I’ve seen a few examples where one can use a bare underscore in an expression, e.g., julia> map(_ -> "hello", 1:4) 4-element Array{String,1}: "hello" "hello" "hello" "hello" julia> ["hello" for _ in 1:4] 4-element Array{String,1}: "hello" "hello" "hello" "hello" julia> for _ in 1:4 …

_ is used when you syntactically need a variable, but never use it. This is because there are lots of times (like your examples above) where you want a loop, which syntactically would need a variable to be defined. The reason for this convention is that knowing that a variable won’t be used makes co…

[image] Oscar_Smith: _ is used when you syntactically need a variable, but never use it. Yeah, I get that. My question is, exactly when does the Julia parser allow a bare underscore and when does it not? Perhaps there are some interesting use cases that I’m missing! :slight_smile:

The error message gives a pretty decent description. You can never use _ as an rvalue, ie as something that goes on the right half of an equal sign. c++11 - What exactly is an R-Value in C++? - Stack Overflow does a pretty good job explaining (for c, but the terminology is the same).

Thanks. That SO post is helpful. That makes sense for the array comprehension and the for loop, since those can be written using assignment syntax: julia> ["hello" for _ = 1:4] 4-element Array{String,1}: "hello" "hello" "hello" "hello" julia> for _ = 1:4 println("hello") end …

[image] CameronBieganek: Why is the underscore in that example considered an lvalue? You don’t ever use its value. By using the name _ for an argument, you are saying “I will never use this”.

[image] StefanKarpinski: You don’t ever use its value. To be honest, I don’t see how that has anything to do with whether something is an lvalue. Presumably, in the expression x -> 2x, the x on the left is an lvalue, but it’s not because “You don’t ever use its value.” My guess here is that f…

when I write f(x) = ... blah blah blah ... the variable x is an l-value because it get’s assigned to when I do a function call. That is, when I say f(2), a function scope will be created and inside that scope, there will be an assignment x = 2. So long as I don’t use x inside the function body, …

Thanks! That makes sense. Though I’m guessing you meant “the variable x is an l-value”.

[image] CameronBieganek: My guess here is that function arguments can be considered lvalues in some sense? Yes, you can think of them as being assigned the arguments when the function is called. Whether something is an lvalue or not is a syntactic property orthogonal to being _. Personally,…

I’m wondering about that as well—when does the Julia parser allow a bare underscore. I have found an example that (I think) is not explained by what has been discussed so far, and it so happens it’s something I’d like to use in a module I’m working on. Say a physical property c is usually a functi…

Usage rules for bare underscores

General Usage

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'_' as an argument in function definition General Usage question	8	950	December 17, 2023
Class of variables General Usage	5	640	August 8, 2023
Why it has "_," in front of some variables New to Julia question	11	975	March 8, 2023
Error: "all-underscore identifier used as rvalue" General Usage	1	796	May 1, 2023
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Usage rules for bare underscores

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