[Possible Bug?] Weird behavior with small negative indices in findnext()

Internals & Design
1

Take a look at this, y’all, and tell me if this is a bug – I’ll step up later this week to fix it and write some @tests for it if so.

julia> versioninfo()
Julia Version 1.2.0
Commit c6da87ff4b (2019-08-20 00:03 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.1 (ORCJIT, skylake)
Environment:
  JULIA_EDITOR = "C:\Users\Andre\AppData\Local\atom\app-1.40.1\atom.exe"  -a
  JULIA_NUM_THREADS = 4


julia> 

julia> 

julia> 

julia> findnext("Kitten", "Kitten", 1)
1:6

julia> # As expected. But...

julia> findnext("Kitten", "Kitten", 0)
1:6

julia> # Weird, but not totally out of the box.

julia> findnext("Kitten", "Kitten", -1)
1:6

julia> # Wait. What?

julia> findnext("Kitten", "Kitten", -2)
1:6

julia> findnext("Kitten", "Kitten", -3)
1:6

julia> findnext("Kitten", "Kitten", -4)
1:6

julia> findnext("Kitten", "Kitten", -5)
ERROR: BoundsError: attempt to access 6-element Array{UInt8,1} at index [0]
Stacktrace:
 [1] getindex at .\array.jl:728 [inlined]
 [2] _nthbyte at .\strings\search.jl:148 [inlined]
 [3] _searchindex(::Array{UInt8,1}, ::Array{UInt8,1}, ::Int64) at .\strings\search.jl:185
 [4] _searchindex(::String, ::String, ::Int64) at .\strings\search.jl:153
 [5] _search(::String, ::String, ::Int64) at .\strings\search.jl:220
 [6] findnext(::String, ::String, ::Int64) at .\strings\search.jl:256
 [7] top-level scope at none:0

julia> # WTF.

julia> findnext("supercalifragilisticexpialidocious", "supercalifragilisticexpialidocious", -5)
1:34

julia> findnext("supercalifragilisticexpialidocious", "supercalifragilisticexpialidocious", -10)
1:34

julia> findnext("supercalifragilisticexpialidocious", "supercalifragilisticexpialidocious", -15)
1:34

julia> findnext("supercalifragilisticexpialidocious", "supercalifragilisticexpialidocious", -20)
ERROR: BoundsError: attempt to access 34-element Array{UInt8,1} at index [-16]
Stacktrace:
 [1] getindex at .\array.jl:728 [inlined]
 [2] _nthbyte at .\strings\search.jl:148 [inlined]
 [3] _searchindex(::Array{UInt8,1}, ::Array{UInt8,1}, ::Int64) at .\strings\search.jl:189
 [4] _searchindex(::String, ::String, ::Int64) at .\strings\search.jl:153
 [5] _search(::String, ::String, ::Int64) at .\strings\search.jl:220
 [6] findnext(::String, ::String, ::Int64) at .\strings\search.jl:256
 [7] top-level scope at none:0

julia> # Seems like we can index up to - floor(half(array)). But I thought we weren't using negative indices for strings?

Is this normal behavior?

3

Further thinking:

IIRC, Strings are stated in the Julia manual IIRC to always have a first index of 1.

This may or may not be the case for other iterables, but since it is the case for Strings, I do think that either throwing a BoundsError for a non-positive argument, or making any non-positive argument reroute to 1 so that any negative number should work, is wise. This behavior is counterintuitive otherwise, because as it is there’s some arbitrary negative Int that once you pass it you get BoundsErrors.

1 Like
4

That does seem buggy. Do you mind filing an issue on GitHub?

2 Likes
5

Done. :slight_smile:

https://github.com/JuliaLang/julia/issues/33100

2 Likes