R/read_idx.R

In idx2r: Convert Files to and from IDX Format to Vectors, Matrices and Arrays

#' Read an IDX file
#'
#' This function allows to read an IDX file
#' 
#' @usage read_idx(file_name, endian = "big") 
#' @param file_name character vector containing the name of
#'        the file to be read
#' @param endian whether the file has "big" or "little" endian
#' @rdname read_idx
#' @examples
#' m = matrix(1:16, nrow = 4)
#' file_name = file.path(tempdir(),"m.idx")
#' write_idx(m, file_name)  
#' mr = read_idx(file_name) 
#' @export

read_idx = function(file_name, endian = "big")
{ if (!is.character(file_name)) stop("File_name must be character")
  file = file(file_name, "rb")
  # Check that the 2 first bytes contain the magic number  
  magic_1 = readBin(file, "raw", n = 1, size = 1, endian = endian)
  magic_2 = readBin(file, "raw", n = 1, size = 1, endian = endian)
  if (any(as.character(c(magic_1, magic_2)) != c("00", "00")))
    stop("The magic number doesn't match") 
  # Detect the type of data
  signed = TRUE
  type = as.character(readBin(file, "raw", n = 1, size = 1, endian = endian))
  if (type == "08")
  { size = 1
    signed = FALSE
    what = "integer"
  }
  else if (type == "09")
  { size = 1
    what = "integer"
  }
  else if (type == "0b")
  { size = 2
    what = "integer"
  }
  else if (type =="0c")
  { size = 4 
    what = "integer"
  }
   else if (type =="0d")
  { size = 4 
    what = "numeric"
  }
  else if (type == "0e")
  { size = 8
    what = "double"
  }
  else stop("Type byte found is not supported")
  # Get the number of dimensions
  number_dim = as.integer(readBin(file, "raw", n = 1, size = 1, endian = endian))
  # Get the actual dimensions
  dims = numeric(number_dim)
  for (i in seq_along(dims))
  { dims[i] = readBin(file, "integer", n = 1, size = 4, endian = endian)
  } 
  # Read the data 
  n_records = prod(dims)
  data = readBin(file, what, n = n_records,  size = size, endian = endian)
  close(file)
  if (number_dim == 1) m = data
  else
  { m = array(data, dim=dims[number_dim:1])
    m = aperm(m)
  }
  return(m)
}