How do I print a double value with full precision using cout?

Question

In my earlier question I was printing a double using cout that got rounded when I wasn't expecting it. How can I make cout print a double using full precision?

Answer 1

In C++20 you can use std::format to do this:

std::cout << std::format("{}", std::numbers::pi_v<double>);

Output (assuming IEEE 754 double):

3.141592653589793

The default floating-point format is the shortest decimal representation with a round-trip guarantee. The advantage of this method compared to the setprecision I/O manipulator is that it doesn't print unnecessary digits and is not affected by global state (see this blog post for more details).

In the meantime you can use the {fmt} library, std::format is based on. {fmt} also provides the print function that makes this even easier and more efficient (godbolt):

fmt::print("{}", M_PI);

The question doesn’t actually define what it means by "full precision". Normally it is understood as the precision enough for a round trip through decimal but there is another possible but unlikely interpretation of the maximum number of (significant) decimal digits. For IEEE 754 double the latter is 767 digits.

Disclaimer: I'm the author of {fmt} and C++20 std::format.

Answer 2

Use std::setprecision:

#include <iomanip>
std::cout << std::setprecision (15) << 3.14159265358979 << std::endl;

Answer 3

Here is what I would use:

std::cout << std::setprecision (std::numeric_limits<double>::digits10 + 1)
          << 3.14159265358979
          << std::endl;

Basically the limits package has traits for all the build in types.
One of the traits for floating point numbers (float/double/long double) is the digits10 attribute. This defines the accuracy (I forget the exact terminology) of a floating point number in base 10.

See: http://www.cplusplus.com/reference/std/limits/numeric_limits.html
For details about other attributes.

Answer 4

How do I print a double value with full precision using cout?

Use hexfloat or
use scientific and set the precision

std::cout.precision(std::numeric_limits<double>::max_digits10 - 1);
std::cout << std::scientific <<  1.0/7.0 << '\n';

// C++11 Typical output
1.4285714285714285e-01

---

Too many answers address only one of 1) base 2) fixed/scientific layout or 3) precision. Too many answers with precision do not provide the proper value needed. Hence this answer to a old question.

1. What base?

A double is certainly encoded using base 2. A direct approach with C++11 is to print using std::hexfloat.
If a non-decimal output is acceptable, we are done.

std::cout << "hexfloat: " << std::hexfloat << exp (-100) << '\n';
std::cout << "hexfloat: " << std::hexfloat << exp (+100) << '\n';
// output
hexfloat: 0x1.a8c1f14e2af5dp-145
hexfloat: 0x1.3494a9b171bf5p+144

---

2. Otherwise: fixed or scientific?

A double is a floating point type, not fixed point.

Do not use std::fixed as that fails to print small double as anything but 0.000...000. For large double, it prints many digits, perhaps hundreds of questionable informativeness.

std::cout << "std::fixed: " << std::fixed << exp (-100) << '\n';
std::cout << "std::fixed: " << std::fixed << exp (+100) << '\n';
// output
std::fixed: 0.000000
std::fixed: 26881171418161356094253400435962903554686976.000000 

To print with full precision, first use std::scientific which will "write floating-point values in scientific notation". Notice the default of 6 digits after the decimal point, an insufficient amount, is handled in the next point.

std::cout << "std::scientific: " << std::scientific << exp (-100) << '\n';  
std::cout << "std::scientific: " << std::scientific << exp (+100) << '\n';
// output
std::scientific: 3.720076e-44
std::scientific: 2.688117e+43

---

3. How much precision (how many total digits)?

A double encoded using the binary base 2 encodes the same precision between various powers of 2. This is often 53 bits.

[1.0...2.0) there are 2⁵³ different double,
[2.0...4.0) there are 2⁵³ different double,
[4.0...8.0) there are 2⁵³ different double,
[8.0...10.0) there are 2/8 * 2⁵³ different double.

