Imlerith - 11 months ago 51

R Question

I want to represent the following number using the log function:

`2.5e-600/1.7e-500`

here is what I did on paper, and what I would like to automate on R for any given number:

`log(2.5e-600/1.7e-500) = log(2.5e-600)-log(1.7e-500)`

= log(2.5)-600*log(10) - log(1.7) + 500*log(10)

= -229.8728

However, I am thinking that on R it won't be as straight forward to go from

`log(10^-600) to -600*log(10)`

`-Inf`

`-1381.511`

My question is how can I remedy to that problem? I am thinking that maybe there is a function that would allow me to retrieve the exponent part of a number ? same question for going from

`log(2.5e-600) to log(2.5)-600*log(10)`

Answer

For this special case, you can do it fairly straightforwardly without using `mpfr`

:

```
x1 <- "2.5e-600"
x2 <- "1.7e-500"
```

Split value into mantissa and exponent (log10):

```
get_num <- function(x) {
as.numeric(strsplit(x,"e")[[1]])
}
val1 <- get_num(x1)
val2 <- get_num(x2)
```

Now take the ratio of the mantissas (log(v1)-log(v2)), but subtract the exponents:

```
log(val1[1])-log(val2[1])+log(10^(val1[2]-val2[2]))
## [1] -229.8728
```

Source (Stackoverflow)