print() and @show for intermediate results

[SubhadityaMukherjee]

For some reason, outputs do not display when it comes to loops.
Say I have this code. I followed the instructions are removed @info but it only prints to console and not here. Is there a workaround or am I doing something wrong? Since this is a local variable I cant even bind it to display elsewhere...

#Package used "OpenAIGym"
begin
	global s
	for i ∈ 1:20
	  T = 0
	  R = run_episode(env, RandomPolicy()) do (s, a, r, s′)
	    #render(env)
	    T += 1
	  end
		R2 = R
	  "Episode $i finished after $T steps. Total reward: $R" #Need this to display here 
	end
	close(env)
end

On the other hand something like this works.

begin 
	temp = "123"
	temp
end

PS. I love this package. ❤️

[fonsp]

Hi Subhaditya,

Thanks for the feedback! Cell output in Pluto is very minimal:
one cell contains one expression that gives one result. This result is displayed in the notebook.

A begin block shows its output because the begin block returns its last contained value

So:

123 == begin
    456
    123
end

is true. The same goes for the if, let and try blocks. (Try it!)

The for and while blocks do not return anything. (Well... they return nothing.) The argument is: the block runs multiple times, so which of those should it return?

Examples

Say that we have

for i in 1:20
    R = i+100
    T = R^2
    md"hello $R and $T"
end

then this will not return anything -> it will not display anything.

If you want to get something from the final iteration:

let
    x =  nothing
    for i in 1:20
        R = i+100
        T = R^2
        # something
        x = md"hello $R and $T"
    end
    x
end

If you want to see all outputs:

map(1:20) do i
    R = i+100
    T = R^2
    md"hello $R and $T"
end

this will show you the 20-element array, you can click on it to see the intermediate values.

To print intermediate progress

This is not yet supported inside Pluto, but there is a plan.

What you can do is print(...) or @show inside your for loop, and look at the terminal where you launched Pluto to check the output.

[SubhadityaMukherjee]

Oh okay that is great then! Thank you for the detailed breakdown :)
Do I close the issue?

[fonsp]

I'd like to leave it open to remind myself to add this to the FAQ :)

[SubhadityaMukherjee]

Okay 👍
There. Reopened it. Sorry about that

[fonsp]

No worries!

Let me know if you have more questions! Sometimes it's hard for me to see which things are clear and which aren't.

[ilyagr]

If you want to discourage people from using print in their notebooks, one option that seems nice to me is if stdout and stderr was redirected to a console presented similarly to the Live docs screen, sort of like the browser's JS console. You could also consider showing the input to julia there (which cell is executing), and reporting when the execution is done and the kernel becomes idle.

Some way to see the Julia console output in the browser seems important, especially so for users who are using Binder, or some other installation that doesn't show the console Pluto started from.

Some things that are nice to be able to see:

• How is the execution of Pkg.add is doing? Did it fail? Is everything frozen because the package is compiling? Did the compilation fail terribly and kill the Julia process?

• Same for importing a package.

• @warn output.

• Some library functions, like Plots.plotattr just print to stdout instead of returning an object.

• Debugging by putting @show in various places. (Update: I should explicitly mention that this is the part addressed by @fonsp's plan.)

Update: Also, I wanted to add that I realize that dealing with this stuff is not quite creating the future of computing. :). It would be really nice, though.

An example: I tried using Gadfly recently and, for some reason, it uses @warn instead of raising an exception when it doesn't recognize an argument to plot. So, until I accidentally pulled up the terminal with the console, I couldn't figure out why my plots aren't working. Luckily, I wasn't using Binder, so at least there was a console to look at.

Update 2: if you are not altogether opposed to the console idea, one more suggestion: it can be closed by default, but have an indicator turn red if anything is printed to stderr (as opposed to stdout), so that the user knows there might be something important to see there.

[SubhadityaMukherjee]

@ilyagr Yeah that would be nice.
The issue is that these kind of outputs are essential while training a machine learning model but not so for other code I guess.
I do have a terminal open and I can view it though so thats not an issue but anyway.

[ixzh]

and to sumplement, how to completely supress any output (such that the cell appears to be a blank line after execution). I call a GC() seems to do the trick but not very nice.

better still if the line itself can be collapsed if there is no output...this is for formatting purpose as such cells create extra spacing between paras

[fonsp]

and to sumplement, how to completely supress any output

add a semicolon ; at the end of your code.

GC is unrelated to displaying output, you shouldn't have to use it yourself. The reason that it suppresses output is that GC() returns nothing, which displays as an empty line in Pluto and the REPL. So adding GC() is equivalent to just adding nothing at the end of a cell.

[fonsp]

@ilyagr Thanks for the summary! These are important reasons to include some form of a terminal in Pluto. I like your suggestion:

it can be closed by default, but have an indicator turn red if anything is printed to stderr (as opposed to stdout), so that the user knows there might be something important to see there.

[fonsp]

Also see the zulip thread about this subject here: https://julialang.zulipchat.com/#narrow/stream/243342-pluto.2Ejl/topic/Show.20messages.20in.20notebook.3F
a copy paste of the messages is here: https://gist.github.com/fonsp/d93cb6cea5ab70bdf5cd67ed033a11e4