Formatting output in C++

In a C++ code I have a matrix of double variables which I print out. However because all of them have different number of digits, the output format is destroyed. One solution is to do
cout.precision(5) but I want different columns have a different precision. Also, because there are negative values in some cases, the presence of the - sign also causes problems. How to get around this and produce a properly formatted output?

cout.precision(5)

-

6 Answers
6

Off the top of my head, you can use setw(int) to specify the width of the output.

like this:

std::cout << std::setw(5) << 0.2 << std::setw(10) << 123456 << std::endl;
std::cout << std::setw(5) << 0.12 << std::setw(10) << 123456789 << std::endl;


gives this:

0.2 123456
0.12 123456789


The key is, as others have said, to use manipulators. What they
neglected to say is that you normally use manipulators that you write
yourself. An FFmt manipulator (which corresponds to the F format in
Fortran is fairly easy:

FFmt

F

class FFmt
{
int myWidth;
int myPrecision;
public:
FFmt( int width, int precision )
: myWidth( width )
, myPrecision( precision )
{
}

friend std::ostream&
operator<<( std::ostream& dest, FFmt const& fmt )
{
dest.setf( std::ios_base::fixed, std::ios_base::formatfield );
dest.precision( myPrecision );
dest.width( myWidth );
return dest;
}
};


This way, you can define a variable for each column, say:

FFmt col1( 8, 2 );
FFmt col2( 6, 3 );
// ...


and write:

std::cout << col1 << value1
<< ' ' << col2 << value2...


In general, except in the simplest programs, you should probably not be
using the standard manipulators, but rather custom manipulators based on
your application; e.g. temperature and pressure if that's the sort of
thing your dealing with. In this way, it's clear in the code what
you're formatting, and if the client suddenly asks for one more digit in
the pressure, you know exactly where to make the change.

temperature

pressure

std::setw

Use manipulators.

From sample here:

#include <iostream>
#include <iomanip>
#include <locale>
int main()
{
std::cout.imbue(std::locale("en_US.utf8"));
std::cout << "Left fill:n" << std::left << std::setfill('*')
<< std::setw(12) << -1.23 << 'n'
<< std::setw(12) << std::hex << std::showbase << 42 << 'n'
<< std::setw(12) << std::put_money(123, true) << "nn";

std::cout << "Internal fill:n" << std::internal
<< std::setw(12) << -1.23 << 'n'
<< std::setw(12) << 42 << 'n'
<< std::setw(12) << std::put_money(123, true) << "nn";

std::cout << "Right fill:n" << std::right
<< std::setw(12) << -1.23 << 'n'
<< std::setw(12) << 42 << 'n'
<< std::setw(12) << std::put_money(123, true) << 'n';
}


Output:

Left fill:
-1.23*******
0x2a********
USD *1.23***

Internal fill:
-*******1.23
0x********2a
USD ****1.23

Right fill:
*******-1.23
********0x2a
***USD *1.23


Take a look at stream manipulators, especially std::setw and std::setfill.

std::setw

std::setfill

float f = 3.1415926535;
std::cout << std::setprecision(5) // precision of floating point output
<< std::setfill(' ') // character used to fill the column
<< std::setw(20) // width of column
<< f << 'n'; // your number


Try using setw manipulator. Please refer http://www.cplusplus.com/reference/iostream/manipulators/setw/ for further information

There is a way using i/o manipulators, but I find it unwieldy. I would just write a function like this:

template<typename T>
std::string RightAligned(int size, const T & val)
{
std::string x = boost::lexical_cast<std::string>(val);
if (x.size() < size)
x = std::string(size - x.size(), ' ') + x;
return x;
}


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