frankly I think ranges on the case constant expressions would be a
>>more useful addition while staying with the basic philosophy of the C
>>switch statement. IOW, "case 2...5:", or something along those
>>lines. But still not something I'm loosing sleep over...

>>
>Then programmers will inevitably write
>>
> case 'A' ... 'Z':
>>
>which is non-portable (under EBCDIC it matches '\' and '}').

>
And why exactly would that be worse than an 'if'-'else' chain
relying on ASCII?
>

An if-else chain in source code wouldn't rely on ASCII, but on the
source character set, which would, presumably, be EBCDIC when you're
targeting a machine that uses EBCDIC.

switch(ch)
{
case 'A':
case 'B':
case 'C':
case 'D':
....
}

Each character will be properly encoded, even though there are extra
characters in the middle of the capital letters in EBCDIC.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

**Hendrik Schober** · Oct 30 '08, 12:05 PM

Re: C/C++ language proposal: Change the 'case expression' from "integralc onstant-expression&quot ; to "integr al expression&quot ;

Pete Becker wrote:

On 2008-10-30 07:10:13 -0400, Hendrik Schober <spamtrap@gmx.d esaid:
>

>Keith Thompson wrote:

>>"robertwessel 2@yahoo.com" <robertwessel2@ yahoo.comwrites :
>>>On Oct 28, 7:42 pm, JoelKatz <dav...@webmast er.comwrote:
>>>>What will be next? "case >=7:"?
>>>Frankly I think ranges on the case constant expressions would be a
>>>more useful addition while staying with the basic philosophy of the C
>>>switch statement. IOW, "case 2...5:", or something along those
>>>lines. But still not something I'm loosing sleep over...
>>Then programmers will inevitably write
>>>
>> case 'A' ... 'Z':
>>>
>>which is non-portable (under EBCDIC it matches '\' and '}').

> And why exactly would that be worse than an 'if'-'else' chain
> relying on ASCII?
>>

>
An if-else chain in source code wouldn't rely on ASCII [...]

I'd expect most of the programmers I worked with, in the
absent of the 'switch' syntax proposed above, to write
if( x>='A' && x<='Z' )
which is just as wrong.

So allowing this for #switch' IMO wouldn't make anything
worse.

Schobi

**Keith Thompson** · Oct 30 '08, 03:45 PM

Re: C/C++ language proposal: Change the 'case expression' from "integr al constant-expression&quot ; to "integr al expression&quot ;

Hendrik Schober <spamtrap@gmx.d ewrites:

Keith Thompson wrote:

>"robertwessel2 @yahoo.com" <robertwessel2@ yahoo.comwrites :

>>On Oct 28, 7:42 pm, JoelKatz <dav...@webmast er.comwrote:
>>>What will be next? "case >=7:"?
>>Frankly I think ranges on the case constant expressions would be a
>>more useful addition while staying with the basic philosophy of the C
>>switch statement. IOW, "case 2...5:", or something along those
>>lines. But still not something I'm loosing sleep over...

>Then programmers will inevitably write
> case 'A' ... 'Z':
>which is non-portable (under EBCDIC it matches '\' and '}').

>
And why exactly would that be worse than an 'if'-'else' chain
relying on ASCII?

It wouldn't. The problem (as I think I've already said in this
thread) is that adding this syntax makes it much easier to check for
characters in the range 'A' to 'Z' *without* making it any easier to
check for characters that are uppercase letters.

It's not the least bit difficult to write bad code in C or C++ (this
is cross-posted), even with their current features, but let's not make
it even easier.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Harald van =?UTF-8?b?RMSzaw==?=** · Oct 30 '08, 06:55 PM

Re: C/C++ language proposal: Change the 'case expression' from"integ ral constant-expression&quot ; to "integr al expression&quot ;

On Wed, 29 Oct 2008 13:45:46 -0700, Keith Thompson wrote:

Caveat: Sometimes ('A' <= c && c <= 'Z') *is* exactly what you want, if
you're writing deliberately non-portable code.

It doesn't need to be *deliberately* non-portable. If you were
implementing your own C library, on an ASCII-based machine with minimal
locale support, this could be the best way to write isupper.

C/C++ language proposal: Change the 'case expression' from "integral constant-expression" to "integral expression"

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