Julia merge-sort implementation not working correctly

user_3204458 0
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I'm not quite sure why my merge-sort implementation is not working.

merge_sort takes as arguments an array A, and starting and final indices p and r. If I try to run merge_sort(A, 1, 9) on A = [1, 64, 64, 315, 14, 2, 3, 4, 5], A will become A = [1, 1, 1, 1, 1, 2, 2, 4, 5]. I'm trying to use a sentinel to detect whether the L and R arrays have been exhausted.

Here is the code:

function merge_sort(A, p, r)
    if p < r
        q = floor(Int, (p+r)/2)
        merge_sort(A, p, q)
        merge_sort(A, q+1, r)
        merge(A, p, q, r)
    end
end



function merge(A, p, q, r)
    n1 = q-p+1
    n2 = r-q
    L = []
    R = []

    for i = 1:n1
      push!(L, A[p+1-1])
    end

    for j = 1:n2
      push!(R, A[q+j])
    end

    sentinel = 123456789
    push!(L, sentinel)
    push!(R, sentinel)
    i=1
    j=1

    for k=p:r
      if L[i] <= R[j]
         A[k] = L[i]
         i = i+1
      else
        A[k] = R[j]
        j = j+1

      end
    end
end
julia mergesort sorting
1 Answers

You have a typo in push!(L, A[p+1-1]) which should be push!(L, A[p+i-1]).

Here is a bit cleaned up version of your code (but I did not try to fully optimize it to retain your logic):

function merge_sort!(A, p = 1, r = length(A))
    if p < r
        q = div(p+r, 2)
        merge_sort!(A, p, q)
        merge_sort!(A, q+1, r)
        merge!(A, p, q, r)
    end
    A
end

function merge!(A, p, q, r)
    sentinel = typemax(eltype(A))
    L = A[p:q]
    R = A[(q+1):r]
    push!(L, sentinel)
    push!(R, sentinel)
    i, j = 1, 1
    for k in p:r
      if L[i] <= R[j]
          A[k] = L[i]
          i += 1
      else
          A[k] = R[j]
          j += 1
      end
    end
end
user_12695670
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