why does as.integer in R decrement the value?

Question

I am doing a simple operation of multiplying a decimal number and converting it to integer but the result seems to be different than expected. Apologies if this is discussed else where, I am not able to find any straight forward answers to this

> as.integer(1190.60 * 100)
[1] 119059

EDIT: So, I have to convert that to character and then do as.integer to get what is expected

> temp <- 1190.60
> temp2 <- 1190.60 * 100
> class(temp)
[1] "numeric"
> class(temp2)
[1] "numeric"
> as.character(temp2)
[1] "119060"
> as.integer(temp2)
[1] 119059
> as.integer(as.character(temp2))
[1] 119060

EDIT2: According to the comments, thanks @andrey-shabalin

> temp2
[1] 119060
> as.integer(temp2)
[1] 119059
> as.integer(round(temp2))
[1] 119060

EDIT3: As mentioned in the comments the question is related to behaviour of as.integer and not about floating calculations

Answer 1

The answer to this is "floating point error". You can see this easily by checking the following:

> temp <- 1190.60
> temp2 <- 1190.60 * 100
> temp2 - 119060
[1] -1.455192e-11

Due to floating point errors, temp2 isn't exactly 119060 but :

> sprintf("%.20f", temp2)
[1] "119059.99999999998544808477"

If you use as.integer on a float, it works the same way as trunc, i.e. it does round the float in the direction of 0. So in this case that becomes 119059.

If you convert to character using as.character(), R will make sure that it uses maximum 15 significant digits. In this example that would be "119059.999999999". The next digit is another 9, so R will round this to 119060 before conversion. I avoid this in the code above by using sprintf() instead of as.character().