Digit limitation from decimal point in C++

Question

I'm new to C++. I have a double variable double a=0.1239857 and I want to limit variable a from decimal point two digits. So a will be 0.12. I know C++ have functions that return largest or smallest integer that is greater or lower than a like ceil or floor.

Is there a function that implements digit limitation of floating-point variable? Or How can I change precision of the a variable?

Answer 1

Are you actually trying to round the number, or just change its displayed precision?

For the former (truncating the extra digits):

double scale = 0.01;  // i.e. round to nearest one-hundreth value = (int)(value / scale) * scale; 

or (rounding up/down as appropriate, per jheriko's answer)

double scale = 0.01;  // i.e. round to nearest one-hundreth value = floor(value / scale + 0.5) * scale; 

For the latter:

cout << setprecision(2) << value; 

where the parameter to setprecision() is the maximum number of digits to show after the decimal point.

Answer 2

This will result in two digits after the decimal place.

a = floor(a * 100.0) / 100.0;