# Map! - but it didn't map!

**URL:** <https://discourse.julialang.org/t/map-but-it-didnt-map/36514>\
**Category:** New to Julia\
**Created:** [March 25, 2020, 7:43pm UTC](https://discourse.julialang.org/t/map-but-it-didnt-map/36514 "2020-03-25T19:43:52Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Klaus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/klaus/32/13623_2.png) [@Klaus](https://discourse.julialang.org/u/Klaus)\
**Post date:** [March 25, 2020, 7:43pm UTC](https://discourse.julialang.org/t/map-but-it-didnt-map/36514/1 "2020-03-25T19:43:52Z")

</div>

I’ m still at the basics, so I wonder why line 2 works fine, but line 3 doesn’t:

1)—listofnumbers = [23, 45, 56, 67, 89]  
2)—listofnumbers = map(x → x + 1, listofnumbers)  
3)—map!(x → x + 1, listofnumbers)  
4)—println(listofnumbers)

I got the following message, but I still don’t know what’s wrong with line number 3:

ERROR: LoadError: BoundsError: attempt to access ()  
at index [1]  
Stacktrace:  
[1] getindex(::Tuple, ::Int64) at .\tuple.jl:24  
[2] map\_n!(::var"#33#34", ::Array{Int64,1}, ::Tuple{}) at .\abstractarray.jl:2123  
[3] map!(::var"#33#34", ::Array{Int64,1}) at .\abstractarray.jl:2152  
[4] top-level scope at C:\julia\Julia-1.3.1\examples\extra001.jl:10  
[5] include at .\boot.jl:328 [inlined]  
[6] include\_relative(::Module, ::String) at .\loading.jl:1105  
[7] include(::Module, ::String) at .\Base.jl:31  
[8] include(::String) at .\client.jl:424  
[9] top-level scope at none:0  
in expression starting at C:\julia\Julia-1.3.1\examples\extra001.jl:10

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<div class="post-metadata">

**Author:** ![bashonubuntu](https://avatars.discourse-cdn.com/v4/letter/b/f19dbf/32.png) [@bashonubuntu](https://discourse.julialang.org/u/bashonubuntu)\
**Post date:** [March 25, 2020, 7:56pm UTC](https://discourse.julialang.org/t/map-but-it-didnt-map/36514/2 "2020-03-25T19:56:41Z")

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Hello,

The first step is always to check the help

````julia
help?> map!
search: map! asyncmap! map_coefficients_inplace! map mapcols mapfoldr mapfoldl map_rows mapslices mapreduce map_coefficients

  map!(function, destination, collection...)

  Like map, but stores the result in destination rather than a new collection. destination must be at least as large as the 
  first collection.

  Examples
  ≡≡≡≡≡≡≡≡≡≡

  julia> a = zeros(3);
  
  julia> map!(x -> x * 2, a, [1, 2, 3]);
  
  julia> a
  3-element Array{Float64,1}:
   2.0
   4.0
   6.0

  ───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────  

  map!(f, values(dict::AbstractDict))

  Modifies dict by transforming each value from val to f(val). Note that the type of dict cannot be changed: if f(val) is not  
  an instance of the key type of dict then it will be converted to the key type if possible and otherwise raise an error.      

  Examples
  ≡≡≡≡≡≡≡≡≡≡

  ```jldoctest julia> d = Dict(:a => 1, :b => 2) Dict{Symbol,Int64} with 2 entries: :a => 1 :b => 2

  julia> map!(v -> v-1, values(d)) Dict{Symbol,Int64} with 2 entries: :a => 0 :b => 1 ```

````

Based on the above, we need a destination for map! (the ! symbol stands for in-place operations so it doesn’t allocate a new array for storing the result. It just mutates the existing array.).

What you need then is

```julia
julia> listofnumbers = [23, 45, 56, 67, 89]
5-element Array{Int64,1}:
 23
 45
 56
 67
 89

julia> map!(x -> x + 1, listofnumbers, listofnumbers)
5-element Array{Int64,1}:
 24
 46
 57
 68
 90

```

You could also create a separate destination object, like so

```julia

julia> listofnumbers = [23, 45, 56, 67, 89]
5-element Array{Int64,1}:
 23
 45
 56
 67
 89

julia> a = fill(0, 5)
5-element Array{Int64,1}:
 0
 0
 0
 0
 0

julia> map!(x -> x + 1, a, listofnumbers)
5-element Array{Int64,1}:
 24
 46
 57
 68
 90

julia> a
5-element Array{Int64,1}:
 24
 46
 57
 68
 90

```

Note that `a = fill(0, 5)` at the beginning and `a = [24 46 57 68 90]` at the end. This is because you were mutating` a` in-place.

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<div class="post-metadata">

**Author:** ![Klaus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/klaus/32/13623_2.png) [@Klaus](https://discourse.julialang.org/u/Klaus)\
**Post date:** [March 25, 2020, 8:15pm UTC](https://discourse.julialang.org/t/map-but-it-didnt-map/36514/3 "2020-03-25T20:15:12Z")

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🙂Thanks a lot! It works!  
(… and I leaned more than one thing - with your help\>? - it was new to me, too)

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<div class="post-metadata">

**Author:** ![bashonubuntu](https://avatars.discourse-cdn.com/v4/letter/b/f19dbf/32.png) [@bashonubuntu](https://discourse.julialang.org/u/bashonubuntu)\
**Post date:** [March 25, 2020, 9:08pm UTC](https://discourse.julialang.org/t/map-but-it-didnt-map/36514/4 "2020-03-25T21:08:57Z")

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Awesome, glad it worked!