C Problem?

>Keil does have some problems with the pre- and
>post- increment/decrement operators when used
>within a more complicated expression

Which problems your take in mind ?



>(x++)<<3
>is equivalent to

>(x)<<3; x += 1
>and it seems that MSVC has the "right" >answer.


This is not exactly the same as original qiestion:

unsigned char x = 1;
x=(x++) << 3;

is equivalent to

unsigned char x = 1;
unsigned char tmp;

tmp = x;          // get old value of 'x'
x   = x + 1;      // x++
x   = (tmp << 3); // Assign('=') have lowest priority

So, right answer is 'x==8' after all.

>Keil does have some problems with the pre- and
>post- increment/decrement operators when used
>within a more complicated expression

Which problems your take in mind ?



>(x++)<<3
>is equivalent to

>(x)<<3; x += 1
>and it seems that MSVC has the "right" >answer.


This is not exactly the same as original qiestion:

unsigned char x = 1;
x=(x++) << 3;

is equivalent to

unsigned char x = 1;
unsigned char tmp;

tmp = x;          // get old value of 'x'
x   = x + 1;      // x++
x   = (tmp << 3); // Assign('=') have lowest priority

So, right answer is 'x==8' after all.

Please see

http://www.eskimo.com/~scs/C-faq/q3.3.html

x = (x++) << 3;

not only are the parens useless since ++ is much higher precedence than << but the resultant value assigned to x is up to the implementation (compiler) you are using. This sort of equation should be written as

x = x + 1 << 3;

Let the compiler figure out that +1 means INC.

Regards,

- Mark

Mark,
thanks for good point:
http://www.eskimo.com/~scs/C-faq/q3.3.html

http://www.eskimo.com/~scs/C-faq/top.html

It is a good place to understand why I intuitively avoid some
ambiguous C-constructions :)

On the other side, this is a 'style' quiestion.
For example, in your case I always write as

x = (x + 1) << 3;

despite that "no need" parens,
just to produce evident readability.


But if we return to original question,

unsigned char x = 1;
x = (x++) << 3;

I think that in this case Keil-implementation "more suitable" :),
because MSVC result '9'
(accordingly author's original message)
can be explained only if suppose that 'x++' evaluated after assigning '8' to x.

As I remember, '=' operation in C:

  LVALUE = EXPRESSION

calculated from right to left.

I think, this means that
at first
EXPRESSION should be COMPLETELY evaluated (with all nested operations, EVEN side-effect operations such as function calls),
then
result of expression saved to place
pointed by LVALUE.


This is just my common-sense opinion :)

unsigned char x = 1;
x = (x++) << 3;

"MSVC result '9' ... can be explained only if suppose that 'x++' evaluated after assigning '8' to x."

Which is what I'd expect from a post increment:

"x++" means "take the value of x and then, when you've done with it, increment x by one"

In this case, "when you've done with it" means, "when you've shifted it"

However, I agree with the consensus that this is ambiguous and best avoided!