Julia: High Performance Programming

Array views

Julia, similarly to most scientific languages, has a very convenient syntax for array slicing. Consider the following example that sums each column of a two-dimensional matrix. First, we will define a function that sums the elements of a vector to produce a scalar. We will then use this function inside a loop to sum the columns of a matrix, passing each column one by one to our vector adder, as follows:

function sum_vector(x::Array{Float64, 1})
   s = 0.0
   for i = 1:length(x)
      s = s + x[i]
   end
   return s
end

function sum_cols_matrix(x::Array{Float64, 2})
   num_cols = size(x, 2)
   s = zeros(num_cols)
   for i = 1:num_cols
      s[i] = sum_vector(x[:, i])
  end
  return s
end

The x[:, j] syntax denotes all the row elements of the jth column. In other words, it ...