# Functions in a loop over string array usually returns nothing

**URL:** <https://discourse.julialang.org/t/functions-in-a-loop-over-string-array-usually-returns-nothing/43689>\
**Category:** New to Julia\
**Created:** [July 26, 2020, 12:49am UTC](https://discourse.julialang.org/t/functions-in-a-loop-over-string-array-usually-returns-nothing/43689 "2020-07-26T00:49:08Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![nstgc](https://avatars.discourse-cdn.com/v4/letter/n/e0b2c6/32.png) [@nstgc](https://discourse.julialang.org/u/nstgc)\
**Post date:** [July 26, 2020, 12:49am UTC](https://discourse.julialang.org/t/functions-in-a-loop-over-string-array-usually-returns-nothing/43689/1 "2020-07-26T00:49:08Z")

</div>

I’m not sure the best way to summarize this (hence the bad title), but I have a list of directions (a string array) and I want to do something with them. First I make a function to generate the list

```julia
function list_dirs()
    return filter(x-> isdir(x) && ! occursin(r"^\.",x), readdir())
end

```

Note that have `return` in there does not seem to make any difference (I guess Julia can infer what I want?). If I then execute

```julia
for directory in list_dirs()
    cd(pwd,directory)
    println("it ran")
end

```

I get a bunch of “it ran” and nothing else. However `cd.(pwd,list_dirs())` works as expected. Also `length(list_dirs())` makes sense. `typeof(list_dirs())` is also outputing `Array{String,1}` as expected. Also `cd(pwd,list_dirs()[1])` works.

To make things stranger (at least for me),

```julia
for directory in list_dirs()
    println(directory)
end

```

prints the array elements. However this is the only function that seems to be able to act on the array. Trying to instead loop over `dir_list = list_dirs()` doesn’t help.

To me it feels like I’m missing something fundamental. If I had to guess, I’d say it’s an issue of scope. This is especially frustrating since I recently wrote another script which loops over a file list and operates on the elements in that list.

---

<div class="post-metadata">

**Author:** ![CameronBieganek](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cameronbieganek/32/6915_2.png) [@CameronBieganek](https://discourse.julialang.org/u/CameronBieganek)\
**Post date:** [July 26, 2020, 1:33am UTC](https://discourse.julialang.org/t/functions-in-a-loop-over-string-array-usually-returns-nothing/43689/2 "2020-07-26T01:33:48Z")

</div>

`pwd()` returns a string, rather than actually printing the working directory. Since `for` loops return `nothing`, you don’t get to see that string unless you explicitly print it. Try this:

```julia
for directory in list_dirs()
    cd(println ∘ pwd, directory)
end

```

---

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [July 26, 2020, 1:36am UTC](https://discourse.julialang.org/t/functions-in-a-loop-over-string-array-usually-returns-nothing/43689/3 "2020-07-26T01:36:31Z")

</div>

> [@nstgc](#):
>
> Note that have `return` in there does not seem to make any difference (I guess Julia can infer what I want?).

Right, the last expression in a function is automatically returned. Using the `return` keyword at the end of a function is optional, since it doesn’t change the behavior of the function at all. The `return` keyword is only necessary if you want to return early, as in:

```julia
function foo()
  if something()
    return
  end
  do_something_else()
end

```

---

<div class="post-metadata">

**Author:** ![nstgc](https://avatars.discourse-cdn.com/v4/letter/n/e0b2c6/32.png) [@nstgc](https://discourse.julialang.org/u/nstgc)\
**Post date:** [July 26, 2020, 12:38pm UTC](https://discourse.julialang.org/t/functions-in-a-loop-over-string-array-usually-returns-nothing/43689/4 "2020-07-26T12:38:21Z")

</div>

Thanks, I feel really stupid now, but that was a good lesson so I also feel less stupid than before.

I tend to proto-type everything in the REPL using basic commands (I’m pretty sure that’s normal). In this case had I just run what I wanted in the first place it would have worked.

Julia can really do some nice things with scripting. Not that I need to tell anyone (just about everyone is more familiar with Julia than I am).

@rdeits

Thanks for clearing that up. That makes sense. I guess I’m overly used to explicitly declaring what needs to be returned. Kind of like how after 4 years I’m still uncomfortable writing `n = 1` rather than `Int64 n = 1`.