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
float 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++.)
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
float 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++.)
With C++ 11, using abs() alone is very dangerous:
#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():
#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.
What is the difference between abs() and fabs() functions?
fabs versus abs? - C++ Forum
c++ - When do I use fabs and when is it sufficient to use std::abs? - Stack Overflow
Python - abs vs fabs - Stack Overflow
In C++, it's always sufficient to use std::abs; it's overloaded for all the numerical types.
In C, abs only works on integers, and you need fabs for floating point values. These are available in C++ (along with all of the C library), but there's no need to use them.
It's still okay to use fabs for double and float arguments. I prefer this because it ensures that if I accidentally strip the std:: off the abs, that the behavior remains the same for floating point inputs.
I just spent 10 minutes debugging this very problem, due to my own mistake of using abs instead of std::abs. I assumed that the using namespace std;would infer std::abs but it did not, and instead was using the C version.
Anyway, I believe it's good to use fabs instead of abs for floating-point inputs as a way of documenting your intention clearly.
math.fabs() converts its argument to float if it can (if it can't, it throws an exception). It then takes the absolute value, and returns the result as a float.
In addition to floats, abs() also works with integers and complex numbers. Its return type depends on the type of its argument.
In [7]: type(abs(-2))
Out[7]: int
In [8]: type(abs(-2.0))
Out[8]: float
In [9]: type(abs(3+4j))
Out[9]: float
In [10]: type(math.fabs(-2))
Out[10]: float
In [11]: type(math.fabs(-2.0))
Out[11]: float
In [12]: type(math.fabs(3+4j))
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
/home/npe/<ipython-input-12-8368761369da> in <module>()
----> 1 type(math.fabs(3+4j))
TypeError: can't convert complex to float
Edit: as @aix suggested, a better (more fair) way to compare the speed difference:
In [1]: %timeit abs(5)
10000000 loops, best of 3: 86.5 ns per loop
In [2]: from math import fabs
In [3]: %timeit fabs(5)
10000000 loops, best of 3: 115 ns per loop
In [4]: %timeit abs(-5)
10000000 loops, best of 3: 88.3 ns per loop
In [5]: %timeit fabs(-5)
10000000 loops, best of 3: 114 ns per loop
In [6]: %timeit abs(5.0)
10000000 loops, best of 3: 92.5 ns per loop
In [7]: %timeit fabs(5.0)
10000000 loops, best of 3: 93.2 ns per loop
In [8]: %timeit abs(-5.0)
10000000 loops, best of 3: 91.8 ns per loop
In [9]: %timeit fabs(-5.0)
10000000 loops, best of 3: 91 ns per loop
So it seems abs() only has slight speed advantage over fabs() for integers. For floats, abs() and fabs() demonstrate similar speed.
In addition to what @aix has said, one more thing to consider is the speed difference:
In [1]: %timeit abs(-5)
10000000 loops, best of 3: 102 ns per loop
In [2]: import math
In [3]: %timeit math.fabs(-5)
10000000 loops, best of 3: 194 ns per loop
So abs() is faster than math.fabs().