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Title: Python % Operator (Video) – Real Python

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Description: In the previous lesson, I introduced you to modular arithmetic. In this lesson, I’ll show you how to use the % operator to accomplish the same thing in Python. The % operator is the modulus operator. Just like in the previous lesson, 13 mod 12 is 1…

Open Graph Description: In the previous lesson, I introduced you to modular arithmetic. In this lesson, I’ll show you how to use the % operator to accomplish the same thing in Python. The % operator is the modulus operator. Just like in the previous lesson, 13 mod 12 is 1…

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00:00https://realpython.com/lessons/python-mod-operator/#t=0.51
In the previous lesson, I introduced you to modular arithmetic. In this lesson,https://realpython.com/lessons/python-mod-operator/#t=0.51
I’ll show you how to use the % operator to accomplish the same thing inhttps://realpython.com/lessons/python-mod-operator/#t=4.8
Python.https://realpython.com/lessons/python-mod-operator/#t=8.61
00:12https://realpython.com/lessons/python-mod-operator/#t=12.53
The % operator is the modulus operator.https://realpython.com/lessons/python-mod-operator/#t=12.53
Just like in the previous lesson,https://realpython.com/lessons/python-mod-operator/#t=15.44
13 mod 12 is 1. 13 divided by 12 has a remainder of 1,https://realpython.com/lessons/python-mod-operator/#t=17.18
hence the result.https://realpython.com/lessons/python-mod-operator/#t=22.31
00:25https://realpython.com/lessons/python-mod-operator/#t=25.46
Here’s another one for 15 mod 4. And a bigger value,https://realpython.com/lessons/python-mod-operator/#t=25.46
240 mod 13. Because the mod operator involves division,https://realpython.com/lessons/python-mod-operator/#t=29.27
you have the same problem if you do a mod by 0,https://realpython.com/lessons/python-mod-operator/#t=34.61
as if you did a divide by 0.https://realpython.com/lessons/python-mod-operator/#t=37.64
00:40https://realpython.com/lessons/python-mod-operator/#t=40.97
You’ll get a ZeroDivisionError raised in this case.https://realpython.com/lessons/python-mod-operator/#t=40.97
In the previous lesson, I showed you that negatives were possible.https://realpython.com/lessons/python-mod-operator/#t=45.62
8 mod -3 is -1.https://realpython.com/lessons/python-mod-operator/#t=49.34
You have to be a little careful with negative numbers.https://realpython.com/lessons/python-mod-operator/#t=52.73
00:54https://realpython.com/lessons/python-mod-operator/#t=54.62
There’s two different ways of calculating mod, and you can get two differenthttps://realpython.com/lessons/python-mod-operator/#t=54.62
results depending on which mechanism you’re using.https://realpython.com/lessons/python-mod-operator/#t=57.8
I’ll show you more about that a little later.https://realpython.com/lessons/python-mod-operator/#t=60.38
01:03https://realpython.com/lessons/python-mod-operator/#t=63.32
The % operator also supports floating-point.https://realpython.com/lessons/python-mod-operator/#t=63.32
Floating-point values going in result in floating-point values coming out.https://realpython.com/lessons/python-mod-operator/#t=67.52
If you haven’t played much with floating-point before,https://realpython.com/lessons/python-mod-operator/#t=73.49
you might be a little surprised at the following little piece of math.https://realpython.com/lessons/python-mod-operator/#t=75.53
01:18https://realpython.com/lessons/python-mod-operator/#t=78.58
.1 + .1 + .1 isn’t 0.3.https://realpython.com/lessons/python-mod-operator/#t=78.58
This has to do with how your computer storeshttps://realpython.com/lessons/python-mod-operator/#t=82.97
these kinds of numbers and the floating-point standard. Mod on floating-point hashttps://realpython.com/lessons/python-mod-operator/#t=84.86
the same kind of precision problem as this addition example I just showed you.https://realpython.com/lessons/python-mod-operator/#t=89.6
01:35https://realpython.com/lessons/python-mod-operator/#t=95.09
Little inaccuracies sneak in.https://realpython.com/lessons/python-mod-operator/#t=95.09
01:39https://realpython.com/lessons/python-mod-operator/#t=99.98
You should always be cautious with floating-point,https://realpython.com/lessons/python-mod-operator/#t=99.98
and this is why you should never use floating-point values to store money.https://realpython.com/lessons/python-mod-operator/#t=102.56
These kinds of errors accumulate over time.https://realpython.com/lessons/python-mod-operator/#t=106.4
01:49https://realpython.com/lessons/python-mod-operator/#t=109.52
Python has a class called Decimal, capital D, inside of the decimal module.https://realpython.com/lessons/python-mod-operator/#t=109.52
It has arbitrary precision and doesn’t have this problem.https://realpython.com/lessons/python-mod-operator/#t=115.04
