ifstream / ofstream precision

[karnavor]

Hello,

I'd like to ask two different questions on a related topic please...

1. When I am writing float data to a text file, I want to set the precision. I know that with std::cout, you can write "std::cout.precision(5)" to set the precision to 5 decimal places. But writing "std:fstream.precision(5)" does not work. How can I do this?

2. When I read the file back again, I want to add a load of zeros to each float, after the 5 decimal places. For example, if my data file has the value "1.23456", which has been limited by setting the ofstream precision to 5 decimal places, I want to read in this value again, such that the float variable becomes "1.234560000000.....". When I have tried just reading it in normally using std::ifstream, it in fact sets my flat variable to the value "1.234599999999999....". This may seem a trivial difference, but it is actually crucial for my program.

Thanks for any help!!

[MrViggy]

Floats and doubles are imprecise due to the way that the value is stored in memory. The computer cannot store 1.23456 exactly, which is why you see '1.2345999999999'.

Viggy

[Lindley]

1. When I am writing float data to a text file, I want to set the precision. I know that with std::cout, you can write "std::cout.precision(5)" to set the precision to 5 decimal places. But writing "std:fstream.precision(5)" does not work. How can I do this?

You should be calling precision() on the ostream object that you're trying to write to; it's not a static method, so you can't call it on the ostream class itself.

Also, precision() only sets the number of decimal places if you've set the mode to std::fixed. In the default mode, precision() instead specifies the number of significant digits.