Yet if code prints in decimal with N significant digits, the number of combinations [1.0...10.0) is 9/10 * 10^N.

Whatever N (precision) is chosen, there will not be a one-to-one mapping between double and decimal text. If a fixed N is chosen, sometimes it will be slightly more or less than truly needed for certain double values. We could error on too few (a) below) or too many (b) below).

3 candidate N:

a) Use an N so when converting from text-double-text we arrive at the same text for all double.

std::cout << dbl::digits10 << '\n';
// Typical output
15

b) Use an N so when converting from double-text-double we arrive at the same double for all double.

// C++11
std::cout << dbl::max_digits10 << '\n';
// Typical output
17

When max_digits10 is not available, note that due to base 2 and base 10 attributes, digits10 + 2 <= max_digits10 <= digits10 + 3, we can use digits10 + 3 to insure enough decimal digits are printed.

c) Use an N that varies with the value.

This can be useful when code wants to display minimal text (N == 1) or the exact value of a double (N == 1000-ish in the case of denorm_min). Yet since this is "work" and not likely OP's goal, it will be set aside.

---

It is usually b) that is used to "print a double value with full precision". Some applications may prefer a) to error on not providing too much information.

With .scientific, .precision() sets the number of digits to print after the decimal point, so 1 + .precision() digits are printed. Code needs max_digits10 total digits so .precision() is called with a max_digits10 - 1.

typedef std::numeric_limits< double > dbl;
std::cout.precision(dbl::max_digits10 - 1);
std::cout << std::scientific <<  exp (-100) << '\n';
std::cout << std::scientific <<  exp (+100) << '\n';
// Typical output
3.7200759760208361e-44
2.6881171418161356e+43
//2345678901234567  17 total digits

Similar C question

Answer 5

The iostreams way is kind of clunky. I prefer using boost::lexical_cast because it calculates the right precision for me. And it's fast, too.

#include <string>
#include <boost/lexical_cast.hpp>

using boost::lexical_cast;
using std::string;

double d = 3.14159265358979;
cout << "Pi: " << lexical_cast<string>(d) << endl;

Output:

Pi: 3.14159265358979

Answer 6

By full precision, I assume mean enough precision to show the best approximation to the intended value, but it should be pointed out that double is stored using base 2 representation and base 2 can't represent something as trivial as 1.1 exactly. The only way to get the full-full precision of the actual double (with NO ROUND OFF ERROR) is to print out the binary bits (or hex nybbles).

One way of doing that is using a union to type-pun the double to a integer and then printing the integer, since integers do not suffer from truncation or round-off issues. (Type punning like this is not supported by the C++ standard, but it is supported in C. However, most C++ compilers will probably print out the correct value anyways. I think g++ supports this.)

union {
    double d;
    uint64_t u64;
} x;
x.d = 1.1;
std::cout << std::hex << x.u64;

This will give you the 100% accurate precision of the double... and be utterly unreadable because humans can't read IEEE double format ! Wikipedia has a good write up on how to interpret the binary bits.

In newer C++, you can do

std::cout << std::hexfloat << 1.1;

Answer 7

Here is how to display a double with full precision:

double d = 100.0000000000005;
int precision = std::numeric_limits<double>::max_digits10;
std::cout << std::setprecision(precision) << d << std::endl;

This displays:

100.0000000000005

max_digits10 is the number of digits that are necessary to uniquely represent all distinct double values. max_digits10 represents the number of digits before and after the decimal point.

