Zo Has - 6 months ago 39

C Question

- How does mod of power of 2 work on only lower order bits of a binary number ()?
`1011000111011010`

- What is this number mod 2 to power 0, 2 to power 4?
- What does power of 2 have to do with the modulo operator? Does it hold a special property?
- Can someone give me an example?

The instructor says "When you take something mod to power of 2 you just take its lower order bits". I was too afraid to ask what he meant =)

Answer

He meant that taking `number mod 2^n`

is equivalent to stripping off all but the `n`

lowest-order *(right-most)* bits of `number`

.

For example, if n == 2,

```
number number mod 4
00000001 00000001
00000010 00000010
00000011 00000011
00000100 00000000
00000101 00000001
00000110 00000010
00000111 00000011
00001000 00000000
00001001 00000001
etc.
```

So in other words, `number mod 4`

is the same as `number & 00000011`

(where `&`

means bitwise-and)

Note that this works exactly the same in base-10: `number mod 10`

gives you the last digit of the number in base-10, `number mod 100`

gives you the last two digits, etc.