Clearing a variable?

New to Julia
1

I believe this has been asked and answered before; however I’ve gone through many of the related posts and I still can’t find how to do it. In my case, I had an array called T, which after a while I didn’t need. What I wanted to do was to create a function called T instead. I was told that

julia> T = nothing

would clear it, which it sort of does - at least it clears all memory allocated to it. But if I then try

julia> T(n) = n+1

say, I receive the error:

ERROR: cannot define function T; it already has a value

Is there any way of completely removing a user variable from the namespace? Or of writing over it?

I know I could stop and restart Julia, but I have a number of variables and functions I’ve been playing around with (I do a lot of experimenting in the REPL), that I don’t want to have to rewrite in a new session. So for me, clearing or removing a variable would be very useful indeed. Thanks!

1 Like
2

No

No, variable can only be declared const at their first definition (and function-defs do this implicitly).

(Also, a variable which was declared const cannot be bound to a value of another type.)

Sorry!

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3

Instead, you can do this way:

T = (n)->n+1
3 Likes
4

Thank you - well, I guess that means I might have to re-consider how I use Julia. In my current session varinfo() lists about 38 user-defined variables and functions. This means that I can’t re-use, as it were, any previously used names for something else. No doubt this is a design choice made for excellent reasons.

5

Oh now that’s clever: so Julia thinks that T is still a variable, even though it is in fact a function?

6

Variable bind to things, which are all instances of types. E.g. 1 is such an instance, or as well the function sin. Variables (or more accurately “bindings”) which are const cannot be redefined. Also a variable which was non-const before cannot become const later. And as I said before, function definitions implicitly make const bindings, thus the problem you ran into.

However, just non-const variable can be re-assigned no problem:

julia> a = 5
5

julia> a = "Asdf"
"Asdf"

julia> const a = [1] # errors because of above explained stuff
ERROR: cannot declare a constant; it already has a value
Stacktrace:
 [1] top-level scope at REPL[3]:1

julia> a() = 1 # errors
ERROR: cannot define function a; it already has a value
Stacktrace:
 [1] top-level scope at none:0
 [2] top-level scope at REPL[4]:1

All in all, it’s sometimes a little bit annoying but not really all that much of a problem.

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7

Note this is also almost never a problem if you follow naming conventions (like those suggested in the style guide) Aside from readability, it helps make sure your bindings don’t bump into each other :wink:

3 Likes
8

Yes, that’s very good advice indeed - thank you! The trouble is that I’m often chopping and changing between different language REPL’s (currently I am running Julia, Python, GNU Maxima, GNU Octave, all in different tmux panes) and I tend to use the same slapdash style in all of them. And of those four, I believe Julia is the only one without a variable clearing mechanism. However, I could certainly do with a slightly more disciplined approach, and maybe if I am more disciplined in my use of Julia I could be equally more disciplined in my use of other systems…