In c, you can have code like this:

static void wtest( void ) {
  int f;
  while ( ( f = some_val( ) ) ) {
    printf(" our value is now: %d\n", f );
  }
}

gcc compiles this without warning or error (at least if you use the double parentheses to assure the compiler that you realize you are testing an assignment, not a comparison).

I would like to do the same thing in d, something like this:

private void wtest( ) {
  int f;
  while ( ( f = some_val( ) ) ) {
    writeln(" our value is now: ", f );
  }
}

or even better:

private void wtest( ) {
  while ( ( auto f = some_val( ) ) ) {
    writeln(" our value is now: ", f );
  }
}

This however does not work, and the gdc compiler says "assignment cannot be used as a condition, perhaps == was meant?"

I don't absolutely have to do it this way, as i guess i could do 'while (true) {...' and then break if the assignment returns zero.

But i really, really would like to use the idiom of assigning a value from inside a while condition.

Is it possible to do this?  Perhaps with extra braces or something, like while ( {something here} ) { .... } ?

TIA for any pointers or advice.

dan

On 03/12/2016 9:55 PM, dan wrote:
> In c, you can have code like this:
>
> static void wtest( void ) {
>   int f;
>   while ( ( f = some_val( ) ) ) {
>     printf(" our value is now: %d\n", f );
>   }
> }
>
> gcc compiles this without warning or error (at least if you use the
> double parentheses to assure the compiler that you realize you are
> testing an assignment, not a comparison).
>
> I would like to do the same thing in d, something like this:
>
> private void wtest( ) {
>   int f;
>   while ( ( f = some_val( ) ) ) {
>     writeln(" our value is now: ", f );
>   }
> }
>
> or even better:
>
> private void wtest( ) {
>   while ( ( auto f = some_val( ) ) ) {
>     writeln(" our value is now: ", f );
>   }
> }
>
> This however does not work, and the gdc compiler says "assignment cannot
> be used as a condition, perhaps == was meant?"
>
> I don't absolutely have to do it this way, as i guess i could do 'while
> (true) {...' and then break if the assignment returns zero.
>
> But i really, really would like to use the idiom of assigning a value
> from inside a while condition.
>
> Is it possible to do this?  Perhaps with extra braces or something, like
> while ( {something here} ) { .... } ?
>
> TIA for any pointers or advice.
>
> dan

If you can use another compiler do so, gdc is on an old frontend/Phobos now. I recommend ldc or you know the reference compiler dmd if performance/platform isn't an issue (not that dmd can't produce decent codegen).

This does compile:

int func() {
	return 0;	
}

void main() {
	int x;
	while((x = func()) != 0) {
			
	}
}

On Saturday, 3 December 2016 at 09:03:25 UTC, rikki cattermole wrote:
> On 03/12/2016 9:55 PM, dan wrote:
>> [...]
>
> If you can use another compiler do so, gdc is on an old frontend/Phobos now. I recommend ldc or you know the reference compiler dmd if performance/platform isn't an issue (not that dmd can't produce decent codegen).
>
> This does compile:
>
> int func() {
> 	return 0;	
> }
>
> void main() {
> 	int x;
> 	while((x = func()) != 0) {
> 			
> 	}
> }

Thanks Rikki, that works great and is nearly ideal (doesn't seem to allow 'auto' but probably that's some scoping issue).

I do prefer gdc because it is gpl'ed, but appreciate any suggestions.

Thanks again for your help!

dan

On 04/12/2016 7:26 AM, dan wrote:
> On Saturday, 3 December 2016 at 09:03:25 UTC, rikki cattermole wrote:
>> On 03/12/2016 9:55 PM, dan wrote:
>>> [...]
>>
>> If you can use another compiler do so, gdc is on an old
>> frontend/Phobos now. I recommend ldc or you know the reference
>> compiler dmd if performance/platform isn't an issue (not that dmd
>> can't produce decent codegen).
>>
>> This does compile:
>>
>> int func() {
>>     return 0;
>> }
>>
>> void main() {
>>     int x;
>>     while((x = func()) != 0) {
>>
>>     }
>> }
>
> Thanks Rikki, that works great and is nearly ideal (doesn't seem to
> allow 'auto' but probably that's some scoping issue).
>
> I do prefer gdc because it is gpl'ed, but appreciate any suggestions.
>
> Thanks again for your help!
>
> dan

The only thing in gdc that is GPL is the backend and glue layer.
The frontend is the same one in ldc and dmd.