function csCumsum (ptr, c, n) {
  // variables
  let i
  let nz = 0

  for (i = 0; i < n; i++) {
    // initialize ptr @ i
    ptr[i] = nz
    // increment number of nonzeros
    nz += c[i]
    // also copy p[0..n-1] back into c[0..n-1]
    c[i] = ptr[i]
  }
  // finalize ptr
  ptr[n] = nz
  // return sum (c [0..n-1])
  return nz
}
It sets the p[i] equal to the sum of c[0] through c[i-1].

@param {Array}   ptr             The Sparse Matrix ptr array
@param {Array}   c               The Sparse Matrix ptr array
@param {Number}  n               The number of columns