Keeping one parameter fixed and search on randomly in caret

Question

I would like to keep the parameter alpha fixed at 1 and use random search for lambda, is this possible?

library(caret)

X <- iris[, 1:4]
Y <- iris[, 5]

fit_glmnet <- train(X, Y, method = "glmnet", tuneLength = 2, trControl = trainControl(search = "random"))

Answer 1

I do not think this can be achieved by specifying directly in caret train but here is how to emulate the desired behavior:

From this link

one can see random search for lambda is achieved by:

lambda = 2^runif(len, min = -10, 3)

where len is the tune length

To emulate random search over one parameter:

len <- 2
fit_glmnet <- train(X, Y,
                    method = "glmnet",
                    tuneLength = len,
                    trControl = trainControl(search = "grid"),
                    tuneGrid = data.frame(alpha = 1, lambda = 2^runif(len, min = -10, 3)))