# Problem writing data to a file after printing it

**URL:** <https://discourse.julialang.org/t/problem-writing-data-to-a-file-after-printing-it/29919>\
**Category:** New to Julia\
**Created:** [October 15, 2019, 3:14am UTC](https://discourse.julialang.org/t/problem-writing-data-to-a-file-after-printing-it/29919 "2019-10-15T03:14:23Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![rverdier](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rverdier/32/9104_2.png) [@rverdier](https://discourse.julialang.org/u/rverdier)\
**Post date:** [October 15, 2019, 3:14am UTC](https://discourse.julialang.org/t/problem-writing-data-to-a-file-after-printing-it/29919/1 "2019-10-15T03:14:23Z")

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The following script, taken from a complicated encoding/decoding application for a course I’m preparing, illustrates a problem with a data vector in which printing it and then writing it to a file creates an empty file, but omitting the printing writes the file correctly, acting as if an internal data pointer is left at the end of the data by the println statement. I want the shortest possible solution and the reason why it works, since this is a course for beginning students. Thanks in advance for any help.

```julia
# codertest.jl: Exhibit Julia problem writing data after it's been printed 
# Usage: include("desktop/codertest.jl") from directory above desktop
# If the user responds y to the question, the modified string is displayed correctly
# but the output file is empty. Otherwise, the output file has the modified data.
  bytes = Vector{UInt8}("Four score and seven years ago our fathers brought forth on this continent a new
 nation, conceived in liberty, and dedicated to the proposition that all men are
 created equal.")
  println("length(bytes)= $(length(bytes))")
  bytes[6] = 'Z' # dummy for encoding operation in real application
  println("Display modified bytes? ('y' or cr): ")
  reply = readline()
  if reply == "y" println("$(String(bytes))\n") end
  println("length(bytes)= $(length(bytes))")
  outfilename = "desktop/ctout.txt"
  outfile = open(outfilename, "w")
  write(outfile, bytes)
  close(outfile)
  println("wrote $(length(bytes)) bytes to $outfilename")
# End of codertest

```

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<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [October 15, 2019, 4:55am UTC](https://discourse.julialang.org/t/problem-writing-data-to-a-file-after-printing-it/29919/2 "2019-10-15T04:55:57Z")

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`String(bytes)` truncates the argument to `0` length. See `?String`.

It is an unfortunate interface design that will probably be fixed in 2.0. See

[https://github.com/JuliaLang/julia/issues/32528](https://github.com/JuliaLang/julia/issues/32528)

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**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [October 15, 2019, 4:59am UTC](https://discourse.julialang.org/t/problem-writing-data-to-a-file-after-printing-it/29919/3 "2019-10-15T04:59:41Z")

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may I ask what is the teaching objective here? to be careful about Julia pitfalls? I think if you have a Vector of UInt you’re probably dealing with IOStream (or like) objects so stream-mutating behavior is common (not this particular one, that is)

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<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [October 15, 2019, 8:33am UTC](https://discourse.julialang.org/t/problem-writing-data-to-a-file-after-printing-it/29919/4 "2019-10-15T08:33:09Z")

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> [@jling](#):
>
> may I ask what is the teaching objective here? to be careful about Julia pitfalls?

I don’t think that running into this issue here was intended. There should be no other pitfalls in code like this, it is pretty standard IO, except that

```julia
open(outfilename, "w") do io
    write(io, bytes)
end

```

or even

```julia
write(outfilename, bytes)

```

would be more concise and take care of accidental errors during IO.

> [@jling](#):
>
> I think if you have a Vector of UInt you’re probably dealing with IOStream (or like) objects so stream-mutating behavior is common (not this particular one, that is)

Here the issue is a surprise deviation from an otherwise commonly used naming convention,

> **[Style Guide · The Julia Language](https://docs.julialang.org/en/v1/manual/style-guide/#Append-!-to-names-of-functions-that-modify-their-arguments-1)**
>
> Documentation for The Julia Language.

There are no streams involved in `String(bytes)`. A reasonably experienced, intermediate Julia user who is not aware of `String` doing this can easily run into this issue. This is a wart that will hopefully be fixed somehow.

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<div class="post-metadata">

**Author:** ![rverdier](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rverdier/32/9104_2.png) [@rverdier](https://discourse.julialang.org/u/rverdier)\
**Post date:** [October 15, 2019, 12:53pm UTC](https://discourse.julialang.org/t/problem-writing-data-to-a-file-after-printing-it/29919/5 "2019-10-15T12:53:51Z")

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Thank you for your replies. My teaching objective is to convert  
to Julia a course that I’ve successfully taught in other  
languages. The full encode/decode app can be very useful in  
protecting files with sensitive information. I don’t search for  
Julia problems, but try to find simple solutions when they occur.  
I’ll continue to do that for a while.

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<div class="post-metadata">

**Author:** ![rverdier](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rverdier/32/9104_2.png) [@rverdier](https://discourse.julialang.org/u/rverdier)\
**Post date:** [October 15, 2019, 8:12pm UTC](https://discourse.julialang.org/t/problem-writing-data-to-a-file-after-printing-it/29919/6 "2019-10-15T20:12:42Z")

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Now that I’ve been told the source of the problem (thanks), a simple fix is to replace the line

```julia
  if reply == "y" println("$(String(bytes))\n") end
with
  if reply == "y" println("$(String(copy(bytes)))\n") end

```

with a footnote explaining to the beginning student why we write a copy rather than the original.