# Another UndefRefError: access to undefined reference

**URL:** <https://discourse.julialang.org/t/another-undefreferror-access-to-undefined-reference/126039>\
**Category:** Data\
**Created:** [February 18, 2025, 5:50pm UTC](https://discourse.julialang.org/t/another-undefreferror-access-to-undefined-reference/126039 "2025-02-18T17:50:17Z")\
**Posts on this page:** 2\
**Page:** 2

<div class="post-metadata">

**Author:** ![rocco\_sprmnt21](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rocco_sprmnt21/32/20127_2.png) [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)\
**Post date:** [April 20, 2025, 6:53am UTC](https://discourse.julialang.org/t/another-undefreferror-access-to-undefined-reference/126039/23 "2025-04-20T06:53:26Z")

</div>

forse qualcosa del genere ha creato Il problema?

```julia
m=Matrix{Array{String}}(undef, 2, 2)
julia> m[1]=["a"]
1-element Vector{String}:
 "a"

julia> m[2]=["b"]
1-element Vector{String}:
 "b"

julia> m[3]=Array{String}(undef,2)
2-element Vector{String}:
 #undef
 #undef

julia> m[4]=["c"]
1-element Vector{String}:
 "c"

julia> m
2×2 Matrix{Array{String}}:
 ["a"] [#undef, #undef]
 ["b"] ["c"]

julia> using DataFrames

using DataFrames

df=DataFrame(m,:auto)

unique(df,[:x2])

```

---

<div class="post-metadata">

**Author:** ![steveo314](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/steveo314/32/205248_2.png) [@steveo314](https://discourse.julialang.org/u/steveo314)\
**Post date:** [April 21, 2025, 12:41am UTC](https://discourse.julialang.org/t/another-undefreferror-access-to-undefined-reference/126039/24 "2025-04-21T00:41:35Z")

</div>

There weren’t `undef` values in the data frame. I need to revisit the dataset from the date I originally posted and see what caused the error.

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