input-output.md

6 - Input - Output

File reading/writing

File reading/writing is similar to other languages where you first open the file, specify the modality (r read, w write or a append) and bind the file to an object, and finally operate on this object and close() it when you are done.

A better alternative is however to encapsulate the file operations in a do block that closes the file automatically when the block ends:

Write:

open("afile.txt", "w") do f  # "w" for writing
  write(f, "test\n")         # \n for newline
end

Read the whole file in a single operation:

open("afile.txt", "r") do f   # "r" for reading
  filecontent = read(f,String) # attention that it can be used only once. The second time, without reopening the file, read() would return an empty string
  print(filecontent)
end

or, reading line by line:

open("afile.txt", "r") do f
  for ln in eachline(f)
    println(ln)
  end
end

or, if you want to keep track of the line numbers:

open("afile.txt", "r") do f
   for (i,ln) in enumerate(eachline(f))
     println("$i $ln")
   end
end

Other IO

Some packages that deals with IO are:

• CSV: CSV.jl
• Web stream: HTTP.jl
• Spreadsheets (OpenDocument): OdsIO.jl
• HDF5: HDF5.jl

Some basic examples that use them are available in the DataFrame section.

While an updated, expanded and revised version of this chapter is available in "Chapter 5 - Input/Output" of Antonello Lobianco (2019), "Julia Quick Syntax Reference", Apress, this tutorial remains in active development.