# How to calculate a mod b in Python?

## Question:

Is there a modulo function in the Python `math`

library?

Isn’t `15 % 4`

, 3? But `15 mod 4`

is 1, right?

## Answers:

There’s the `%`

sign. It’s not just for the remainder, it is the modulo operation.

Why don’t you use % ?

```
print 4 % 2 # 0
```

`mod = a % b`

This stores the result of `a mod b`

in the variable `mod`

.

And you are right, `15 mod 4`

is 3, which is exactly what python returns:

```
>>> 15 % 4
3
```

`a %= b`

is also valid.

```
>>> 15 % 4
3
>>>
```

The modulo gives the remainder after integer division.

you can also try `divmod(x, y)`

which returns a tuple `(x // y, x % y)`

I don’t think you’re fully grasping modulo. `a % b`

and `a mod b`

are just two different ways to express modulo. In this case, python uses `%`

. No, `15 mod 4`

is not `1`

, `15 % 4 == 15 mod 4 == 3`

.

```
A = [3, 1, 2, 4]
for a in A:
print(a % 2)
```

output:

```
1
1
0
0
```