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With arrays, you can filter based on a condition:
[i for i=1:10 if isodd(i) ]
returns:
5-element Vector{Int64}:
1
3
5
7
9
but attempting something similar with map results in nothing values:
julia> map(1:10) do i
isodd(i) ? i : nothing
end
reutrns:
10-element Vector{Union{Nothing, Int64}}:
1
nothing
3
nothing
5
nothing
7
nothing
9
nothing
566
List comprehension is more concise and easier to read as compared to map. List comprehension are used when a list of results is required as map only returns a map object and does not return any list. Map is faster in case of calling an already defined function (as no lambda is required).
Generally "map" means converting a series of inputs to an equal length series of outputs while "reduce" means converting a series of inputs into a smaller number of outputs. What people mean by "map-reduce" is usually construed to mean "transform, possibly in parallel, combine serially".
map creates a new array by transforming every element in an array individually. filter creates a new array by removing elements that don't belong. reduce , on the other hand, takes all of the elements in an array and reduces them into a single value. Just like map and filter , reduce is defined on Array.
map is a one-to-one mapping. So I'm afraid you can't do the same with map. Maybe what you are looking for is just filter?
julia> a = 1:10
1:10
julia> filter(isodd, a)
5-element Vector{Int64}:
1
3
5
7
9
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