Global and Local Scope inside the for and while loop

[Dsantra92]

The notebook throws a UndefVarError while accessing the global variables inside the for loop.

[shashankp]

Its not an issue with the for loop as such

[fonsp]

As @shashankp pointed out, you need to keep all assignments and modifications (+=) within a single cell - otherwise reactivity can't work.

So:

begin
    s = 1
    for i in 1:10
        s += i
    end
    s
end

but this still won't work, because for introduces a scope. If you assign to s within the for loop, then you are assigning to a local variable, also called s, that is only defined within that loop. If you want to read more, this is called an ambiguous assignment inside a soft scope: https://docs.julialang.org/en/v1/manual/variables-and-scoping/#Local-Scope Confusingly, Pluto falls in the category "non-interactive"

The solution is to use local:

s = begin
    local s = 1
    for i in 1:10
        s += i
    end
    s
end

or:

s = begin
    local my_s = 1
    for i in 1:10
        my_s += i
    end
    my_s
end

But for and while loops have so-called "functional" alternatives. The big ones are map, filter, reduce and sum:

begin
    s = 1
    s += sum(1:10) do i
        i
    end
end

Functional programming works very nicely together with Pluto - try it out!

[fonsp]

Sorry for all that strange syntax!

[Dsantra92]

@fonsp @shashankp thank you for this wonderful clarification.