How can I force division to be floating point? Division keeps rounding down to 0?

Question

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

Solution 1

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

Solution 2

You 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.

Solution 3

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?"

Upgrade to Python 3

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.

If Using Python 2

In Python 2, it's not so simple. Some ways of dealing with classic Python 2 division are better and more robust than others.

Recommendation for Python 2

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.

Other Options for Python 2

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

Not Recommended for Python 2

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

Solution 4

c = a / (b * 1.0)

Solution 5

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.

View more solutions

Comments

[hobs]

But you might later be tempted to do 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.

[Mark Dickinson]

That's not ideal, though, since it doesn't work in the case where 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).

[JoeCondron]

Yeah you're right. I was assuming both integers as this is the only time when the division ops differ but you really want a general solution. I didn't actually know you could import the operator or that it doesn't work at all for float divisors. Answer edited.

[gsbabil]

@JoeCondron Did you try python -c 'a=10; b=3.0; print a/b'?

[JoeCondron]

I didn't have to because it obviously works in this scenario. However, what if a and 'b', e.g., are the outputs of an integer-value function? E.g., a = len(list1), b = len(list2).

[gsbabil]

@JoeCondron: good point. I just updated the answer to include float(..). I think multiplying by 1.0, as @Pinochle suggested below, could also be useful.

[stackoverflowuser2010]

And how are you going to apply this approach if the numerator and denominator are both variables?

[stackoverflowuser2010]

How are you going to apply this approach if the numerator and denominator are both variables?

[Alexander]

I assume you refer to the first example, if that is so , i would just use float() on one of the variables.

[Keir Simmons]

Because it doesn't work when variables are used for abstraction. Almost no meaningful code has values hardcoded like that.

[TimZaman]

This has little votes because this answer doesn't answer the question, and isn't a general answer at all. In an answer it's also important first to show why this works. It's very simple: if the numerator or denominator is a float, the result will be a float. Usually you don't use python as a plaintext calculator, so you want an answer for variables a and b.

[yannis]

Note that from __future__ import division must be at the very beginning of the file

[R. Navega]

"1 // 2 = 0", "1 // 2.0 = 0.0" -- interesting little gotcha, even if it's an integer division, if any of the operands is float then the result is a whole number but also float. I was using an integer division to calculate a list index and getting an error because of that.

[mercury0114]

Also the problem is if you want the integer division in one place but the float division in another place of the file...

[smac89]

For the longest time, I actually thought ./ 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 language

[ShadowRanger]

@mercury0114: Not a problem; you just use // when you want floor division, and / when you want "true" (float) division.

[xxSithRagexx]

Thanks for this answer! Was driving me crazy. Why on earth did they choose to do that in Python 2? I've never seen that before.