fabs is one of a family of type-specific names from C. The C++ standard libraries provide these names, fabsf for float, fabs for double and fabsl for long double, for compatibility with C, so that barring other issues C code that uses these names can be compiled as C++. However, the C++ library provides corresponding overloads of the single name abs, which one should use in C++. There is an issue with abs, namely that the overloads for integral argument types reside in (or for the name placed in the std namespace, in ). So, for using abs one should better include both and . And since in C++17 and later the header provides some functions that are not provided by , if one writes a wrapper to include standard library numerical stuff headers then it should best include also . Answer from alfps on reddit.com
Reddit
reddit.com › r/cpp_questions › difference between abs() and fabs(), and can i replace with each other in the program?? and abs() fn is kinda not working.....
r/cpp_questions on Reddit: Difference between abs() and fabs(), and can i replace with each other in the program?? And abs() fn is kinda not working.....
November 26, 2019 - fabs is one of a family of type-specific names from C. The C++ standard libraries provide these names, fabsf for float, fabs for double and fabsl for long double, for compatibility with C, so that barring other issues C code that uses these ...
Quora
quora.com › What-is-the-difference-between-abs-and-fabs-function
What is the difference between abs() and fabs() function? - Quora
The only difference between both of them is, abs() is used to calculate the absolute value for integer type numbers whereas fabs() are used for floating type numbers. abs() function is use un...
Videos
Cplusplus
cplusplus.com › forum › beginner › 136274
fabs versus abs? - C++ Forum
Well, if programming in C++, someone ... of abs() for all arithmetic types. fabs() and labs() are provided for backward compatibility with C, which does not have function overloading and had to have different fucntions for different types....
Youth4work
youth4work.com › talent › c language › forum › what is the difference between abs() and fabs() functions?
What is the difference between abs() and fabs() functions?
April 16, 2018 - Both functions are to retrieve absolute value. abs() is for integer values and fabs() is for floating type numbers. Prototype for abs() is under the library file < stdlib.h > and fabs() is under < math.h >. ... Didn't get the answer.
Programiz
programiz.com › c-programming › library-function › math.h › fabs
C fabs() - C Standard Library
Become a certified C programmer. Try Programiz PRO! ... The fabs() function takes a single argument (in double) and returns the absolute value of that number (also in double).
Top answer 1 of 3
49
In C++, std::abs is overloaded for both signed integer and floating point types. std::fabs only deals with floating point types (pre C++11). Note that the std:: is important; the C function ::abs that is commonly available for legacy reasons will only handle int!
The problem with
Copyfloat f2= fabs(-9);
is not that there is no conversion from int (the type of -9) to double, but that the compiler does not know which conversion to pick (int -> float, double, long double) since there is a std::fabs for each of those three. Your workaround explicitly tells the compiler to use the int -> double conversion, so the ambiguity goes away.
C++11 solves this by adding double fabs( Integral arg ); which will return the abs of any integer type converted to double. Apparently, this overload is also available in C++98 mode with libstdc++ and libc++.
In general, just use std::abs, it will do the right thing. (Interesting pitfall pointed out by @Shafik Yaghmour. Unsigned integer types do funny things in C++.)
2 of 3
19
With C++ 11, using abs() alone is very dangerous:
Copy#include <iostream>
#include <cmath>
int main() {
std::cout << abs(-2.5) << std::endl;
return 0;
}
This program outputs 2 as a result. (See it live)
Always use std::abs():
Copy#include <iostream>
#include <cmath>
int main() {
std::cout << std::abs(-2.5) << std::endl;
return 0;
}
This program outputs 2.5.
You can avoid the unexpected result with using namespace std; but I would adwise against it, because it is considered bad practice in general, and because you have to search for the using directive to know if abs() means the int overload or the double overload.
Sololearn
sololearn.com › en › Discuss › 2784928 › fabs-and-abs
fabs and abs | Sololearn: Learn to code for FREE!
f in fabs stands for float The difference is that math.fabs(number) will always return a floating point number even if the argument is integer, whereas abs() will return a floating point or an integer depending upon the argument. src: geeksforgeeks ...
GeeksforGeeks
geeksforgeeks.org › c language › fabs-function-in-c
fabs() Function in C - GeeksforGeeks
July 23, 2025 - Parameter: It will take a single parameter which is to be converted to an absolute value. Return Value: While passing values to fabs(), we can convert the number explicitly to double.
YouTube
youtube.com › watch
Absolute Value Functions abs() And fabs() | C Programming Tutorial - YouTube
How to find the absolute value of a number using the functions abs() and fabs() in C. Source code: https://github.com/portfoliocourses/c-example-code/blob/m...
