I have two integer values a and b, but I need their ratio in floating point. I know that a < b and I want to calculate a / b, so if I use integer division I'll always get 0 with a remainder of a.
How can I force c to be a floating point number in Python 2 in the following?
c = a / b In 3.x, the behaviour is reversed; see Why does integer division yield a float instead of another integer? for the opposite, 3.x-specific problem.
In Python 2, division of two ints produces an int. In Python 3, it produces a float. We can get the new behaviour by importing from __future__.
>>> from __future__ import division >>> a = 4 >>> b = 6 >>> c = a / b >>> c 0.66666666666666663 
a // b
from __future__ import division must be at the very beginning of the fileYou can cast to float by doing c = a / float(b). If the numerator or denominator is a float, then the result will be also.
A caveat: as commenters have pointed out, this won't work if b might be something other than an integer or floating-point number (or a string representing one). If you might be dealing with other types (such as complex numbers) you'll need to either check for those or use a different method.
__future__. b might lose information when cast as float! The correct workaround is to multiply other multiplicand by 1.0.
float doesn't work but __future__ does? I'd expect future division to use this same method as its implementation strategy. And how is multiplying by 1.0 different than casting to float?float or int already. The correct approach is discussed in detail in PEP 238.How can I force division to be floating point in Python?
I have two integer values a and b, but I need their ratio in floating point. I know that a < b and I want to calculate a/b, so if I use integer division I'll always get 0 with a remainder of a.
How can I force c to be a floating point number in Python in the following?
c = a / b
What is really being asked here is:
"How do I force true division such that a / b will return a fraction?"
In Python 3, to get true division, you simply do a / b.
>>> 1/2 0.5 Floor division, the classic division behavior for integers, is now a // b:
>>> 1//2 0 >>> 1//2.0 0.0 However, you may be stuck using Python 2, or you may be writing code that must work in both 2 and 3.
In Python 2, it's not so simple. Some ways of dealing with classic Python 2 division are better and more robust than others.
You can get Python 3 division behavior in any given module with the following import at the top:
from __future__ import division which then applies Python 3 style division to the entire module. It also works in a python shell at any given point. In Python 2:
>>> from __future__ import division >>> 1/2 0.5 >>> 1//2 0 >>> 1//2.0 0.0 This is really the best solution as it ensures the code in your module is more forward compatible with Python 3.
If you don't want to apply this to the entire module, you're limited to a few workarounds. The most popular is to coerce one of the operands to a float. One robust solution is a / (b * 1.0). In a fresh Python shell:
>>> 1/(2 * 1.0) 0.5 Also robust is truediv from the operator module operator.truediv(a, b), but this is likely slower because it's a function call:
>>> from operator import truediv >>> truediv(1, 2) 0.5 Commonly seen is a / float(b). This will raise a TypeError if b is a complex number. Since division with complex numbers is defined, it makes sense to me to not have division fail when passed a complex number for the divisor.
>>> 1 / float(2) 0.5 >>> 1 / float(2j) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: can't convert complex to float It doesn't make much sense to me to purposefully make your code more brittle.
You can also run Python with the -Qnew flag, but this has the downside of executing all modules with the new Python 3 behavior, and some of your modules may expect classic division, so I don't recommend this except for testing. But to demonstrate:
$ python -Qnew -c 'print 1/2' 0.5 $ python -Qnew -c 'print 1/2j' -0.5j 
a / (b + 0.0) just because addition is usually faster than multiplication, even if the difference is negligible.c = a / (b * 1.0) In Python 3.x, the single slash (/) always means true (non-truncating) division. (The // operator is used for truncating division.) In Python 2.x (2.2 and above), you can get this same behavior by putting a
from __future__ import division at the top of your module.
Just making any of the parameters for division in floating-point format also produces the output in floating-point.
Example:
>>> 4.0/3 1.3333333333333333 or,
>>> 4 / 3.0 1.3333333333333333 or,
>>> 4 / float(3) 1.3333333333333333 or,
>>> float(4) / 3 1.3333333333333333 
1.0 + 1/3 or float(c) + a/b or float(a/b) and you'll be disappointed with the answer. Better to use python 3+ or import the __future__.division module, (see accepted answer), to always get the answer you expect. The existing division rules create insidious, hard-to-trace math error.
python -c 'a=10; b=3.0; print a/b'?
a and 'b', e.g., are the outputs of an integer-value function? E.g., a = len(list1), b = len(list2).float(..). I think multiplying by 1.0, as @Pinochle suggested below, could also be useful.Add a dot (.) to indicate floating point numbers
>>> 4/3. 1.3333333333333333 
float() on one of the variables.This will also work
>>> u=1./5 >>> print u 0.2 
a and b.
./ was a valid operator in python that allows you to do floating point division. This is why it is good to use white space judiciously, in any programming languageIf you want to use "true" (floating point) division by default, there is a command line flag:
python -Q new foo.py There are some drawbacks (from the PEP):
It has been argued that a command line option to change the default is evil. It can certainly be dangerous in the wrong hands: for example, it would be impossible to combine a 3rd party library package that requires -Qnew with another one that requires -Qold.
You can learn more about the other flags values that change / warn-about the behavior of division by looking at the python man page.
For full details on division changes read: PEP 238 -- Changing the Division Operator
from operator import truediv c = truediv(a, b) 
a is an int and b is a float. A better solution along the same lines is to do from operator import truediv and then use truediv(a, b).from operator import truediv c = truediv(a, b) where a is dividend and b is the divisor. This function is handy when quotient after division of two integers is a float.
a = a / 3: Ifa / 3were a float type, you couldn't assign it to the original integer type. Most type languages also do "type promotion": If one is float, a float operator is used, and to be able to use it, the other will be promoted (i.e.: converted) to a float. This may seem handy, but in object-oriented programming this can become quite messy if you define your own numeric types. Also this can have an unintended performance impact.