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:rolleyes:
Does the () take precedence over the pre increment or not?Code:#include <stdio.h>
void main()
{
float a = 4, b = 4, c = 8;
a = (a + c) / ++c;
printf("\n a = %f", a);
}
In some text books the () appears higher up in the table of precedence.
I ran this program in VS 2003 and I got 1.444, which is 13 / 9.
Why don't I get (12) / 9 if the parens come first?
:confused:
It does take higher precedence.
But you have to consider the precedence of all operators involved along with their associativity rules. You have 4 operators in effect on that formula. (), +, / and ++.
So... let's order them by precedence and associativity.
1st ()
2nd +
3rd ++
4th /
The addition operator is moved up because the grouping operator forces evaluation of the expression contained inside. So, the first thing that happens is the sum of a with c.
The pre-increment has higher precedence over the division. As such, it is evaluated first. c is incremented.
Only then is the division made.
> I ran this program in VS 2003 and I got 1.444, which is 13 / 9.
I didn't see this, sorry.
That may come from the fact a, b and c are floats and some manner of rounding is happening. Remember that floats have a very low precision.
Change b and c to int and a to double. the result will have a much higher precision which I believe will be acceptable for your problem
I don't understand. If the first thing that happens is the sum of a with c then a+c should be 12. Instead, a+c is giving 13 which implies that c has been incremented already.Quote:
Originally Posted by Mario F.
What am I missing?
I suspect the answer is that it is undefined, though I am not entirely sure.
If you are using the ++ operator before the variable name, the first thing that happens is the variable is incremented. Which is the exact opposite of using ++ after the variable name, it will increment it after the other operations are preformed.
I've always used ++ after the variable and I have never had any problems with it.
Now when I start thinking using ++ before the variable seems to be very useful in some cases...
> Does the () take precedence over the pre increment or not?
Do not confuse the fact that you have used () with "This is always done first".
> In some text books the () appears higher up in the table of precedence.
Sounds like some other books are wrong then.
> I ran this program in VS 2003 and I got 1.444, which is 13 / 9.
Good for you.
Your expression is undefined, so any answer (or indeed no answer) is entirely plausable.
http://c-faq.com/expr/index.html
NEVER EVER use ++ on a variable, and attempt to use that same variable in another part of the expression. It's undefined - no if's, but's or maybe's.
Also read the part of that FAQ which talks about "sequence points".
Given
f() + g() / h();
No amount of () or operator precedence will tell the compiler which function to call first. Precedence and () only describe how sub-expressions are combined, not evaluated.
It is perfectly valid for a compiler to call f() first.
Oh yeah, the void main is wrong as well.
Imagine it as a series of function calls like this:C++ doesn't specify which function argument will be evaluated first, so for the divide function, you don't know whether the add(a,c) will run first or the increment(c). The result is undefined.Code:a = divide(add(a, c), increment(c));
All the parentheses do is stop it from being this:Code:a = add(a, divide(c, increment(c)));
5275: Told ya!Quote:
Originally Posted by Salem