Question

This is a reproducible example:

a <- 0.05
za.2 <- qnorm(1-a/2)
b <- 0.20
zb <- qnorm(1-b)

lambda12 <- -log(1/2)/12
lambda18 <- -log(1/2)/18
theta <- lambda18/lambda12
(d = round(4*(za.2+zb)^2/log(theta)^2))   

Tf<-36
library(Ryacas)
n <- Sym("n")

Solve(n/2*(2-exp(-lambda12*Tf)-exp(-lambda18*Tf))==d , n)

The last line returns

expression(list(n == 382/1.625))

Is there a way to extract the quotient and assign it to another variable (235.0769)?

Answer 1

G.Grothendieck pointed out in comments that you’ll need to first to capture the expression to be operated upon below:

soln <- Solve(n/2*(2-exp(-lambda12*Tf)-exp(-lambda18*Tf))==d , n) 
X <- yacas(soln)$text

Then, to extract the quotient, you can take advantage of the fact that many R language objects either are or can be coerced to lists.

 X <- expression(list(n == 382/1.625))
 res <- eval(X[[1]][[2]][[3]])
 res
 [1] 235.0769

The following just shows why that sequence of indices extracts the right piece of the expression:

as.list(X)
# [[1]]
# list(n == 382/1.625)

as.list(X[[1]])
# [[1]]
# list
# 
# [[2]]
# n == 382/1.625

as.list(X[[1]][[2]])
# [[1]]
# `==`
# 
# [[2]]
# n
# 
# [[3]]
# 382/1.625