append values by dataframe row in a loop - r

Question

I'm running models with various initial values, and I'm trying to append values (3 estimators) by rows to a dataframe in a loop. I assign values to estimators within the loop, but I can't recall them to produce a dataframe.
My code: f is the model for the estimation. Three parameters: alpha, rho, and lambda in the model. I want to output these 3 values.
library("maxLik")
f <- function(param) {
alpha <- param[1]
rho <- param[2]
lambda <- param[3]
u <- 0.5 * (dataset$v_50_1)^alpha - 0.5 * lambda * (dataset$v_50_2)^alpha
p <- 1/(1 + exp(-rho * u))
logl <- sum(dataset$gamble * log(p) + (1 - dataset$gamble) * log(1 - p))
}
df <- data.frame(alpha = numeric(), rho = numeric(), lambda = numeric())
for (j in 1:20) {
tryCatch({
ml <- maxLik(f, start = c(alpha = runif(1, 0, 2), rho = runif(1, 0, 4), lambda = runif(1,
0, 10)), method = "NM")
alpha[j] <- ml$estimate[1]
rho[j] <- ml$estimate[2]
lambda[j] <- ml$estimate[3]
}, error = function(e) {NA})
}
output <- data.frame(alpha, rho, lambda)
error occurs:
Error in data.frame(alpha, rho, lambda) : object 'alpha' not found
Expected output
alpha rho lambda
0.4 1 2 # estimators append by row.
0.6 1.1 3 # each row has estimators that are estimated
0.7 1.5 4 # by one set of initial values, there are 20
# rows, as the estimation loops for 20 times.

Answer 1

I am running an example, by changing the function f
library("maxLik")
t <- rexp(100, 2)
loglik <- function(theta) log(theta) - theta*t
df <- data.frame(alpha = numeric(), rho = numeric(), lambda = numeric())
for (j in 1:20){
tryCatch({
ml <- maxLik(loglik, start = c(alpha = runif(1, 0, 2), rho = runif(1, 0, 4),
lambda = runif(1, 0, 10)), method = "NM")
df <- rbind(df, data.frame(alpha = ml$estimate[1],
rho = ml$estimate[2],
lambda = ml$estimate[3]))
# I tried to append values for each column
}, error = function(e) {NA})}
> row.names(df) <- NULL
> head(df)
alpha rho lambda
1 2.368739 2.322220 2.007375
2 2.367607 2.322328 2.007093
3 2.368324 2.322105 2.007597
4 2.368515 2.322072 2.007334
5 2.368269 2.322071 2.007142
6 2.367998 2.322438 2.007391