glmnet: at what lambda is each coefficient shrunk to 0?

I am using LASSO (from package glmnet) to select variables. I have fitted a glmnet model and plotted coefficients against lambda's.

library(glmnet)
set.seed(47)
x = matrix(rnorm(100 * 3), 100, 3)
y = rnorm(100)
fit = glmnet(x, y)
plot(fit, xvar = "lambda", label = TRUE)

Now I want to get the order in which coefficients become 0. In other words, at what lambda does each coefficient become 0?

I don't find a function in glmnet to extract such result. How can I get it?

Function glmnetPath in my initial answer is now in an R package called solzy.

## you may need to first install package "remotes" from CRAN
remotes::install_github("ZheyuanLi/solzy")

## Zheyuan Li's R functions on Stack Overflow
library(solzy)

## use function `glmnetPath` for your example
glmnetPath(fit)

#$enter
#  i  j ord var     lambda
#1 3  2   1  V3 0.15604809
#2 2 19   2  V2 0.03209148
#3 1 24   3  V1 0.02015439
#
#$leave
#  i  j ord var     lambda
#1 1 23   1  V1 0.02211941
#2 2 18   2  V2 0.03522036
#3 3  1   3  V3 0.17126258
#
#$ignored
#[1] i   var
#<0 rows> (or 0-length row.names)

Interpretation of enter

As lambda decreases, variables (see i for numeric ID and var for variable names) enter the model in turn (see ord for the order). The corresponding lambda for the event is fit$lambda[j].

• variable 3 enters the model at lambda = 0.15604809, the 2nd value in fit$lambda;

• variable 2 enters the model at lambda = 0.03209148, the 19th value in fit$lambda;

• variable 1 enters the model at lambda = 0.02015439, the 24th value in fit$lambda.

Interpretation of leave

As lambda increases, variables (see i for numeric ID and var for variable names) leave the model in turn (see ord for the order). The corresponding lambda for the event is fit$lambda[j].

• variable 1 leaves the model at lambda = 0.02211941, the 23rd value in fit$lambda;

• variable 2 leaves the model at lambda = 0.03522036, the 18th value in fit$lambda;

• variable 3 leaves the model at lambda = 0.17126258, the 1st value in fit$lambda.

Interpretation of ignored

If not an empty data.frame, it lists variables that never enter the model. That is, they are effectively ignored. (Yes, this can happen!)

Note: fit$lambda is decreasing, so j is in ascending order in enter but in descending order in leave.

To further explain indices i and j, take variable 2 as an example. It leaves the model (i.e., its coefficient becomes 0) at j = 18 and enters the model (i.e., its coefficient becomes non-zero) at j = 19. You can verify this:

fit$beta[2, 1:18]
## all zeros

fit$beta[2, 19:ncol(fit$beta)]
## all non-zeros

See Obtain variable selection order from glmnet for a more complicated example.