Julia waits for function to finish before printing message in a loop

Question

I have a function in Julia that requires to do things in a loop. A parameter is passed to the loop, and the bigger this parameter, the slower the function gets. I would like to have a message to know in which iteration it is, but it seems that Julia waits for the whole function to be finished before printing anything. This is Julia 1.4. That behaviour was not on Julia 1.3. A example would be like this

function f(x) rr=0.000:0.0001:x aux=0 for r in rr print(r, " ") aux+=veryslowfunction(r) end return aux end

As it is, f, when called, does not print anything until it has finished.

Przemyslaw Szufel · Accepted Answer · 2020-04-02 00:13:07Z

8

You need to add after the print command:

flush(stdout)

Explanation

The standard output of a process is usually buffered. The particular buffer size and behavior will depend on your system setting and perhaps the terminal type. By flushing the buffer you make sure that the contents is actually sent to the terminal.

edited Apr 2, 2020 at 0:13

answered Apr 2, 2020 at 0:02

Przemyslaw Szufel

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3 Comments

Przemyslaw Szufel Over a year ago

I replicated your example and on my machine I do not need to flush. The behavior of your code will differ across operating systems and consoles. But this "strangeness" is not Julia-specific. Is my answer flush(stdout) working for you?

wpkzz Over a year ago

Yeah, it is... but with julia 1.3 was not needed. And not expected.

Przemyslaw Szufel Over a year ago

When you upgraded other things might have changed. For example on Julia 1.4 I can not replicate it. Such things happen when you work with streams - be it pipes or files - always flush if you want to be sure and do not rely on system config.

phipsgabler · Accepted Answer · 2020-04-02 08:08:35Z

Alternatively, you can also use a library like ProgressLogging.jl (needs TerminalLoggers.jl to see actual output), or ProgressMeter.jl, which will automatically update a nicely formatted status bar during each step of the loop.

For example, with ProgressMeter, a call to

function f(x) rr=0.000:0.0001:x aux=0 @showprogress for r in rr aux += veryslowfunction(r) end return aux end

will show something like (in the end):

Progress: 100%|██████████████████████████████████████████████████████████████| Time: 0:00:10

Nils Gudat · Accepted Answer · 2020-04-02 06:42:50Z

Again I can't reproduce the behaviour in my terminal (it always prints), but I wanted to add that for these types of situations the @show macro is quite neat:

julia> function f(x) rr=0.000:0.0001:x aux=0 for r in rr @show r aux+=veryslowfunction(r) end return aux end f (generic function with 1 method) julia> f(1) r = 0.0 r = 0.0001 r = 0.0002 ...

It uses println under the hood:

julia> using MacroTools julia> a = 5 5 julia> prettify(@expand(@show a)) quote Base.println("a = ", Base.repr($(Expr(:(=), :ibex, :a)))) ibex end

println without flushing will be shown or not. However, many caching mechanism implementations might cache line-by-line and hence it has a bigger chance of being executed than print.

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