Published November 4, 2022
TechOnTheNet
techonthenet.com › c_language › standard_library_functions › math_h › fabs.php
C Language: fabs function (Absolute Value of Floating-Point Number)
The fabs function returns the absolute value of a floating-point number represented by x. In the C Language, the required header for the fabs function is:
Scaler
scaler.com › home › topics › fabs() function in c
fabs() Function in C - Scaler Topics
November 7, 2023 - The fabs in C is a library function provided within the math.h header file of the C programming language. It returns the absolute value of the given input, i.e., it will return the positive value of the given input irrespective of its sign (positive/negative).
Sololearn
sololearn.com › en › Discuss › 1799301 › qus10what-is-the-difference-between-abs-and-fabs-functions
Qus10:-What is the difference between abs() and fabs() functions? | Sololearn: Learn to code for FREE!
Hi if you have a float number, you have to use fabs() if you have an integer number, you have to use abs() exemple with fabs() : fabs(239.14) return 239.14 fabs(-239.14) return 239.14 exemple with abs() : abs(239) return 239 abs(-239) return 239
Krayonnz
krayonnz.com › user › doubts › detail › 61894a1591cca20041786b9f › difference-between-abs-and-fabs-functions-in-C
Difference between abs() and fabs() functions in C ?
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O'Reilly
oreilly.com › library › view › c-in-a › 0596006977 › re57.html
fabs - C in a Nutshell [Book]
December 16, 2005 - float x = 4.0F * atanf( 1.0F ); long double y = 4.0L * atanl( 1.0L ); if ( x == y ) printf( "x and y are exactly equal.\n" ); else if (fabs( x − y ) < 0.0001 * fabsl( y ) ) printf( "x and y are approximately equal:\n" "x is %.8f; y is %.8Lf.\n", x, y ); ... The absolute value functions for integer types: abs(), labs(), llabs(), and imaxabs(); the absolute value functions for complex numbers: cabs(), cabsf(), cabsl(); the C99 functions fdim() and copysign(); the functions fmax() and fmin()
Authors Peter PrinzTony Crawford
Published 2005
Pages 618
Codecademy
codecademy.com › docs › c++ › math functions › fabs()
C++ (C Plus Plus) | Math Functions | fabs() | Codecademy
April 13, 2025 - In this example, fabs() is used ... floating-point precision issues: ... fabs() is designed for floating-point types (float, double, long double) while abs() is typically used with integer types....
Cplusplus
cplusplus.com › reference › cmath › fabs
fabs
double fabs (double x); float fabs (float x);long double fabs (long double x); double fabs (T x); // additional overloads for integral types ... Returns the absolute value of x: |x|. Header <tgmath.h> provides a type-generic macro version of this function. Additional overloads are provided in this header (<cmath>) for the integral types: These overloads effectively cast x to a double (defined for T being any integral type).
Microsoft Learn
learn.microsoft.com › en-us › cpp › c-runtime-library › reference › fabs-fabsf-fabsl
fabs, fabsf, fabsl | Microsoft Learn
The fabs functions return the absolute value of the argument x. There's no error return. C++ allows overloading, so you can call overloads of fabs if you include the <cmath> header.
Reddit
reddit.com › r/askprogramming › simple c question on abs() and fabs()
r/AskProgramming on Reddit: Simple C Question on abs() and fabs()
October 22, 2020 -
Hi guys, I know that abs() takes in an int and returns an int, while fabs() takes in a float/double and returns a float/double.
However, if I pass in an int to fabs, it works as per usual, but not when I pass a double into abs. I would expect some syntax error to appear or smth, or does type promotion occur here, where the int gets promoted to a double and so fabs() work as per usual?
Cppreference
en.cppreference.com › w › cpp › numeric › math › fabs.html
std::abs(float), std::fabs, std::fabsf, std::fabsl - cppreference.com
1-4) Computes the absolute value of the floating-point value num. The library provides overloads of std::abs and std::fabs for all cv-unqualified floating-point types as the type of the parameter num.(since C++23) For integral arguments, the integral overloads of std::abs are likely better matches.
Apple Developer
developer.apple.com › forums › thread › 90710
What does the function "fabs(_:)" … | Apple Developer Forums
This is a common gotcha in C programming, because if "myValue" is a variable of type "double" (say), and you write "double result = abs (myValue)", it looks like you are taking the absolute value, but the C compiler [correctly and automatically] converts the double to an int by truncating it, computes the absolute value of the truncated integer, converts it back to a double, and this isn't the answer you wanted.