Round a double to 3 decimalplaces

I am trying to round a double to 3 decimalplaces like this example.
The output will be 12.345 but when I am compiling the program it says:
´floor´: identifier not found.

Perheps I have to #include a class for this and if, which one can be used for this ?

Code:

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double d = 12.3446;
d = floor(d * 1000.0 + 0.5 ) / 1000.0;

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I'm sure you have to use #include <iomanip>. In the cout statement when displaying the output, you should have something like this (I'm using dev- C++):
cout << fixed << showpoint << setprecision(3) << d;

Or if you want the value itself to be changed, you can include <cmath>.

The <cmath> did work for me... thanks a lot...

..........
coding