How to use formatC so that numbers as a result of a simple arithmetic operation are correct?

The function formatC() produces wrong formats of numbers when a simple arithmetic operation is passed as x.

Example:

formatC(29, width = 3, format = "d", flag = "0")
> "029"

This is correct. But when I do:

formatC(100*0.29, width = 3, format = "d", flag = "0")
> "028"

What am I missing here? (R version 4.3.2 (2023-10-31 ucrt))

>Solution :

You are using format = "d". The docs set out the various options for the format argument:

equal to "d" (for integers), "f", "e", "E", "g", "G", "fg" (for reals), or "s" (for strings). Default is "d" for integers, "g" for reals.

Neither 29 nor 0.29 * 100 are stored by R as an integer and they are not equal. You can see this here:

class(29) # numeric
class(29L) # integer
sprintf("%.20f", 29)
# [1] "29.00000000000000000000"
sprintf("%.20f", 100 * 0.29)
# [1] "28.99999999999999644729"

Using integer formatting is truncating the second value to 28. Use an appropriate format for reals e.g.:

formatC(100 * 0.29, width = 3, format = "g", flag = "0")
# [1] "029"

For more on why this occurs see: Why are these numbers not equal?