Split vector into N (potentially unequal length) subvectors?

General Usage
#1

Is there a convenience function for splitting a vector into N subvectors? For example, if you had a length-10 vector and called the function to split it into 3 parts, you would get three vectors back, two with length 3 and another with length 4. The subvectors contain all the original items. I see many references to IterTools.partition but it performs a slightly different task.

I could write a little function to do this, but I wanted to see if one is already available.

#2

Iterators.partition is the only builtin Iā€™m aware of that comes close to this purpose. To get the ā€œlong tailā€ partition, rather than the ā€œshort tailā€ given by Iterators.partition, Iā€™d do something like this:

function partitionvec(x,stride,longtail::Bool=true)
    # longtail=true to lengthen the last entry with the leftovers
    # longtail=false to place the leftovers in their own entry
    stride > 0 || error("stride must be positive") # doesn't handle negative strides
    starts = firstindex(x):stride:lastindex(x)-longtail*stride # where to start each subvector
    return [view(x,starts[i]:get(starts,i+1,lastindex(x)+1)-1) for i in eachindex(starts)]
end
#3

Yeah I decided just to write a really explicit version

function makechunks(X::AbstractVector{T}, n::Int) where {T}
    L = length(X)
    c = L Ć· n
    Y = Vector{Vector{T}}(undef, n)
    idx = 1
    for i āˆˆ 1:n-1
        Y[i] = X[idx:idx+c-1]
        idx += c 
    end
    Y[end] = X[idx:end]
    return Y
end
#4

You might want to use views instead to avoid copies. Also, the whole thing can be made more compact with a comprehension:

@views function makechunks(X::AbstractVector, n::Integer)
    c = length(X) Ć· n
    return [X[1+c*k:(k == n-1 ? end : c*k+c)] for k = 0:n-1]
end
3 Likes
#5

Yes thatā€™s all fair. Itā€™s not a performance-critical bit of code for me at the moment so I wasnā€™t bothering. Iā€™m just using it to split up some items before calling a function on the groups within a @threads loop.