C++ String to float with precision

Question

I have a file with number readings (example 5.513208E-05 / 1.146383E-05) I read the file and store the entries in a temporary string. After that I convert the temporary string into float variable (which I store in an Multi Dimensional Array). I use the code below to convert.

getline(infile, temporary_string, ',');

Array[i][0] = ::atof(temporary_string.c_str());

getline(infile, temporary_string);

Array[i][1] = ::atof(temporary_string.c_str());

The problem is that when I print the floats to the screen

5.51321e-05 1.14638e-05 instead of 5.513208E-05 1.146383E-05

How can I get the precise numbers stored ???

What's the variable type of Array? If it's float, try changing it to double. — fuenfundachtzig
– fuenfundachtzig, Commented Jul 4, 2013 at 12:31
Ehm, float will give you approximately log10(2^23) digits, which is "6.92", so between 6 and 7 digits. — Mats Petersson
– Mats Petersson, Commented Jul 4, 2013 at 12:34
follow this link may help you stackoverflow.com/questions/10605653/… — beyrem
– beyrem, Commented Jul 4, 2013 at 12:35
Floats are not precise. Six significant figures of precision is about normal for float on a 32-bit system. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jul 4, 2013 at 12:39

Some programmer dude · Accepted Answer · 2013-07-04 12:33:24Z

4

You don't specify precision when you read or convert the string. Instead you set the precision when you output the value:

std::cout << std::setprecision(3) << 1.2345 << '\n';

The above will produce the following oputput:

1.23

See e.g. this reference.

answered Jul 4, 2013 at 12:33

Some programmer dude

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1 Comment

ZeW Over a year ago

Thank you ... the precision was fine, it was the cout that was rounding it up

Joop Eggen · Accepted Answer · 2013-07-04 12:36:42Z

0

Ensure you have double Array[][], not float. A text presentation (base 10) is always approximated by the binary floating point number (base 2), but with luck approximated number of atof has the same presentation, when using the same format. In general one does not do much calculation, and on output uses a reduced precision with setprecision or formats.

answered Jul 4, 2013 at 12:36

Joop Eggen

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Comments

Paulo1205 · Accepted Answer · 2013-07-04 14:15:48Z

Every floating-point representation of numbers has limited precision. In particular, float has 24 bits (1 fixed+23 variable) for its binary mantissa, thus implying a precision of roughly seven decimal digits.

If you need more precision for the stored number, you may wish to consider using double instead of float. On normal PCs, double has 53 bits (1+52) for the binary mantissa, thus allowing a 15-decimal digit precision.

But remember that there's also a problem when those numbers are output. I think the default precision for both printf() and std::ostream is only 6 digits, both for float and for double, unless you specify otherwise. There is no point, however, in demanding a higher precision during output than what the data type provides. So, even though you can say printf("%0.30g", some_float), the extra 23 digits beyond the seven actually supported by the data type might not really produce useful information.

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