Set back default floating point print precision in C++

set precision in c++
c++ round to significant figures
c++ precision double
ofstream precision
c++ fixed
setprecision stream manipulator
setprecision not declared in this scope c++
setprecision(0)

I want to control the precision for a double during a comparison, and then come back to default precision, with C++.

I intend to use setPrecision() to set precision. What is then syntax, if any, to set precision back to default?

I am doing something like this

std::setPrecision(math.log10(m_FTOL));

I do some stuff, and I would like to come back to default double comparison right afterwards.

I modified like this, and I still have some errors

std::streamsize prec = std::ios_base::precision();
std::setprecision(cmath::log10(m_FTOL));

with cmath false at compilation, and std::ios_base also false at compilation. Could you help?


You can get the precision before you change it, with std::ios_base::precision and then use that to change it back later.

You can see this in action with:

#include <ios>
#include <iostream>
#include <iomanip>

int main (void) {
    double d = 3.141592653589;
    std::streamsize ss = std::cout.precision();
    std::cout << "Initial precision = " << ss << '\n';

    std::cout << "Value = " << d << '\n';

    std::cout.precision (10);
    std::cout << "Longer value = " << d << '\n';

    std::cout.precision (ss);
    std::cout << "Original value = " << d << '\n';

    std::cout << "Longer and original value = "
        << std::setprecision(10) << d << ' '
        << std::setprecision(ss) << d << '\n';

    std::cout << "Original value = " << d << '\n';

    return 0;
}

which outputs:

Initial precision = 6
Value = 3.14159
Longer value = 3.141592654
Original value = 3.14159
Longer and original value = 3.141592654 3.14159
Original value = 3.14159

The code above shows two ways of setting the precision, first by calling std::cout.precision (N) and second by using a stream manipulator std::setprecision(N).


But you need to keep in mind that the precision is for outputting values via streams, it does not directly affect comparisons of the values themselves with code like:

if (val1== val2) ...

In other words, even though the output may be 3.14159, the value itself is still the full 3.141592653590 (subject to normal floating point limitations, of course).

If you want to do that, you'll need to check if it's close enough rather than equal, with code such as:

if ((fabs (val1 - val2) < 0.0001) ...

std::defaultfloat - 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  For example, 5.48958123 should be printed as 5.4895 if given precision is 4. For example, below program sets the precision for 4 digits after the decimal point: In C, there is a format specifier in C. To print 4 digits after dot, we can use 0.4f in printf (). Below is program to demonstrate the same.


You need to keep track of your current precison and then reset back to the same once done with your operations with required modified precison. For this you can use std::ios_base::precision:

streamsize precision ( ) const;
streamsize precision ( streamsize prec );

The first syntax returns the value of the current floating-point precision field for the stream. The second syntax also sets it to a new value.

How to use std::setprecision in C++, I want to print b with the default precision. the previous setting for std::​setprecision (not the std::fixed probably), I'm not sure if it violates any rules, a <​< std::endl; // 2.15 std::cout << std::setprecision(-1) << b; // 3.152220 return 0; } You can then use std::defaultfloat to reset to default floating point notation:  The precision of a floating point number defines how many significant digits it can represent without information loss. When outputting floating point numbers, cout has a default precision of 6 and it truncates anything after that. Given below are few libraries and methods which are used to provide precision to floating point numbers in C++: floor():


Save the entire state with std::ios::copyfmt

You might also want to restore the entire previous state with std::ios::copyfmt in these situations, as explained at: Restore the state of std::cout after manipulating it

main.cpp

#include <iomanip>
#include <iostream>

int main() {
    constexpr float pi = 3.14159265359;
    constexpr float e  = 2.71828182846;

    // Sanity check default print.
    std::cout << "default" << std::endl;
    std::cout << pi << std::endl;
    std::cout << e  << std::endl;
    std::cout << std::endl;

    // Change precision format to scientific,
    // and restore default afterwards.
    std::cout << "modified" << std::endl;
    std::ios cout_state(nullptr);
    cout_state.copyfmt(std::cout);
    std::cout << std::setprecision(2);
    std::cout << std::scientific;
    std::cout << pi << std::endl;
    std::cout << e  << std::endl;
    std::cout.copyfmt(cout_state);
    std::cout << std::endl;

    // Check that cout state was restored.
    std::cout << "restored" << std::endl;
    std::cout << pi << std::endl;
    std::cout << e  << std::endl;
    std::cout << std::endl;
}

GitHub upstream.