Don't use set_precision(max_digits10) with std::fixed.
On fixed notation, set_precision() sets the number of digits only after the decimal point. This is incorrect as max_digits10 represents the number of digits before and after the decimal point.

double d = 100.0000000000005;
int precision = std::numeric_limits<double>::max_digits10;
std::cout << std::fixed << std::setprecision(precision) << d << std::endl;

This displays incorrect result:

100.00000000000049738

Note: Header files required

#include <iomanip>
#include <limits>

Answer 8

C++20 std::format

This great new C++ library feature has the advantage of not affecting the state of std::cout as std::setprecision does:

#include <format>
#include <string>

int main() {
    std::cout << std::format("{:.2} {:.3}\n", 3.1415, 3.1415);
}

Expected output:

3.14 3.142

As mentioned at https://stackoverflow.com/a/65329803/895245 if you don't pass the precision explicitly it prints the shortest decimal representation with a round-trip guarantee. TODO understand in more detail how it compares to: dbl::max_digits10 as shown at https://stackoverflow.com/a/554134/895245 with {:.{}}:

#include <format>
#include <limits>
#include <string>

int main() {
    std::cout << std::format("{:.{}}\n",
        3.1415926535897932384626433, dbl::max_digits10);
}

See also:

• Set back default floating point print precision in C++ for how to restore the initial precision in pre-c++20
• std::string formatting like sprintf
• https://en.cppreference.com/w/cpp/utility/format/formatter#Standard_format_specification

Answer 9

IEEE 754 floating point values are stored using base 2 representation. Any base 2 number can be represented as a decimal (base 10) to full precision. None of the proposed answers, however, do. They all truncate the decimal value.

This seems to be due to a misinterpretation of what std::numeric_limits<T>::max_digits10 represents:

The value of std::numeric_limits<T>::max_digits10 is the number of base-10 digits that are necessary to uniquely represent all distinct values of the type T.

In other words: It's the (worst-case) number of digits required to output if you want to roundtrip from binary to decimal to binary, without losing any information. If you output at least max_digits10 decimals and reconstruct a floating point value, you are guaranteed to get the exact same binary representation you started with.

What's important: max_digits10 in general neither yields the shortest decimal, nor is it sufficient to represent the full precision. I'm not aware of a constant in the C++ Standard Library that encodes the maximum number of decimal digits required to contain the full precision of a floating point value. I believe it's something like 767 for doubles¹. One way to output a floating point value with full precision would be to use a sufficiently large value for the precision, like so², and have the library strip any trailing zeros:

#include <iostream>

int main() {
    double d = 0.1;
    std::cout.precision(767);
    std::cout << "d = " << d << std::endl;
}

This produces the following output, that contains the full precision:

d = 0.1000000000000000055511151231257827021181583404541015625

Note that this has significantly more decimals than max_digits10 would suggest.

---

While that answers the question that was asked, a far more common goal would be to get the shortest decimal representation of any given floating point value, that retains all information. Again, I'm not aware of any way to instruct the Standard I/O library to output that value. Starting with C++17 the possibility to do that conversion has finally arrived in C++ in the form of std::to_chars. By default, it produces the shortest decimal representation of any given floating point value that retains the entire information.

Its interface is a bit clunky, and you'd probably want to wrap this up into a function template that returns something you can output to std::cout (like a std::string), e.g.

#include <charconv>
#include <array>
#include <string>
#include <system_error>

#include <iostream>
#include <cmath>

template<typename T>
std::string to_string(T value)
{
    // 24 characters is the longest decimal representation of any double value
    std::array<char, 24> buffer {};
    auto const res { std::to_chars(buffer.data(), buffer.data() + buffer.size(), value) };
    if (res.ec == std::errc {})
    {
        // Success
        return std::string(buffer.data(), res.ptr);
    }

    // Error
    return { "FAILED!" };
}

int main()
{
    auto value { 0.1f };
    std::cout << to_string(value) << std::endl;
    value = std::nextafter(value, INFINITY);
    std::cout << to_string(value) << std::endl;
    value = std::nextafter(value, INFINITY);
    std::cout << to_string(value) << std::endl;
}

This would print out (using Microsoft's C++ Standard Library):

0.1
0.10000001
0.10000002

---

¹ From Stephan T. Lavavej's CppCon 2019 talk titled Floating-Point <charconv>: Making Your Code 10x Faster With C++17's Final Boss. (The entire talk is worth watching.)

