Richard F wrote:
Hi
Call me a newbie, but is there anything wrong with the following statement?
rtc.time_element.tm_wday = (rtc.time_element.tm_wday < 6) ?
rtc.time_element.tm_wday++ : 0;
I checked K&R and it looks OK to me.
In WINAVR 20081205 the expression rtc.tm_wday++ doesn't appear to be
evaluated.
The assembler looks like:
rtc.time_element.tm_wday = (rtc.time_element.tm_wday < 6) ?
rtc.time_element.tm_wday++ : 0;
1ca: 80 91 77 01 lds r24, 0x0177
1ce: 86 30 cpi r24, 0x06 ; 6
1d0: 08 f0 brcs .+2 ; 0x1d4 <rt_clock+0x42>
1d2: 80 e0 ldi r24, 0x00 ; 0
1d4: 80 93 77 01 sts 0x0177, r24
Thanks
The generated assembly is correct - your statement is wrong (as well as
being amazingly bad style - mixing side effects, assignments and the
conditional operator).
If tm_wday < 6, then you are asking the compiler to make a temporary
copy of tm_wday, then increment it, then assign the temporary copy
(pre-increment) to tm_wday. Clearly, the compiler doesn't have to
bother with the increment.
My advice, whether you are a newbie or not, is to avoid the ?: operator
unless you have very specific reasons for using it - an explicit if ()
{} else {} is almost always clearer. Only use the ?: if the code really
is clearer and neater in that form.
And never use assignment operators (like ++ or +=) inside an assignment
- your statement will probably be wrong, possibly undefined, and
certainly confusing.
mvh.,
David
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