(This explanation is only for positive numbers since it depends on the language otherwise)
Definition
The Modulus is the remainder of the euclidean division of one number by another. %
is called the modulo operation.
For instance, 9
divided by 4
equals 2
but it remains 1
. Here, 9 / 4 = 2
and 9 % 4 = 1
.
In your example: 5 divided by 7 gives 0 but it remains 5 (5 % 7 == 5
).
Calculation
The modulo operation can be calculated using this equation:
a % b = a - floor(a / b) * b
floor(a / b)
represents the number of times you can dividea
byb
floor(a / b) * b
is the amount that was successfully shared entirely- The total (
a
) minus what was shared equals the remainder of the division
Applied to the last example, this gives:
5 % 7 = 5 - floor(5 / 7) * 7 = 5
Modular Arithmetic
That said, your intuition was that it could be -2 and not 5. Actually, in modular arithmetic, -2 = 5 (mod 7)
because it exists k in Z such that 7k - 2 = 5
.
You may not have learned modular arithmetic, but you have probably used angles and know that -90° is the same as 270° because it is modulo 360. It’s similar, it wraps! So take a circle, and say that it’s perimeter is 7. Then you read where is 5. And if you try with 10, it should be at 3 because 10 % 7
is 3.