# Boolean array in comprehensions
**URL:** https://discourse.julialang.org/t/boolean-array-in-comprehensions/14545
**Category:** New to Julia
**Created:** [September 4, 2018, 6:36pm UTC](https://discourse.julialang.org/t/boolean-array-in-comprehensions/14545 "2018-09-04T18:36:18Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@bwilliges](https://discourse.julialang.org/u/bwilliges)
#### Post date: [September 4, 2018, 6:36pm UTC](https://discourse.julialang.org/t/boolean-array-in-comprehensions/14545/1 "2018-09-04T18:36:18Z")
I am trying to understand Julia comprehensions and their filter systems (in version 1.0):
```julia
julia> aa = [2,2,1,2]
4-element Array{Int64,1}:
2
2
1
2
bb = [1,1,1,0]
4-element Array{Int64,1}:
1
1
1
0
julia> [i for i=1:4 if aa[i] == bb[i] ] #works as expected, returns only the third element
1-element Array{Int64,1}:
3
```
However, using the .== command does not work:
```julia
julia> [i for i=1:4 if aa .== bb]
ERROR: TypeError: non-boolean (BitArray{1}) used in boolean context
Stacktrace:
[1] iterate at ./iterators.jl:434 [inlined]
[2] iterate at ./generator.jl:44 [inlined]
[3] grow_to!(::Array{Int64,1}, ::Base.Generator{Base.Iterators.Filter{getfield(Main, Symbol("##36#38")),UnitRange{Int64}},getfield(Main, Symbol("##35#37"))}) at ./array.jl:674
[4] collect(::Base.Generator{Base.Iterators.Filter{getfield(Main, Symbol("##36#38")),UnitRange{Int64}},getfield(Main, Symbol("##35#37"))}) at ./array.jl:617
[5] top-level scope at none:0
#although the .== commands works by itself:
julia> aa .== bb
4-element BitArray{1}:
false
false
true
false
```
Can someone please explain this behaviour? In the [documentation](https://docs.julialang.org/en/v1/manual/noteworthy-differences/) it says:
```julia
In Julia, if A and B are arrays, logical comparison operations like A == B do not return an array of booleans. Instead, use A .== B, and similarly for the other boolean operators like <, > and =.
```
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [September 4, 2018, 6:41pm UTC](https://discourse.julialang.org/t/boolean-array-in-comprehensions/14545/2 "2018-09-04T18:41:43Z")
The `if` clause of a filtered comprehension is executed on every iteration of the `for` clause, and it needs to return a single `true` or `false` that determines if that specific iteration should be filtered from the output.
Using `.==` is not what you want on two counts: It doesn’t return a single true/false value, and it’ll be slower because it’s computing _all_ values across the _entire_ arrays on every single iteration!
Remember that Julia is fast at working element-wise across collections — there’s no need to bend over backwards trying to vectorize things.