Compile and run:

g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out

Output:

default
3.14159
2.71828

modified
3.14e+00
2.72e+00

restored
3.14159
2.71828

Tested on Ubuntu 19.04, GCC 8.3.0.

4.8, As with integers, C++ does not define the actual size of these types (but it When outputting floating point numbers, std::cout has a default precision of 6 9876543.21f << '\n';. std::cout << 0.0000987654321f << '\n';. return 0;. } Because we set the precision to 16 digits, each of the above numbers is printed with 16 digits. Number of digits of precision for floating point output (default 8). May be None if floatmode is not fixed, to print as many digits as necessary to uniquely specify the value. thresholdint, optional. Total number of array elements which trigger summarization rather than full repr (default 1000).


setprecision() can be used only for output operations and cannot be used for comparisons

To compare floats say a and b , you have to do it explicitly like this:

  if( abs(a-b) < 1e-6) {   
  }
  else {
  } 

Precision of floating point numbers in C++ (floor(), ceil(), trunc , When outputting floating point numbers, cout has a default precision of 6 and it truncates anything after that. Given below are few libraries and methods which  Set decimal precision Sets the decimal precision to be used to format floating-point values on output operations. Behaves as if member precision were called with n as argument on the stream on which it is inserted/extracted as a manipulator (it can be inserted/extracted on input streams or output streams ).


You can use cout << setprecision(-1)

std::setprecision - cppreference.com, Input/output library When used in an expression out << setprecision(n) or in >> setprecision(n), sets the precision parameter of the stream out or in to exactly n . [edit] Return value Output: default precision (6): 3.14159 std::setprecision(10​): manages decimal precision of floating point operations When printing them, I see the floats with a lot of decimal digits, but when printing, I don't need all of them. So I would like to define a custom format (e.g. 2 or 3 decimals) when floats are printed. I need to use floats and not Decimal. Also, I'm not allowed to truncate/round floats. Is there a way to change the default behavior?


Output Formatting, The standard C++ output manipulators are: std::endl; int main() { cout << "Hello world 1" << endl; cout << "Hello world 2\n"; return 0; } A point for advanced users: In a Unix environment changing the default session The manipulator fixed will set up the output stream for displaying floating point values in fixed format. You can set the precision directly on std::cout and use the std::fixed format specifier. double d = 3.14159265358979; cout.precision(17); cout << "Pi: " << fixed << d << endl; You can #include <limits> to get the maximum precision of a float or double.


printf(), #include <stdio.h> int printf(const char *format-string, argument-list); Kopiraj For decimal floating-point numbers, if the precision is missing, either the f or e style that was printed out by a different activation group, the scanf() family function will set Default precision is six. c. No effect. The character is printed. lc. No effect. new value for precision [ edit ] Return value Returns an object of unspecified type such that if str is the name of an output stream of type std:: basic_ostream < CharT, Traits > or an input stream of type std:: basic_istream < CharT, Traits > , then the expression str << setprecision ( n ) or str >> setprecision ( n ) behaves as if the following code was executed:


5. Built-in Types, The latter function is implicitly used when an object is written by the print() function. using long in C, which gives them at least 32 bits of precision ( sys.​maxint is always set to the Floating point numbers are usually implemented using double in C; Class method to return the float represented by a hexadecimal string s. If True, always print floating point numbers using fixed point notation, in which case numbers equal to zero in the current precision will print as zero. If False, then scientific notation is used when absolute value of the smallest number is < 1e-4 or the ratio of the maximum absolute value to the minimum is > 1e3.