Julia: what's the difference between map and broadcast?

Question

I have found some discussions on Julia Users group but they were too technical for me.

I would like to know what are the criteria to choose between the two.

I am following the JuliaBox tutorial but it doesn't explain much. Thanks

Answer 1

Both methods apply a function to their arguments. The difference between the two is how they treat multidimensional arrays.

map tries to zip the arrays column first, creating an iterator of tuples, which are used as arguments to the given function. Thus

julia> map(+, [1,2,3,4], [10, 20, 30, 40])
4-element Array{Int64,1}:
 11
 22
 33
 44

while

julia> map(+, [1,2,3,4], [10 20; 30 40])
4-element Array{Int64,1}:
 11
 32
 23
 44

Notice the element pairing when changing the layout of the second array. On the other hand, broadcast does an element-wise application of the given function. If the two array arguments have the same dimensions, the pairing of elements is similar to map.

julia> broadcast(+, [1,2,3,4], [10, 20, 30, 40])
4-element Array{Int64,1}:
 11
 22
 33
 44

Otherwise, broadcast checks if the two arrays match in at least one dimension and that the other dimension of at least one array is equal to 1. If this condition is satisfied, broadcast extends the arrays to a common size by duplicating the array data along the dimension equal to 1. This is how you get

julia> broadcast(+, [1, 2, 3, 4], [10 20 30 40])
4×4 Array{Int64,2}:
 11  21  31  41
 12  22  32  42
 13  23  33  43
 14  24  34  44

Answer 2

map and broadcast are different when dealing with multiple collections of different dimensions. While broadcast will try to cast all the objects to a common dimension, map will directly apply the given function elementwise.

julia> map(+, 1, [2,2,2])
1-element Array{Int64,1}:
 3

julia> broadcast(+, 1, [2,2,2])
3-element Array{Int64,1}:
 3
 3
 3

the broadcast example has the same result as map(+, [1,1,1], [2,2,2]).

Also note the behavior of broadcast when failing at finding a common dimension between two arguments:

julia> broadcast(+, [1,2,3], [2 2; 2 0])
ERROR: DimensionMismatch("arrays could not be broadcast to a common size")
Stacktrace:
 [1] _bcs1 at ./broadcast.jl:439 [inlined]
 [2] _bcs at ./broadcast.jl:433 [inlined]
 [3] broadcast_shape at ./broadcast.jl:427 [inlined]
 [4] combine_axes at ./broadcast.jl:422 [inlined]
 [5] instantiate at ./broadcast.jl:266 [inlined]
 [6] materialize at ./broadcast.jl:748 [inlined]
 [7] broadcast(::typeof(+), ::Array{Int64,1}, ::Array{Int64,2}) at ./broadcast.jl:702
 [8] top-level scope at none:0

julia> map(+, [1,2,3], [2 2; 2 0])
3-element Array{Int64,1}:
 3
 4