v[f(x)]+=j;

[pupillo0]

Hi, v is an array; in v[f(x)]+=j; will f(x) be evaluated once or twice (one for reading and one for writing)? I guess once (at least due to optimization) but I'd like to know if C language ensures that it will be once and not twice. Thanks Tom

[Stefan_Lang]

A good question, and one that yould be tricky to answer under different circumstances*. But for your purposes it's safe to answer that any of the compound assignment statements evaluate the adress of the modified value just once! *: it would be tricky indeed if f() could modify v! ;)

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

[David Crow]

I'm inclined to think that it took you longer to type your question than it would have to just compile that piece of code and find out.

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles

[pupillo0]

I must write portable code (x86, ARM etc.....) and I was interested in understanding what C rules exactly say.

[David Crow]

It's going to depend on the particular compiler's implementation (of sequence points).

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles

[jschell]

Per the following it suggests that there is an implicit requirement in C that it must be so. http://en.wikipedia.org/wiki/Sequence_point#cite_note-6[^] That said of course the simple way to insure it is to rewrite the code to insure it.

[pupillo0]

Perhaps, I found out the right answer, form the C99 standard, about compound ass.: Semantics 3 A compound assignment of the form E1 op= E2 differs from the simple assignment expression E1 = E1 op (E2) only in that the lvalue E1 is evaluated only once. So E1 is evaluated ONCE

[jschell]

No that isn't what that means. E1 appears on both side of the equals. It is referring to whether E1 is evaluated once for the lvalue side and once for the rvalue side. And it says no.