Hi everyone:
I’m one of the interns at RStudio this summer working on a project that
helps teachers grade student code. I found an unexpected behaviour with
the |==| operator when comparing |quote|d expressions.
Example 1:
|u <- quote(tidyr::gather(key = key, value = value,
new_sp_m014:newrel_f65, na.rm = TRUE)) s <- quote(tidyr::gather(key =
key, value = value, new_sp_m014:newrel_f65, na.rm = FALSE)) u == s #
TRUE u <- quote(tidyr::gather(key = key, value = value, na.rm = TRUE)) s
<- quote(tidyr::gather(key = key, value = value, na.rm = FALSE)) u == s
# FALSE |
Example 2:
|u <-
quote(f(x123456789012345678901234567890123456789012345678901234567890,
1)) s <-
quote(f(x123456789012345678901234567890123456789012345678901234567890,
2)) u == s #> [1] TRUE |
Winston Chang pointed out in the help page for |==|:
Language objects such as symbols and calls are deparsed to character
strings before comparison.
and in the source code that does the comparison [1] shows that It
deparses each language object and then only extracts the first element
from the resulting character vector:
|SET_STRING_ELT(tmp, 0, (iS) ? PRINTNAME(x) : STRING_ELT(deparse1(x, 0,
DEFAULTDEPARSE), 0)); |
Is this a fix that needs to happen within the |==| documentation? or an
actual bug with the operator?
For more context the original issue we had is here:
https://github.com/rstudio-education/grader/issues/28
Workaround:
You can get around this issue by using |all.equal| or |identical|
|u <- quote(tidyr::gather(key = key, value = value,
new_sp_m014:newrel_f65, na.rm = TRUE)) s <- quote(tidyr::gather(key =
key, value = value, new_sp_m014:newrel_f65, na.rm = FALSE)) u == s #
TRUE all.equal(u, s) # "target, current do not match when deparsed"
identical(u, s) # FALSE |
Thanks,
Dan
[1]
https://github.com/wch/r-source/blob/e647f78cb85282263f88ea30c6337b77a30743d9/src/main/relop.c#L140-L155
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