Scope of variables in Julia

问题:

When I ran the Julia code below, there was an error: "UndefVarError: globalOptiValue not defined...". I thought that the globalValue is a global variable, but it is not. Thus, if I add the command "global globalValue" inside the for loop, my code will work. So, could anyone please have a look at it let me know what happened? Thanks in advance!

globalValue = 1.0;
tempValue   = 0.1;
for ii = 1:10
    # global globalValue; if I add this command, my code will work
    tempValue = 5.0;## I have a function to update "tempValue"
    if globalValue < tempValue
        globalValue = tempValue;
    end
end

回答1:

It seems you are on Julia >= 0.7, where the scoping rules have changed.

Long story short, in a local scope, such as your for-loop, global variables are only inherited for reading but not for writing. There are two ways around it:

• put an explicit global in front of the assignment (what you figured out yourself)
• wrap everything in a "global local scope" like let ... end block (globalValue isn't really a global variable anymore)

In your case, the second option would look like

let
globalValue = 1.0;
tempValue   = 0.1;
for ii = 1:10
    tempValue = 5.0;## I have a function to update "tempValue"
    if globalValue < tempValue
        globalValue = tempValue;
    end
end
end

You can find more information here:

• https://discourse.julialang.org/t/repl-and-for-loops-scope-behavior-change/ (in particular this post by Stefan, who is one of the authors of Julia
• https://docs.julialang.org/en/stable/manual/variables-and-scoping/#scope-of-variables-1

Although I find this a bit annoying myself, there are good reasons for why the change has been made. Also, on should try to avoid changing globals anyway. Let me quote the manual here (see link above):

Avoiding changing the value of global variables is considered by many to be a programming best-practice. One reason for this is that remotely changing the state of global variables in other modules should be done with care as it makes the local behavior of the program hard to reason about. This is why the scope blocks that introduce local scope require the global keyword to declare the intent to modify a global variable.