What is the result of % in Python?

Question

What does the % in a calculation? I can\'t seem to work out what it does.

Does it work out a percent of the calculation for example: 4 % 2

Answer 1

It was hard for me to readily find specific use cases for the use of % online ,e.g. why does doing fractional modulus division or negative modulus division result in the answer that it does. Hope this helps clarify questions like this:

Modulus Division In General:

Modulus division returns the remainder of a mathematical division operation. It is does it as follows:

Say we have a dividend of 5 and divisor of 2, the following division operation would be (equated to x):

dividend = 5
divisor = 2

x = 5/2 

1. The first step in the modulus calculation is to conduct integer division:

   x_int = 5 // 2 ( integer division in python uses double slash)

   x_int = 2

2. Next, the output of x_int is multiplied by the divisor:

   x_mult = x_int * divisor x_mult = 4

3. Lastly, the dividend is subtracted from the x_mult

   dividend - x_mult = 1

4. The modulus operation ,therefore, returns 1:

   5 % 2 = 1

Application to apply the modulus to a fraction

Example: 2 % 5 

The calculation of the modulus when applied to a fraction is the same as above; however, it is important to note that the integer division will result in a value of zero when the divisor is larger than the dividend:

dividend = 2 
divisor = 5

The integer division results in 0 whereas the; therefore, when step 3 above is performed, the value of the dividend is carried through (subtracted from zero):

dividend - 0 = 2  —> 2 % 5 = 2 

Application to apply the modulus to a negative

Floor division occurs in which the value of the integer division is rounded down to the lowest integer value:

import math 

x = -1.1
math.floor(-1.1) = -2 

y = 1.1
math.floor = 1

Therefore, when you do integer division you may get a different outcome than you expect!

Applying the steps above on the following dividend and divisor illustrates the modulus concept:

dividend: -5 
divisor: 2 

Step 1: Apply integer division

x_int = -5 // 2  = -3

Step 2: Multiply the result of the integer division by the divisor

x_mult = x_int * 2 = -6

Step 3: Subtract the dividend from the multiplied variable, notice the double negative.

dividend - x_mult = -5 -(-6) = 1

Therefore:

-5 % 2 = 1