Appreciate the symbolic expression of Ryacas

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) 

Last line returns

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

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

Answer 1

G.Grothendieck indicated in the comments that you need to first write down the expression that will be used below:

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

Then, to extract the factor, you can take advantage of the fact that many objects of the R-language are either, or can be forced into lists.

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

The following shows why this index sequence retrieves the right side 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