R 中的指数函数返回 0 作为值

Question

who can tell me a solution to avoid a result of 0 in y in R program

y= 1-(1/(1+exp(-x)))

exp(-x) is > 0 but for x values like 100 the result of y is 0. I need to get small numbers > 0 to apply log(y) without getting -Inf result.

Answer 1

You can solve this with some simple maths:

x <- 1
1-(1/(1+exp(-x)))
#[1] 0.2689414
1/(exp(x) + 1)
#[1] 0.2689414

x <- 100
1-(1/(1+exp(-x)))
#[1] 0
1/(exp(x) + 1)
#[1] 3.720076e-44

If you are interested in log(y) for large values of x, just ignore the + 1 , y is approximately exp(-x) with a very small error for large x. Thus, log(y) == -x for large x.

Your teacher probably wants you to do the maths.