# Indexing a DataFrame with a boolean DataFrame

**URL:** <https://discourse.julialang.org/t/indexing-a-dataframe-with-a-boolean-dataframe/83108>\
**Category:** Data\
**Tags:** dataframes\
**Created:** [June 21, 2022, 8:26am UTC](https://discourse.julialang.org/t/indexing-a-dataframe-with-a-boolean-dataframe/83108 "2022-06-21T08:26:49Z")\
**Posts on this page:** 1\
**Showing post:** 1

<div class="post-metadata">

**Author:** ![CarloLucibello](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/carlolucibello/32/3278_2.png) [@CarloLucibello](https://discourse.julialang.org/u/CarloLucibello)\
**Post date:** [June 21, 2022, 8:26am UTC](https://discourse.julialang.org/t/indexing-a-dataframe-with-a-boolean-dataframe/83108/1 "2022-06-21T08:26:49Z")

</div>

I would like to set some values across a whole datafame using a boolean mask, similarly to this example with plain arrays:

```julia
julia> x = rand(3, 3)
3×3 Matrix{Float64}:
 0.0559191 0.249678 0.165318
 0.845124 0.83681 0.248268
 0.291518 0.0380153 0.205101

julia> x[x .< 0.5] .= 0;

julia> x
3×3 Matrix{Float64}:
 0.0 0.0 0.0
 0.845124 0.83681 0.0
 0.0 0.0 0.0

```

It turns out that this pattern is not allowed with dataframes though:

```julia
julia> df = DataFrame(rand(3,3), :auto)
3×3 DataFrame
 Row │ x1 x2 x3       
     │ Float64 Float64 Float64  
─────┼───────────────────────────────
   1 │ 0.045519 0.468771 0.387336
   2 │ 0.0133922 0.383619 0.418809
   3 │ 0.870746 0.898979 0.628106

julia> df .< 0.5
3×3 DataFrame
 Row │ x1 x2 x3    
     │ Bool Bool Bool  
─────┼─────────────────────
   1 │ true true true
   2 │ true true true
   3 │ false false false

julia> df[df .< 0.5] .= 0
ERROR: MethodError: no method matching getindex(::DataFrame, ::DataFrame)
Closest candidates are:
  getindex(::AbstractDataFrame, ::CartesianIndex{2}) at ~/.julia/packages/DataFrames/zqFGs/src/other/broadcasting.jl:3
  getindex(::AbstractDataFrame, ::Integer, ::Colon) at ~/.julia/packages/DataFrames/zqFGs/src/dataframerow/dataframerow.jl:210
  getindex(::AbstractDataFrame, ::Integer, ::Union{Colon, Regex, AbstractVector, All, Between, Cols, InvertedIndex}) at ~/.julia/packages/DataFrames/zqFGs/src/dataframerow/dataframerow.jl:208
  ...
Stacktrace:
 [1] maybeview
   @ ./views.jl:145 [inlined]
 [2] dotview(::DataFrame, ::DataFrame)
   @ Base.Broadcast ./broadcast.jl:1200
 [3] top-level scope
   @ REPL[39]:1

```

What is an alternative syntax?

---

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