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C++ question(Decimal places)very weird

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Join Date: May 2005

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Reputation: maherddd is an unknown quantity at this point

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maherddd

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C++ question(Decimal places)very weird

May 29th, 2005

this is an example of what is happening

consider the program:

#include<iostream.h>

void main(void)
{

double x=2.3767553235;
cout<<x<<endl;

}

when i compile it

the output is only giving 2.37676
Why?

I dont want only 5 decimal places,plz someone tell me why is this happening.

i want it to be say for example 8 or 10 decimal places

thank u so much

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Join Date: May 2005

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Reputation: maherddd is an unknown quantity at this point

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maherddd

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Re: C++ question(Decimal places)very weird

May 29th, 2005

so my point is i want 2.3767553235 in the output,why is the pc only putting 2.37676

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Join Date: May 2005

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Reputation: Dogtree is an unknown quantity at this point

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Dogtree

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Re: C++ question(Decimal places)very weird

May 29th, 2005

>> Why?
Why not? It's concievable that there would be a default precision for the stream, one that would be valid everywhere so that it could be portable. It just so happens that that's the case. The default is 6:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
 
int main()
{
  double d = 123.4567890123;
 
  std::cout << "Precision: " << std::cout.precision() << '\n';
  std::cout << "Value: " << d << '\n';
  std::cout.precision(4);
  std::cout << "Value: " << d << '\n';
}
#include <iostream>

int main()
{
  double d = 123.4567890123;

  std::cout << "Precision: " << std::cout.precision() << '\n';
  std::cout << "Value: " << d << '\n';
  std::cout.precision(4);
  std::cout << "Value: " << d << '\n';
}

>> i want it to be say for example 8 or 10 decimal places
Assuming your system is capable of holding an accurate precision to 10 digits past the radix including an arbitrary number of digits before the radix, you would do it like this:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
 
int main()
{
  double x = 2.3767553235;
 
  std::cout.precision(11);
  std::cout << x << '\n';
}
#include <iostream>

int main()
{
  double x = 2.3767553235;

  std::cout.precision(11);
  std::cout << x << '\n';
}

However, because the precision is all digits in the number, to get an exact precision of 10 for any whole value, you would need a way to determine the number of digits before the radix. An easy to understand solution is to make a string and take its length:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <sstream>
 
namespace jrd {
  int digits(double val)
  {
    std::ostringstream oss;
 
    oss << static_cast<int>(val);
 
    return oss.str().length();
  }
}
 
int main()
{
  double x = 2.3767553235;
 
  std::cout.precision(10 + jrd::digits(x));
  std::cout << x << '\n';
}
#include <iostream>
#include <sstream>

namespace jrd {
  int digits(double val)
  {
    std::ostringstream oss;

    oss << static_cast<int>(val);

    return oss.str().length();
  }
}

int main()
{
  double x = 2.3767553235;

  std::cout.precision(10 + jrd::digits(x));
  std::cout << x << '\n';
}

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Join Date: May 2005

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maherddd

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Re: C++ question(Decimal places)very weird

May 29th, 2005

whats the difference between <iostream> and <iostream.h>?

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Join Date: May 2005

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Reputation: Dogtree is an unknown quantity at this point

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Dogtree

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Posting Whiz in Training

Re: C++ question(Decimal places)very weird

May 29th, 2005

<iostream.h> is not standard, and therefore is becoming increasingly less supported by compilers as standard C++ matures. This isn't a theoretical difference, by the way. My compiler will refuse to compile <iostream.h>.

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Join Date: Jun 2005

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little nell little nell is offline

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Re: C++ question(Decimal places)very weird

Jun 7th, 2005

Another important difference between iostream.h and iostream is that iostream.h has its declarations in the global namespace, while iostream has them in the std namespace.

dogtree's method is quite inefficient. It is possible to directly set the number of decimal places, but you have to make sure the output mode is "fixed" first.

This example produces 10 decimal places.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include<iostream>
 
using namespace std;
 
int main()
{
	double x = 2.3767553235;
	cout << fixed << showpoint << setprecision(10) << x << endl;
}
#include<iostream>

using namespace std;

int main()
{
	double x = 2.3767553235;
	cout << fixed << showpoint << setprecision(10) << x << endl;
}

If you google for "C++ decimal places" you will find this article

http://www.cs.fsu.edu/~myers/c++/notes/basics1.html

and right at the end is a short explanation of formatting decimal numbers.