01:58https://realpython.com/lessons/python-mod-operator/#t=118.31
I’ll show you more about these later.https://realpython.com/lessons/python-mod-operator/#t=118.31
There are two ways of doing modular arithmetic in Python: the % operatorhttps://realpython.com/lessons/python-mod-operator/#t=121.37
and inside of the math library, fmod().https://realpython.com/lessons/python-mod-operator/#t=125.72
02:14https://realpython.com/lessons/python-mod-operator/#t=134.84
Generally, it’s recommended to use the % operator when dealing withhttps://realpython.com/lessons/python-mod-operator/#t=134.84
integers and the fmod() function when dealing with floating-point.https://realpython.com/lessons/python-mod-operator/#t=138.41
02:22https://realpython.com/lessons/python-mod-operator/#t=142.73
Remember how I mentioned earlier that negatives are a problem?https://realpython.com/lessons/python-mod-operator/#t=142.73
It turns out that % for integers and floats and fmod() calculate negativehttps://realpython.com/lessons/python-mod-operator/#t=145.49
modulus differently, and so you’ll get inconsistent results.https://realpython.com/lessons/python-mod-operator/#t=151.04
02:36https://realpython.com/lessons/python-mod-operator/#t=156.95
Here’s a large exponent, and here’s the same using the % operator.https://realpython.com/lessons/python-mod-operator/#t=156.95
You have a significantly different value showing up there.https://realpython.com/lessons/python-mod-operator/#t=162.32
This problem with negatives that I’ve shown you isn’t just within Python.https://realpython.com/lessons/python-mod-operator/#t=166.91
02:49https://realpython.com/lessons/python-mod-operator/#t=169.97
It’s also between different languages.https://realpython.com/lessons/python-mod-operator/#t=169.97
Not all programming languages calculate the negative modulus the same way.https://realpython.com/lessons/python-mod-operator/#t=172.43
8 % -3 in JavaScript is 2. In Python,https://realpython.com/lessons/python-mod-operator/#t=176.42
the same calculation is -1.https://realpython.com/lessons/python-mod-operator/#t=180.67
03:04https://realpython.com/lessons/python-mod-operator/#t=184.18
There are two common ways of calculating mod.https://realpython.com/lessons/python-mod-operator/#t=184.18
Here’s how Python’s % operator does it.https://realpython.com/lessons/python-mod-operator/#t=187.09
The remainder is calculated as a minus n times the floor of a divided by n.https://realpython.com/lessons/python-mod-operator/#t=190.15
03:18https://realpython.com/lessons/python-mod-operator/#t=198.37
Here’s an example.https://realpython.com/lessons/python-mod-operator/#t=198.37
13 mod 12 is 13 minus 12 times the floor of 13https://realpython.com/lessons/python-mod-operator/#t=199.9
over 12.https://realpython.com/lessons/python-mod-operator/#t=204.94
Or 13 minus 12 times the floor of 1.083.https://realpython.com/lessons/python-mod-operator/#t=207.28
The floor of 1.083 is 1,https://realpython.com/lessons/python-mod-operator/#t=212.68
so you end up with 13 minus 12 times 1,https://realpython.com/lessons/python-mod-operator/#t=214.93
which is a remainder of 1. Now let’s do that againhttps://realpython.com/lessons/python-mod-operator/#t=218.8
with a negative number. 8 mod -3 gives you 8 minus -3https://realpython.com/lessons/python-mod-operator/#t=222.58
times the floor of 8 divided by -3.https://realpython.com/lessons/python-mod-operator/#t=228.64
03:52https://realpython.com/lessons/python-mod-operator/#t=232.75
8 divided by -3 gives you minus 2.6 repeated,https://realpython.com/lessons/python-mod-operator/#t=232.75
the floor of which is -3,https://realpython.com/lessons/python-mod-operator/#t=237.0
giving you 9 on the right-hand side and remainder of -1.https://realpython.com/lessons/python-mod-operator/#t=239.35
JavaScript’s % operator calculates things slightly differently. For thehttps://realpython.com/lessons/python-mod-operator/#t=246.25
same operation, instead of using floor(),https://realpython.com/lessons/python-mod-operator/#t=250.99
it uses trunc(). For whole numbers, this isn’t a problem.https://realpython.com/lessons/python-mod-operator/#t=253.69
04:18https://realpython.com/lessons/python-mod-operator/#t=258.579
The end result is the same calculation as you saw before.https://realpython.com/lessons/python-mod-operator/#t=258.579
Unfortunately, for negative numbershttps://realpython.com/lessons/python-mod-operator/#t=263.83
it is a problem. When you get to the trunc() step of 8 divided by -3,https://realpython.com/lessons/python-mod-operator/#t=265.66
the truncation of -2.6 repeated is -2.https://realpython.com/lessons/python-mod-operator/#t=272.08
04:36https://realpython.com/lessons/python-mod-operator/#t=276.49
This changes the value on the right-hand side and thus changes the result.https://realpython.com/lessons/python-mod-operator/#t=276.49
04:42https://realpython.com/lessons/python-mod-operator/#t=282.67
