Since the associativity of '?' is from right to left,any 2 consecutive '?' operators must be treated as such,Right?
Now,
int x=-1;
int y=x?x++?x:-1:1;
I expect this to be executed as:
int y = x ? (x++?x:-1) : 1;
Now since its being executed from right to left,when encountering the first '?' in the statement,x's value is 0 and the expression is as
int y= x? 0 : 1;
hence i expected y to be 1,but it shows Zero on my dev-cpp.Where am i wrong?
You have the order of evaluation wrong. In a ? b : c
a ? b : c
, a
is always evaluated first, then either b
or c
is evaluated.
I've marked up your example so that I can identify subexpressions:
c
int y=x?x++?x:-1:1;
a bbbbbbbb
(a) is evaluated, yielding -1, so (b) is evaluated. There, x++
is evaluated, yielding -1 again, so (c) is evaluated. At this point, x
is 0.
Or, with more verbose, clearer code, it's as if you said:
int x = -1;
int y;
if (x != 0)
{
int new_x = x + 1;
if (x != 0)
{
y = new_x;
}
else
{
y = -1;
}
}
else
{
y = 1;
}
Operations:
Assign y to value =
if(x): --> x = -1, so true as it is non-zero
{
if(x): --> x = -1 ,so true as x will increment later due to post increment
x= x +1; --> increment x, so x = 0 . This is the value assigned. So y = 0;
else:
-1
}
else:
{
1
}
Hope this helps!
The answer to your question is that in C/C++ int y = x ? (x++?x:-1) : 1;
int y = x ? (x++?x:-1) : 1;
we will hit two sequence points at ?
. Any update operations to variable with in a sequence point will be effective after that sequence is over. So lets look at our example in hand.
First sequence point is first ?
from left.
x=-1; (Actual Value)
x=-1; (Value used in expression)
y=-1?(x++?x:-1):1;
Second sequence point is second ?
from left. As mentioned above the update operations are effective after sequence so even though x++
is there the value used in this sequence is -1
and updated value will be used in following.
x=0; (Actual Value, bcoz of x++)
x=-1; (Value used in expression)
y=-1?x:-1;
Now it will be
x=0; (Actual Value)
x=0; (Value used in expression)
y=x;
y=0;
Hope this make sense now.
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