python – Exercise on Summing Digits | Whats with n // = 10

python – Exercise on Summing Digits | Whats with n // = 10

That implements what is called floor division. Floor division (indicated by // here) truncates the decimal and returns the integer result, while normal division returns the answer you may expect (with decimals). In Python 3.x, a greater distinction was made between the two, meaning that the two operators return different results. Here is an example using Python 3:

>>> 10 / 3
3.3333333333333335
>>> 10 // 3
3

Prior to Python 3.x, there is no difference between the two, unless you use the special built-in from __future__ import division, which then makes the division operators perform as they would in Python 3.x (this is using Python 2.6.5):

In [1]: 10 / 3
Out[1]: 3

In [2]: 10 // 3
Out[2]: 3

In [3]: from __future__ import division

In [4]: 10 / 3
Out[4]: 3.3333333333333335

In [5]: 10 // 3
Out[5]: 3

Therefore when you see something like n //= 10, it is using the same +=/-=/*=/etc syntax that you may have seen, where it takes the current value of n and performs the operation before the equal sign with the following variable as the second argument, returning the result into n. For example:

In [6]: n = 50

In [7]: n += 10

In [8]: n
Out[8]: 60

In [9]: n -= 20

In [10]: n
Out[10]: 40

In [11]: n //= 10

In [12]: n
Out[12]: 4

// is the floor division operator. It always truncates the return value to the largest integer smaller than or equal to the answer.

python – Exercise on Summing Digits | Whats with n // = 10

The second to last line is a combination of operators, in a way, including an uncommon one, which is why its a little confusing.

Lets piece it apart.

First, // in Python is floor division, which basically is division rounded down to the nearest whole number. Thus,

>>> 16//5
3
>>> 2//1
2
>>> 4//3
1
>>> 2//5
0

Finally, the = is there because of a Python syntax that allows one to perform an operation on a variable, and then immediately reassign the result to the variable. Youve probably seen it most commonly in +=, as:

>>> a = 5
>>> a += 7
>>> a
12

In this case, //= means perform floor division, floor dividing the variable by the second argument, then assign the result to the original input variable. Thus:

>>> a = 10
>>> a //= 6
>>> a
1