solved

[ne555]

L B wrote:

you can't put -b. You CAN however do 0-b, which is what you want.

Are you sure that doubles don't have the unary operator - ?

[LB]

They do, but it gives me a compiler error if I try to do x = (-b). I guess I'm doing it wrong.

[Galik]

LB wrote:

They do, but it gives me a compiler error if I try to do x = (-b).

What compiler are you using and what error does it give? That's looks perfectly legal to me.

[LB]

It tells that it expected a 'primary expression' before '-'.

[jumper007]

It tells that it expected a 'primary expression' before '-'.
mate, try with
x==-b
ok ?
this is working in all C++ programming languages. trust me.
and oh, dont use () , and do == instead of =. this works for me. i hope i helped