Software Development

Quick Tip: How To Format Floating Point Output In C++

Quick Tip Graphic
Written by John Woolsey

Skill Level: Intermediate

I am sure many of you are familiar with the venerable printf() function in C. However, using the cout (standard output stream) object from the iostream standard library is the preferred method for printing in C++. Most programmers quickly pick up the basics of using cout, but often get tripped up when trying to format floating point numbers. I thought I would share how that can be accomplished with a simple example.

Since C++ is generally a superset of C, technically you can continue using the printf() function in the same fashion in C++ that you are accustomed to in C. Nonetheless, cout is still the preferred approach due to better type safety and extensibility over printf().

By default, cout prints six significant digits of a floating point number, regardless of where the decimal point falls. The setprecision() function is used to change the number of significant digits. Adding the fixed manipulator changes the precision setting to apply specifically to the number of decimal places after the decimal point. Lastly, the setw() function sets the minimum width of a field and adds extra padding when necessary.

The example C++ code shown below illustrates how to use both methods, printf() and cout, to print formatted floating point numbers with a field width of 5 and the number of decimal places set to 2.

#include <iomanip>   // std::setprecision, std::setw
#include <iostream>  // std::cout, std::fixed

int main() {
   float floatA = 3.14159;
   float floatB = 12.3456;

   // Standard C formatting method
   printf("floatA = %5.2f\n", floatA);
   printf("floatB = %5.2f\n", floatB);

   std::cout << "==============" << '\n';  // separator

   // Preferred C++ formatting method
   std::cout << std::fixed             // fix the number of decimal digits
             << std::setprecision(2);  // to 2
   std::cout << "floatA = " << std::setw(5) << floatA << '\n';
   std::cout << "floatB = " << std::setw(5) << floatB << std::endl;

The output of the program is shown below.

floatA =  3.14
floatB = 12.35
floatA =  3.14
floatB = 12.35

About the author

John Woolsey

John is an electrical engineer who loves science, math, and technology and teaching it to others even more.
He knew he wanted to work with electronics from an early age, building his first robot when he was in 8th grade. His first computer was a Timex/Sinclair 2068 followed by the Tandy 1000 TL (aka really old stuff).
He put himself through college (The University of Texas at Austin) by working at Motorola where he worked for many years afterward in the Semiconductor Products Sector in Research and Development.
John started developing mobile app software in 2010 for himself and for other companies. He has also taught programming to kids for summer school and enjoyed years of judging kids science projects at the Austin Energy Regional Science Festival.
Electronics, software, and teaching all culminate in his new venture to learn, make, and teach others via the Woolsey Workshop website.

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