[R] unexpected 'else' in " else"

[Jorgen Harmse]

Andrew Simmons is correct but doesn't explain why the code works in the package. This is one of only two differences I have found between running code at the command line and running it from a file. (The other difference is that code in a file is often executed in an environment other than .GlobalEnv. There is some related sugar around packages that usually makes things work the way a user would want.) At the command line, R executes code whenever RETURN could be interpreted as the end of a statement. "If(�.) �. RETURN" is ambiguous: will it be followed by "else", or is it a complete statement? If it's in a file or wrapped in a block or other structure that obviously hasn't ended yet then R will wait to see the next line of input, but if it could be a complete statement then not executing it would cause a lot of frustration for users. Once the statement is executed, R expects another statement, and no statement begins with "else". (Maybe the interpreter could be enhanced to keep the "if" open under some conditions, but I haven't thought it through. In particular, "if" without "else" is NULL if the condition is FALSE, so it might be necessary to undo an assignment, and that seems very difficult.)

Regards,
Jorgen Harmse.


On Fri., Oct. 21, 2022, 05:29 Jinsong Zhao, <jszhao using yeah.net> wrote:

> Hi there,
>
> The following code would cause R error:
>
>  > w <- 1:5
>  > r <- 1:5
>  >         if (is.matrix(r))
> +             r[w != 0, , drop = FALSE]
>  >         else r[w != 0]
> Error: unexpected 'else' in "        else"
>
> However, the code:
>          if (is.matrix(r))
>              r[w != 0, , drop = FALSE]
>          else r[w != 0]
> is extracted from stats::weighted.residuals.
>
> My question is why the code in the function does not cause error?
>
> Best,
> Jinsong
>
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