# Shifting the bit in the array

**URL:** <https://discourse.julialang.org/t/shifting-the-bit-in-the-array/100586>\
**Category:** General Usage\
**Created:** [June 20, 2023, 3:14am UTC](https://discourse.julialang.org/t/shifting-the-bit-in-the-array/100586 "2023-06-20T03:14:56Z")\
**Posts on this page:** 1\
**Showing post:** 27

<div class="post-metadata">

**Author:** ![moble](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/moble/32/23535_2.png) [@moble](https://discourse.julialang.org/u/moble)\
**Post date:** [June 21, 2023, 3:05pm UTC](https://discourse.julialang.org/t/shifting-the-bit-in-the-array/100586/27 "2023-06-21T15:05:53Z")

</div>

I’m still not sure I understand what you want to do, but if the arrays just represent the bits in an integer then this is easy:

> [@divon](#):
>
> I just want to arrange the array based on the number of 1s in it.

As several people have suggested, don’t bother creating the arrays, just create the integers and arrange the integers. The built-in function [`count_ones`](https://docs.julialang.org/en/v1/base/numbers/#Base.count_ones) gives you the number of 1s in the binary representation of the integer, and [`sort`](https://docs.julialang.org/en/v1/base/sort/#Base.sort) will sort them by that:

```julia
julia> sort(1:16, by=count_ones)
16-element Vector{Int64}:
  1
  2
  4
  8
 16
  3
  5
  6
  9
 10
 12
  7
 11
 13
 14
 15

```

To see what this looks like in binary, we can also use [`bitstring`](https://docs.julialang.org/en/v1/base/numbers/#Base.bitstring) to show the 1s and 0s (converting to `UInt8` just to make things easier to read):

```julia
julia> println.(bitstring.(UInt8.(sort(1:16, by=count_ones))));
00000001
00000010
00000100
00001000
00010000
00000011
00000101
00000110
00001001
00001010
00001100
00000111
00001011
00001101
00001110
00001111

```

A lot of the operations you’re talking about on these arrays of bits can be done directly on integers, because they’re very common operations. For example, `xor(3,5)` just works out of the box. They’re called [“bitwise operators”](https://docs.julialang.org/en/v1/manual/mathematical-operations/#Bitwise-Operators). It’s standard to use unsigned integers (`UInt` and friends) so you don’t have to worry about the sign bit. More generally, “bit twiddling” also works on integers. I wouldn’t be surprised if there are more, but there’s at least [one package just for some bit twiddling tricks](https://docs.juliahub.com/Twiddle/FGh1V/1.1.2/). There’s also [the classic page of “Bit Twiddling Hacks”](https://graphics.stanford.edu/~seander/bithacks.html) that any good nerd should know about.

One example that might be relevant for you is from [your other question](https://discourse.julialang.org/t/check-the-sum-of-a-certain-array-index/100552). I would translate

```julia
a = [0, 0, 0, 1, 0, 1, 0, 1]
b = [1, 2, 3, 5]

```

into the integer equivalents (ignoring a possible endianness ambiguity)

```julia
a = 21
b = 232 # Has 1s in binary representation at indices [1,2,3,5]

```

Then the sum over values in `a` at indices `b` translates simply to

```julia
count_ones(a & b)

```

(Bit twiddlers would call `b` the “mask”.)

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