² This would also require using a combination of scientific and fixed, whichever is shorter. I'm not aware of a way to set this mode using the C++ Standard I/O library.

Answer 10

printf("%.12f", M_PI);

%.12f means floating point, with precision of 12 digits.

Answer 11

In this question there is a description on how to convert a double to string losselessly (in Octave, but it can be easily reproduced in C++). De idea is to have a short human readable description of the float and a losseless description in hexa form, for instance: pi -> 3.14{54442d18400921fb}.

Answer 12

The best option is to use std::setprecision, and the solution works like this:

# include <iostream>
# include <iomanip>

int main()
{
    double a = 34.34322;
    std::cout<<std::fixed<<a<<std::setprecision(0)<<std::endl;
    return 0;
}

Note: you do not need to use cout.setprecision to do it and I fill up 0 at std::setprecision because it must have a argument.

Answer 13

Here is a function that works for any floating-point type, not just double, and also puts the stream back the way it was found afterwards. Unfortunately it won't interact well with threads, but that's the nature of iostreams. You'll need these includes at the start of your file:

#include <limits>
#include <iostream>

Here's the function, you could it in a header file if you use it a lot:

template <class T>
void printVal(std::ostream& os, T val)
{
    auto oldFlags = os.flags();
    auto oldPrecision = os.precision();

    os.flags(oldFlags & ~std::ios_base::floatfield);
    os.precision(std::numeric_limits<T>::digits10);
    os << val;
    
    os.flags(oldFlags);
    os.precision(oldPrecision);
}

Use it like this:

double d = foo();
float f = bar();
printVal(std::cout, d);
printVal(std::cout, f);

If you want to be able to use the normal insertion << operator, you can use this extra wrapper code:

template <class T>
struct PrintValWrapper { T val; };
template <class T>
std::ostream& operator<<(std::ostream& os, PrintValWrapper<T> pvw) {
    printVal(os, pvw.val);
    return os;
}
template <class T>
PrintValWrapper<T> printIt(T val) {
    return PrintValWrapper<T>{val};
}

Now you can use it like this:

double d = foo();
float f = bar();
std::cout << "The values are: " << printIt(d) << ", " << printIt(f) << '\n';

Answer 14

This will show the value up to two decimal places after the dot.

#include <iostream>
#include <iomanip>

double d = 2.0;
int n = 2;
cout << fixed << setprecision(n) << d;

See here: Fixed-point notation

std::fixed

Use fixed floating-point notation Sets the floatfield format flag for the str stream to fixed.

When floatfield is set to fixed, floating-point values are written using fixed-point notation: the value is represented with exactly as many digits in the decimal part as specified by the precision field (precision) and with no exponent part.

std::setprecision

Set decimal precision Sets the decimal precision to be used to format floating-point values on output operations.

If you're familiar with the IEEE standard for representing the floating-points, you would know that it is impossible to show floating-points with full-precision out of the scope of the standard, that is to say, it will always result in a rounding of the real value.

You need to first check whether the value is within the scope, if yes, then use:

cout << defaultfloat << d ;

std::defaultfloat

Use default floating-point notation Sets the floatfield format flag for the str stream to defaultfloat.

When floatfield is set to defaultfloat, floating-point values are written using the default notation: the representation uses as many meaningful digits as needed up to the stream's decimal precision (precision), counting both the digits before and after the decimal point (if any).

That is also the default behavior of cout, which means you don't use it explicitly.

Answer 15

With ostream::precision(int)

cout.precision( numeric_limits<double>::digits10 + 1);
cout << M_PI << ", " << M_E << endl;

will yield

3.141592653589793, 2.718281828459045

Why you have to say "+1" I have no clue, but the extra digit you get out of it is correct.

Answer 16

Most portably...

#include <limits>

using std::numeric_limits;

    ...
    cout.precision(numeric_limits<double>::digits10 + 1);
    cout << d;