I saw this in someone's code:
y = img_index // num_images
where img_index
is a running index and num_images
is 3.
When I mess around with //
in IPython, it seems to act just like a division sign (i.e. one forward slash). I was just wondering if there is any reason for having double forward slashes?
To complement Alex's response, I would add that starting from Python 2.2.0a2, from __future__ import division
is a convenient alternative to using lots of float(…)/…
. All divisions perform float divisions, except those with //
. This works with all versions from 2.2.0a2 on.
//
can be considered an alias to math.floor() for divisions with return value of type float
. It operates as no-op
for divisions with return value of type int
.
import math
# let's examine `float` returns
# -------------------------------------
# divide
>>> 1.0 / 2
0.5
# divide and round down
>>> math.floor(1.0/2)
0.0
# divide and round down
>>> 1.0 // 2
0.0
# now let's examine `integer` returns
# -------------------------------------
>>> 1/2
0
>>> 1//2
0
To complement these other answers, the //
operator also offers significant (3x) performance benefits over /
, presuming you want integer division.
$ python -m timeit '20.5 // 2'
100,000,000 loops, best of 3: 14.9 nsec per loop
$ python -m timeit '20.5 / 2'
10,000,000 loops, best of 3: 48.4 nsec per loop
$ python -m timeit '20 / 2'
10,000,000 loops, best of 3: 43.0 nsec per loop
$ python -m timeit '20 // 2'
100,000,000 loops, best of 3: 14.4 nsec per loop
//
is unconditionally "flooring division", e.g:
>>> 4.0//1.5
2.0
As you see, even though both operands are float
s, //
still floors -- so you always know securely what it's going to do.
Single /
may or may not floor depending on Python release, future imports, and even flags on which Python's run, e.g.:
$ python2.6 -Qold -c 'print 2/3'
0
$ python2.6 -Qnew -c 'print 2/3'
0.666666666667
As you see, single /
may floor, or it may return a float, based on completely non-local issues, up to and including the value of the -Q
flag...;-).
So, if and when you know you want flooring, always use //
, which guarantees it. If and when you know you don't want flooring, slap a float()
around other operand and use /
. Any other combination, and you're at the mercy of version, imports, and flags!-)
In Python 3, they made the /
operator do a floating-point division, and added the //
operator to do integer division (i.e., quotient without remainder); whereas in Python 2, the /
operator was simply integer division, unless one of the operands was already a floating point number.
In Python 2.X:
>>> 10/3
3
>>> # To get a floating point number from integer division:
>>> 10.0/3
3.3333333333333335
>>> float(10)/3
3.3333333333333335
In Python 3:
>>> 10/3
3.3333333333333335
>>> 10//3
3
For further reference, see PEP238.
Source: Stackoverflow.com