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digitalmars.D - bug in grammar with ?:

reply Brett <Brett gmail.com> writes:
Fails(loop is not taken):

for(ulong i = len - 1; i >= (true) ? 0 : 1; i--)


passes

for(ulong i = len - 1; i >= ((true) ? 0 : 1); i--)

Only difference is parenthesis(yes, that is all, doesn't matter 
if it's true or false)
Sep 17 2019
next sibling parent reply ketmar <ketmar ketmar.no-ip.org> writes:
Brett wrote:


 Fails(loop is not taken):

 for(ulong i = len - 1; i >= (true) ? 0 : 1; i--)


 passes

 for(ulong i = len - 1; i >= ((true) ? 0 : 1); i--)

 Only difference is parenthesis(yes, that is all, doesn't matter if it's 
 true or false)


not a bug. the whole thing before `?` is a condition, so

	i >= (true) ? 0 : 1

returns 0, of course, because `i >= 1` (your `len` is obviously >= 1), and 
the result of the expression is therefor 0.
Sep 17 2019
parent reply Brett <Brett gmail.com> writes:
On Tuesday, 17 September 2019 at 18:06:55 UTC, ketmar wrote:

 Brett wrote:


 Fails(loop is not taken):

 for(ulong i = len - 1; i >= (true) ? 0 : 1; i--)


 passes

 for(ulong i = len - 1; i >= ((true) ? 0 : 1); i--)

 Only difference is parenthesis(yes, that is all, doesn't 
 matter if it's true or false)


 not a bug. the whole thing before `?` is a condition, so

 	i >= (true) ? 0 : 1

 returns 0, of course, because `i >= 1` (your `len` is obviously

= 1), and the result of the expression is therefor 0.




Makes sense, seems bad though... can introduce severe bugs in 
code if allowed. While using ?: there is not such a common idiom, 
using >= (or <= is)

Might be better for the compiler to detect such things and give a 
warning to use parenthesis? It could introduce subtle and hard to 
detect bugs.

Probably 99% of for loops use the syntax of var >= ...

I use it so much that way that I always think of ... as it's own 
expression rather than the entire part being an expression... 
it's my fault but I imagine 95% of programmers also think along 
similar ways. All it takes is forgetting this one time and one 
could have serious bugs(if the condition is complex and resolves 
in a hard to detect way it could produce such cases, rare but 
possible).
Sep 17 2019
next sibling parent Dennis <dkorpel gmail.com> writes:
On Tuesday, 17 September 2019 at 18:16:41 UTC, Brett wrote:

 All it takes is forgetting this one time and one could have 
 serious bugs(if the condition is complex and resolves in a hard 
 to detect way it could produce such cases, rare but possible).


To be honest, your entire loop is one big code smell.

for(ulong i = len - 1; i >= ((true) ? 0 : 1); i--)

if len = 0, i will underflow and be initialized to ulong.max.
i >= 0 is vacuously true for an unsigned integer, resulting in 
and endless loop

I don't know your original situation, but if you can, please use 
foreach or foreach_reverse.
Sep 17 2019
prev sibling parent ketmar <ketmar ketmar.no-ip.org> writes:
Brett wrote:


 Makes sense, seems bad though... can introduce severe bugs in code if 
 allowed. While using ?: there is not such a common idiom, using >= (or <= 
 is)


yeah. that's why ternary is not recommended to use -- it is not always 
obvious what is a condition there. ternary has one of the lowest 
priorities, and it is right-associative too.
Sep 17 2019
prev sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Tuesday, 17 September 2019 at 18:03:20 UTC, Brett wrote:

 for(ulong i = len - 1; i >= (true) ? 0 : 1; i--)
 for(ulong i = len - 1; i >= ((true) ? 0 : 1); i--)


Those are two different things by design. The >= binds more 
tightly than the ?:, it lets you do stuff like

a > 0 ? "greater than" : "not greater than"


Thus the first  one is more like

if(i >= (true)) {
   return 0;
} else {
   return 1; {
}

and the second one is

if(i >= ((true) ? 0 : 1)


Though I personally like to use extra parens though just remember 
that true and (true) are no different, regardless of context - 
those parens are totally redundant..
Sep 17 2019