Why does integer division yield a float instead of another integer?
Consider this division in Python:
Python 3.1 (r31:73574, Jun 26 2009, 20:21:35) [MSC v.1500 32 bit (Intel)] on win32
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>>> 2/2
1.0
Is this intended? I strongly remember earlier versions returning int/int = int. What should I do? Is there a new division operator or must I always cast?
Take a look at PEP-238: Changing the Division Operator
The // operator will be available to request floor division unambiguously.
Oops, immediately found 2//2. This will output an int rather than a float.
// instead of / because of this answer and cause code that depends on a float to break, not realizing the implications.
Behavior of the division operator in Python 2.7 and Python 3
In Python 2.7: By default, division operator will return integer output.
To get the result in double, multiply the dividend or divisor by 1.0.
100/35 => 2 # Expected is 2.857142857142857
(100*1.0)/35 => 2.857142857142857
100/(35*1.0) => 2.857142857142857
In Python 3
// => used for integer output
/ => used for double output
100/35 => 2.857142857142857
100//35 => 2
100.//35 => 2.0 # Floating-point result if the divisor or dividend is real
// is not "used for integer output". // is the result of the floor() function to the result of the division, which in turns yields an integer if the two operands are integers and a float if at least one of them is a float, for types coherence.
The accepted answer already mentions PEP 238. I just want to add a quick look behind the scenes for those interested in what's going on without reading the whole PEP.
Python maps operators like +, -, * and / to special functions, such that e.g. a + b is equivalent to
a.__add__(b)
Regarding division in Python 2, there is by default only / which maps to __div__ and the result is dependent on the input types (e.g. int, float).
Python 2.2 introduced the __future__ feature division, which changed the division semantics the following way (TL;DR of PEP 238):
/ maps to __truediv__ which must "return a reasonable approximation of the mathematical result of the division" (quote from PEP 238)
// maps to __floordiv__, which should return the floored result of /
With Python 3.0, the changes of PEP 238 became the default behaviour and there is no more special method __div__ in Python's object model.
If you want to use the same code in Python 2 and Python 3 use
from __future__ import division
and stick to the PEP 238 semantics of / and //.
According to Python 3 documentation, Python when divided by integer, will generate float despite expected to be integer.
For exclusively printing integer,use floor division method. Floor division is rounding off zero and removing decimal point. Represented by //
Hence, instead of 2/2 ,use 2//2
You can also import division from __future__ irrespective of using Python 2 or Python 3.
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/to be enough of a language design "mistake" that needed to be fixed in Python 3. It may have been a controversial decision, but enough people felt it was the correct one. Count me in the group that loves the change. It is nice we can all agree to disagree. Cheers. :)