I am wondering if converting numerics to characters and then back again in R can ever change the number? For example, does as.character round off numerics after a certain amount of decimal places (if so, how many)?
1 Answers
@jogo thanks for the suggestion :)
Here is the comment as an answer:
From ?as.character():
as.character represents real and complex numbers to 15 significant digits (technically the compiler's setting of the ISO C constant DBL_DIG, which will be 15 on machines supporting IEC60559 arithmetic according to the C99 standard). This ensures that all the digits in the result will be reliable (and not the result of representation error), but does mean that conversion to character and back to numeric may change the number. If you want to convert numbers to character with the maximum possible precision, use format.
So yes it does change the number if you have more than 15 significant digits. See:
> as.character(1.000000000000001) # more than 15 significant digits
[1] "1"
> as.character(1.00000000000001) # less than 15 significant digits
[1] "1.00000000000001"
Here are some other examples:
y <- as.numeric(as.character(pi))
identical(y, pi) ### gives FALSE
or
x <- 1/7
y <- as.numeric(as.character(x))
x-y
or
as.numeric(as.character(.Machine$double.xmax)) ## result: Inf
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