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Document map behavior on sparse data structures #44233

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Document map behavior on sparse data structures #44233

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a71efe0
Document `map` behavior
Feb 17, 2022
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clearer wording
Feb 17, 2022
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Merge branch 'master' into Document-sparsity-behavior
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Merge branch 'master' into Document-sparsity-behavior
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document other functions
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5 changes: 5 additions & 0 deletions base/abstractarray.jl
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Expand Up @@ -2940,6 +2940,11 @@ mapany(f, itr) = Any[f(x) for x in itr]
Transform collection `c` by applying `f` to each element. For multiple collection arguments,
apply `f` elementwise, and stop when when any of them is exhausted.

Note that `f` makes no guarantees about the order in which `f` is called on elements. In
addition, `f` is assumed to be [pure](https://en.wikipedia.org/wiki/Pure_function). Using
an impure function together with `f` can cause bugs when working with some data structures,
e.g. sparse or diagonal arrays, as `f` will only be called once on duplicated elements.
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it will be called once on default 0 but if you have duplicates that are non-default zeros it will be called multiple times.

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it will be called once on default 0 but if you have duplicates that are non-default zeros it will be called multiple times.

I've edited to say that it "May" call it only once per duplicate element, and clarify that calling map with an impure function is undefined behavior.


See also [`map!`](@ref), [`foreach`](@ref), [`mapreduce`](@ref), [`mapslices`](@ref), [`zip`](@ref), [`Iterators.map`](@ref).

# Examples
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