In Python, the % operator uses floor(),https://realpython.com/lessons/python-mod-operator/#t=282.67
but the math.fmod() uses trunc().https://realpython.com/lessons/python-mod-operator/#t=285.88
This is what causes the difference that I showed you earlier between negativehttps://realpython.com/lessons/python-mod-operator/#t=289.24
values for % and fmod(). You need to be aware of thishttps://realpython.com/lessons/python-mod-operator/#t=293.59
if you’re doing modulus math on negative values.https://realpython.com/lessons/python-mod-operator/#t=297.7
05:01https://realpython.com/lessons/python-mod-operator/#t=301.54
8 % -3 is -1,https://realpython.com/lessons/python-mod-operator/#t=301.54
whereas the fmod() on the same value is 2.https://realpython.com/lessons/python-mod-operator/#t=304.27
This is the same as the JavaScript versus Python example I just showed you.https://realpython.com/lessons/python-mod-operator/#t=307.69
05:13https://realpython.com/lessons/python-mod-operator/#t=313.15
In addition to the mod methods I’ve shown you so far,https://realpython.com/lessons/python-mod-operator/#t=313.15
Python also has a function called divmod().https://realpython.com/lessons/python-mod-operator/#t=315.55
05:20https://realpython.com/lessons/python-mod-operator/#t=320.2
divmod() returns two values—both the result of the division and the remainder.https://realpython.com/lessons/python-mod-operator/#t=320.2
12 goes into 13 once,https://realpython.com/lessons/python-mod-operator/#t=325.51
so you get the first part of the tuple, and has a remainder of 1, giving you thehttps://realpython.com/lessons/python-mod-operator/#t=327.82
second part of the tuple.https://realpython.com/lessons/python-mod-operator/#t=331.54
05:34https://realpython.com/lessons/python-mod-operator/#t=334.9
5 goes into 37 7 times and has a remainder of 2https://realpython.com/lessons/python-mod-operator/#t=334.9
when it’s done. The divmod() function is the equivalent of running two differenthttps://realpython.com/lessons/python-mod-operator/#t=339.43
Python operations at the same time.https://realpython.com/lessons/python-mod-operator/#t=344.5
05:47https://realpython.com/lessons/python-mod-operator/#t=347.26
First off, strict integer division, expressed by the double slashes (//).https://realpython.com/lessons/python-mod-operator/#t=347.26
37 // 5 is 7. And secondly, % operator. 37 % 5 is 2,https://realpython.com/lessons/python-mod-operator/#t=353.03
divmod(37, 5) giving you the same as these two resultshttps://realpython.com/lessons/python-mod-operator/#t=361.89
but in one function call. Note that divmod() uses the same mechanism ashttps://realpython.com/lessons/python-mod-operator/#t=365.99
%. So if you’re dealing with negative values,https://realpython.com/lessons/python-mod-operator/#t=371.36
you’ll get the kind of calculation that the % operator does—not the kindhttps://realpython.com/lessons/python-mod-operator/#t=374.21
of calculation that fmod() uses. In Python,https://realpython.com/lessons/python-mod-operator/#t=378.32
the % operator is considered to have the same operator precedence ashttps://realpython.com/lessons/python-mod-operator/#t=382.49
division and multiplication.https://realpython.com/lessons/python-mod-operator/#t=386.96
06:31https://realpython.com/lessons/python-mod-operator/#t=391.1
4 times 10 is 40, 12https://realpython.com/lessons/python-mod-operator/#t=391.1
goes into 40 3 times with a remainder of 4,https://realpython.com/lessons/python-mod-operator/#t=393.5
4 minus 9 gives you -5.https://realpython.com/lessons/python-mod-operator/#t=396.44
Adding some brackets to the same statement changes the value.https://realpython.com/lessons/python-mod-operator/#t=400.58
Now you’re doing 40 mod 3 instead.https://realpython.com/lessons/python-mod-operator/#t=404.93
06:49https://realpython.com/lessons/python-mod-operator/#t=409.88
That’s the basic usage of % and fmod()https://realpython.com/lessons/python-mod-operator/#t=409.88
in Python. In the next lesson,https://realpython.com/lessons/python-mod-operator/#t=412.88
I’ll show you some common uses of these operators inside of code.https://realpython.com/lessons/python-mod-operator/#t=414.98
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https://realpython.com/videos/modular-arithmetic/
Overviewhttps://realpython.com/courses/python-modulo-operator/
https://realpython.com/lessons/common-uses-mod/
Python Modulo: Using the % Operator (Overview) 01:44 https://realpython.com/videos/python-modulo-overview/
Modular Arithmetic 03:36 https://realpython.com/videos/modular-arithmetic/
Python % Operator 07:00 https://realpython.com/lessons/python-mod-operator/
Common Uses of % 08:57 https://realpython.com/lessons/common-uses-mod/
Advanced Uses and Overriding .__mod__ 09:00 https://realpython.com/lessons/overriding-dunder-mod/
Python Modulo: Using the % Operator (Summary) 03:40 https://realpython.com/lessons/python-modulo-summary/
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