using stringstream to print a rounded floating point number

Question

i have floating point variables "lmin" and "lmax". i wish to display only 4 significant digits. i am currently using something i have found online of the form ...

string textout;
stringstream ss;

ss << lmin;
textout = ss.str();
output(-0.5, -0.875, textout);

ss.str("");
ss << lmax;
textout = ss.str();
output(0.2, -0.875, textout);

where "output" is simply a function i wrote to parse the string and print it to the screen. the important point, is how do i print only a ROUNDED version of lmin and lmax to ss?

Possible duplicate

Ayjay
– Ayjay

2011-10-12 02:34:18 +00:00
Commented Oct 12, 2011 at 2:34 — Ayjay
– Ayjay, Commented Oct 12, 2011 at 2:34

Praetorian · Accepted Answer · 2011-10-12 02:36:34Z

13

Use std::setprecision to specify the number of digits after the decimal point.

#include <sstream>
#include <iostream>
#include <iomanip>

int main()
{
  double d = 12.3456789;
  std::stringstream ss;

  ss << std::fixed << std::setprecision( 4 ) << d;

  std::cout << ss.str() << std::endl;
}

Output:

12.3457

answered Oct 12, 2011 at 2:36

Praetorian

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3 Comments

drjrm3 Over a year ago

while this almost perfectly answers my question, i'm going to push the envelop a bit farther and ask if there is a simple way to print it to scientific notation? if i have 12.3457, i will want only 4 digits, not necessarily 4 digits after the decimal, so i would want 12.34 (or equivalently, 1.234e+1)

Praetorian Over a year ago

@Laurbert515 Just take out the std::fixed part, default is scientific :). Or you can explicitly use std::scientific

Praetorian Over a year ago

@Laurbert515 Seems I was wrong about the default part, sorry. ideone.com/a07Z0

Potatoswatter · Accepted Answer · 2011-10-12 02:35:21Z

1

Simply use ss.precision( 4 ) or ss << std::setprecision( 4 ) before inserting the output.

answered Oct 12, 2011 at 2:35

Potatoswatter

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