How to format a float using sprintf where precision may vary

Question

I have to print a floating point value, however the precision is not known at compile time. So this parameter has to be passed as agument. How this can be achieved. In windows, with CString, the format function helps to achieve this. How to achieve this without CString. Code:

int main()
{

   /* This below code segment works fine.*/
   char str[80];
   sprintf(str, "Value of Pi = %.3f", 3.147758);
   std::cout << str << std::endl; //Prints "Value of Pi = 3.148"

   /* When the precision may vary, henc It needs to be printed required on that.*/
   char str2[80];
   std::string temp = "%.3f";   //This is required as the precision may change. 
                                    // i.e I may have 4 instead 3 decimal points.
   sprintf(str2, "Value = %s", temp, 3.148257);
   std::cout << str2 << std::endl;  //Prints "Value = <null>"
   return 0;
}

Are you sure you need to use sprintf? This is easy with streams. — eerorika
– eerorika, Commented Apr 29, 2014 at 9:48
Have you found appropriate solution? If so, you should accept the most useful answer. — ST3
– ST3, Commented Aug 5, 2014 at 12:14
what about using %g or %G? -> sprintf(str, "Value of Pi = %g", 3.147758); — cabbi
– cabbi, Commented Oct 16, 2024 at 12:12

Parker Coates · Accepted Answer · 2019-05-08 15:32:22Z

6

You need the

"%.*f"

syntax, where the "*" refers to the next argument.

This is all documented.

edited May 8, 2019 at 15:32

Parker Coates

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answered Apr 29, 2014 at 9:49

user207421

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Comments

evilruff · Accepted Answer · 2014-04-29 09:49:55Z

1

If I understand your question right, you can use setPrecision(..) to achieve desired result:

#include <iostream>
#include <iomanip> // for setprecision()
int main()
{
    using namespace std;

    cout << setprecision(16); // show 16 digits
    float fValue = 3.33333333333333333333333333333333333333f;
    cout << fValue << endl;
    double dValue = 3.3333333333333333333333333333333333333;
    cout << dValue << endl;
}

answered Apr 29, 2014 at 9:49

evilruff

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Comments

Community · Accepted Answer · 2017-05-23 11:57:29Z

1

You can use std::stringstream to convert float to std::string. If you want to use sprint. than use * for width. Details here.

edited May 23, 2017 at 11:57

CommunityBot

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answered Apr 29, 2014 at 9:49

Mohit Jain

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Comments

FrostyZombi3 · Accepted Answer · 2014-04-29 10:00:10Z

0

If you can use iomanip then do this:

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
    int numOfDecimals = 2;// this controls precision. change to what ever you want
    float PI = 3.147758;

    cout << fixed << setprecision(numOfDecimals);
    cout << PI << endl;
    return 0;
}

answered Apr 29, 2014 at 10:00

FrostyZombi3

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Comments

Mike · Accepted Answer · 2014-04-29 14:03:41Z

0

if you wanted to find the amount of decimal places on the fly, you could do something like the following which counts the the amount of decimal places. This would mean you do not have to pass in the format string at all.

#include <iostream>
using namespace std;

int main() 
{
double dNum = 3.14159265359, threshold = 0.00000000005;
double temp = dNum;
int count = 0;
while((temp - (int)temp) > threshold) 
{
    temp *= 10;  
    threshold *=10;
    count++;              
} 
char buffer[50];
sprintf(buffer,"value is = %.*f",count,dNum);
return 0;
}

edited Apr 29, 2014 at 14:03

answered Apr 29, 2014 at 10:37

Mike

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2 Comments

Mohit Jain Over a year ago

For most of the numbers, while will loop infinitely.

Mike Over a year ago

fixed - although I had to limit it to 10 decimal places.

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