C++ How to output number with at least one number behind the decimal mark

C++ How to output number with at least one number behind the decimal mark

How can I make my program output a number with at least one number behind
the decimal mark C++? Output: 1 = 1.0 or 1.25 = 1.25 or 2.2 = 2.2 or
3.456789 = 3.456789
Thanks in advance