# Discrepancy between .~ and .==false for BitArrays

**URL:** <https://discourse.julialang.org/t/discrepancy-between-and-false-for-bitarrays/11787>\
**Category:** Modelling & Simulations\
**Created:** [June 19, 2018, 4:41pm UTC](https://discourse.julialang.org/t/discrepancy-between-and-false-for-bitarrays/11787 "2018-06-19T16:41:20Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![tyleransom](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tyleransom/32/530_2.png) [@tyleransom](https://discourse.julialang.org/u/tyleransom)\
**Post date:** [June 19, 2018, 4:41pm UTC](https://discourse.julialang.org/t/discrepancy-between-and-false-for-bitarrays/11787/1 "2018-06-19T16:41:20Z")

</div>

Hi,

I’m having trouble understanding why the following don’t yield equivalent objects:

```julia
srand(1234)
x = rand(50,1); y=rand(50,1).>0.5; z=rand(50,1).>0.5;
size(x[.~y .& .~z])
size(x[y.==false .& z.==false])

size(x[.~y])
size(x[y.==false])

```

which at the REPL yields

```julia
julia> srand(1234)
MersenneTwister(UInt32[0x000004d2], Base.dSFMT.DSFMT_state(Int32[-1393240018, 1073611148, 45497681, 1072875908, 436273599, 1073674613, -2043716458, 1073445557, -254908435, 1072827086 … -599655111, 1073144102, 367655457, 1072985259, -1278750689, 1018350124, -597141475, 249849711, 382, 0]), [1.19499, 1.28289, 1.02616, 1.15641, 1.0607, 1.92415, 1.88804, 1.81203, 1.56299, 1.17436 … 1.68022, 1.61886, 1.25895, 1.32248, 1.8879, 1.97766, 1.21481, 1.08087, 1.96411, 1.23217], 382)

julia> x = rand(50,1); y=rand(50,1).>0.5; z=rand(50,1).>0.5;

julia> size(x[.~y .& .~z])
(15,)

julia> size(x[y.==false .& z.==false])
(27,)

julia> size(x[.~y])
(27,)

julia> size(x[y.==false])
(27,)

```

Does anyone know the difference between `.~` and `.==false`?

Here is my version info:

```julia
julia> versioninfo()
Julia Version 0.6.0
Commit 903644385b* (2017-06-19 13:05 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: AMD A9-9420 RADEON R5, 5 COMPUTE CORES 2C+3G
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Prescott)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, amdfam15)

```

---

<div class="post-metadata">

**Author:** ![tyleransom](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tyleransom/32/530_2.png) [@tyleransom](https://discourse.julialang.org/u/tyleransom)\
**Post date:** [June 19, 2018, 4:46pm UTC](https://discourse.julialang.org/t/discrepancy-between-and-false-for-bitarrays/11787/2 "2018-06-19T16:46:45Z")

</div>

OK, it seems to be an order-of-operations issue:

```julia
size(x[(y.==false) .& (z.==false)])

```

gives

```julia
julia> size(x[(y.==false) .& (z.==false)])
(15,)

```

Can anyone elaborate on what’s going on?

---

<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [June 19, 2018, 4:57pm UTC](https://discourse.julialang.org/t/discrepancy-between-and-false-for-bitarrays/11787/3 "2018-06-19T16:57:37Z")

</div>

Yes, it is exactly a precedence issue. `&` and `|` have precedences geared for bitwise arithmetic — like accessing flags out of a bitmask. There you’re often wanting to compare the result to something (e.g., `if bits & 0x07 == 0x03`). Here, you’re wanting to use it as an elementwise logical operator like `&&`, which is commonly used to chain comparisons like you’re doing. This is a longstanding conflict that dates back to previous languages.

I’m hoping we’ll resolve this by allowing the parsing of `.&&` and `.||` with the expected precedences and behaviors.