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From: Alan Didey <a_didey AT hotmail DOT com>
Newsgroups: comp.lang.c++,comp.os.msdos.djgpp
Subject: Re: setw & notation
Date: 24 Apr 2002 12:19:46 +0100
Organization: Oxford University, England
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"Alex Vinokur" <alexvn AT bigfoot DOT com> writes:

> "Alan Didey" <a_didey AT hotmail DOT com> wrote in message news:3CC59DBB DOT 1000201 AT hotmail DOT com...
> 
> [snip]
> 
> |
> | This program prints numbers without their fractional part (though
> | rounded not truncated), or if they are smaller than one, prints only the
> | first non-zero decimal place.  You have to calculate the precision
> | required to do this yourself.  Note the use of epsilon() - this is
> | important.
> |
> | #include <iostream>
> | #include <limits>
> | #include <iomanip>
> | #include <cmath>
> | using namespace std;
> |
> | int main()
> | {
> |      // remember the original format of the stream
> |      ios_base::fmtflags flags = cout.flags();
> |      streamsize prec = cout.precision();
> |
> |      // we never want scientific notation
> |      cout.setf(ios_base::fixed, ios_base::floatfield);
> |
> |      // my implementation won't ever print more than 18 decimal places
> |      for (double d=100000000000000.0; d>=1e-18; d/=10.0) {
> | if(d < 1)
> |     cout.precision(
> | // enough precision for one decimal place
> | static_cast<streamsize>(
> |     -log10(d) + 1 - numeric_limits<double>::epsilon()
> |     )
> | );
> | else // don't print fractional part of numbers greater than 1
> |     cout.precision(0);
> | cout << d << '\n';
> |      }
> |
> |      // force the output on to the screen
> |      cout.flush();
> |
> |      // restore the stream to its former glory
> |      cout.precision(prec);
> |      cout.setf(ios_base::fmtflags(0), ios_base::floatfield);
> | }
> |
> | Its output:
> 
> [snip]
> 
> |
> | 0.1
> | 0.01
> | 0.001
> | 0.0001
> | 0.00001
> | 0.000001
> | 0.0000001
> | 0.00000001
> | 0.000000001
> | 0.0000000001
> | 0.00000000001
> | 0.000000000001
> | 0.0000000000001
> | 0.00000000000001
> | 0.000000000000001
> | 0.0000000000000001
> | 0.00000000000000001
> | 0.000000000000000001
> |
> |
> [snip]
> 
> Thanks. It is almost OK.
> The only thing I want to change is to see output as following :
>         0.1
>       0.01
>     0.001
>   0.0001
> 0.00001
> ............

Did you really mean:

        0.1
      0.01
    0.001
  0.0001
0.00001

or did you mean:

    0.1
   0.01
  0.001
 0.0001
0.00001

(monospaced typeface needed to tell the difference)

If you meant the latter, then just put a setw(7) manipulator on the
stream before outputting the number.

If you really meant the former, then you can calculate how many spaces
you want from the figure you pass to the precision, and output a
temporary std::string(the_right_number_of_spaces, ' ').

I had kind of hoped you might figure this last bit out for yourself
from the information in some of the replies from other people.