Slicing the matrix should give a sparse array

[giordano]

julia> m = 10 # Number rows
10

julia> n = 10 # Number columns
10

julia> nz = floor(Int, m * 1.5) # Number of non-zero elements
15

julia> rowptr = vcat(sort!(rand(0:nz, m)), nz);

julia> colval = rand(0:(n-1), nz);

julia> nzval = randn(nz);

julia> A = SparseMatrixCSR{0}(m, n, rowptr, colval, nzval)
10×10 SparseMatrixCSR{0, Float64, Int64} with 15 stored entries:
  [1,  5]  =  0.48371
  [2,  3]  =  1.72291
  [3,  8]  =  0.707065
  [3,  5]  =  0.622569
  [3,  7]  =  1.57136
  [4,  8]  =  -0.262692
  [4,  5]  =  -0.00643953
  [5,  6]  =  -1.15259
  [6,  3]  =  0.0652688
  [6,  2]  =  1.02735
  [7,  2]  =  0.169683
  [9,  6]  =  -0.648538
  [9,  4]  =  -0.647156
  [10,  6]  =  -1.28313
  [10,  7]  =  0.116641

julia> A[1:3, :]
3×10 Matrix{Float64}:
 0.0  0.0  0.0      0.0  0.48371  0.0  0.0      0.0  0.0  0.0
 0.0  0.0  1.72291  0.0  0.0      0.0  0.0      0.0  0.0  0.0
 0.0  0.0  0.0      0.0  0.0      0.0  1.57136  0.0  0.0  0.0

This isn't very convenient, especially since this data structure should allow having a compact representation of a slice of rows. The standard SparseMatrixCSC has compact slicing for both rows and columns:

julia> B = sprandn(rng, 10, 10, 0.2)
10×10 SparseMatrixCSC{Float64, Int64} with 19 stored entries:
   ⋅           ⋅           ⋅         ⋅   -0.0684167   0.0414316    ⋅          ⋅        1.79099    ⋅ 
   ⋅           ⋅           ⋅         ⋅     ⋅           ⋅           ⋅          ⋅         ⋅        0.451416
   ⋅           ⋅           ⋅         ⋅     ⋅          0.245215     ⋅          ⋅         ⋅         ⋅ 
   ⋅         -0.880614   -0.149248   ⋅     ⋅           ⋅           ⋅          ⋅         ⋅         ⋅ 
   ⋅           ⋅         -0.673498   ⋅     ⋅           ⋅           ⋅          ⋅       -0.48382    ⋅ 
   ⋅           ⋅           ⋅         ⋅     ⋅           ⋅         -0.679016  -1.93712  -0.468799   ⋅ 
   ⋅           ⋅           ⋅         ⋅     ⋅           ⋅           ⋅          ⋅         ⋅         ⋅ 
  0.0442601   0.0607505    ⋅         ⋅     ⋅           ⋅           ⋅          ⋅         ⋅         ⋅ 
 -0.839911     ⋅           ⋅         ⋅     ⋅           ⋅           ⋅          ⋅        0.715919  0.151704
   ⋅           ⋅           ⋅         ⋅     ⋅         -1.83668     0.267189    ⋅         ⋅         ⋅ 

julia> B[1:3, :]
3×10 SparseMatrixCSC{Float64, Int64} with 5 stored entries:
  ⋅    ⋅    ⋅    ⋅   -0.0684167  0.0414316   ⋅    ⋅   1.79099   ⋅ 
  ⋅    ⋅    ⋅    ⋅     ⋅          ⋅          ⋅    ⋅    ⋅       0.451416
  ⋅    ⋅    ⋅    ⋅     ⋅         0.245215    ⋅    ⋅    ⋅        ⋅ 

julia> B[:, 1:3]
10×3 SparseMatrixCSC{Float64, Int64} with 6 stored entries:
   ⋅           ⋅           ⋅ 
   ⋅           ⋅           ⋅ 
   ⋅           ⋅           ⋅ 
   ⋅         -0.880614   -0.149248
   ⋅           ⋅         -0.673498
   ⋅           ⋅           ⋅ 
   ⋅           ⋅           ⋅ 
  0.0442601   0.0607505    ⋅ 
 -0.839911     ⋅           ⋅ 
   ⋅           ⋅           ⋅