# Main message

I am trying apply `loo_subsample()`

to a model with several parameter arrays of differing dimensionality. Following the procedure outlined in this vignette, I wrote a function for the log-likelihood whose `draws`

argument accepts the output of `extract(stanfit)`

, which is a list. This log-likelihood function works fine when fed to `loo_i()`

, but `loo_subsample()`

itself throws the following error:

```
Error in .ndraws.default(draws) :
.ndraws() has not been implemented for objects of class 'list'
```

The error occurs when the line

`checkmate::assert_int(loo_approximation_draws, lower = 1, upper = .ndraws(draws), null.ok = TRUE)`

is called. I tried to solve this error by instead adding to my log-likelihood function separate (array) arguments for each set of parameters. This again works with `loo_i()`

but results in another error in `loo_subsample()`

:

```
Error in .llfun(data_i = data[i, , drop = FALSE], draws = point_est) :
argument "beta" is missing, with no default
```

This seems to occur after `loo_subsample()`

's calls `elpd_loo_approximation()`

, which does not seem to make use of `loo_subsample()`

's `...`

argument.

I would appreciate any thoughts on how to get around this problem. Thanks!

~Devin

# More details

The following modification of the code in the loo vignette produces a similar error:

```
logistic_stan <- "//
data {
int<lower=0> N; // number of data points
int<lower=0> P; // number of predictors (including intercept)
matrix[N,P] X; // predictors (including 1s for intercept)
int<lower=0,upper=1> y[N]; // binary outcome
}
parameters {
vector[P] beta;
vector[P] alpha;
}
model {
beta ~ normal(0, 1);
alpha ~ normal(0, 1);
y ~ bernoulli_logit(X * beta);
}
"
llfun_logistic <- function(data_i, draws, log = TRUE, alpha) {
if (missing(alpha)) stop("No alpha found.")
x_i <- as.matrix(data_i[, which(grepl(colnames(data_i), pattern = "X")),
drop=FALSE])
logit_pred <- draws %*% t(x_i)
dbinom(x = data_i$y, size = 1, prob = 1/(1 + exp(-logit_pred)), log = log)
}
url <- "http://stat.columbia.edu/~gelman/arm/examples/arsenic/wells.dat"
wells <- read.table(url)
wells$dist100 <- with(wells, dist / 100)
X <- model.matrix(~ dist100 + arsenic, wells)
stan_mod <- stan_model(model_code = logistic_stan)
fit_1 <- sampling(stan_mod, data = standata, seed = 4711)
print(fit_1, pars = c("beta", "alpha"))
parameter_draws_1 <- extract(fit_1)$beta
stan_df_1 <- as.data.frame(standata)
r_eff <- relative_eff(llfun_logistic,
log = FALSE,
chain_id = rep(1:4, each = 1000),
data = stan_df_1,
draws = parameter_draws_1,
alpha = extract(fit_1)$alpha,
cores = 2)
loo_i(i = 1, llfun_logistic, r_eff = r_eff, data = stan_df_1,
draws = parameter_draws_1, alpha = extract(fit_1)$alpha)
set.seed(4711)
loo_ss_1 <-
loo_subsample(
llfun_logistic,
draws = parameter_draws_1,
alpha = extract(fit_1)$alpha,
data = stan_df_1,
observations = 100, # take a subsample of size 100
cores = 2,
r_eff = r_eff
)
```

- Operating System: MacOS 11.2.3
- RStan Version: 2.21.2
- Output of
`devtools::session_info("rstan")`

:

```
β Session info βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
setting value
version R version 4.0.3 (2020-10-10)
os macOS Big Sur 10.16
system x86_64, darwin17.0
ui X11
language (EN)
collate en_US.UTF-8
ctype en_US.UTF-8
tz America/New_York
date 2021-04-26
β Packages βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
package * version date lib source
backports 1.2.1 2020-12-09 [1] CRAN (R 4.0.2)
BH 1.75.0-0 2021-01-11 [1] CRAN (R 4.0.2)
callr 3.7.0 2021-04-20 [1] CRAN (R 4.0.3)
checkmate 2.0.0 2020-02-06 [1] CRAN (R 4.0.2)
cli 2.4.0 2021-04-05 [1] CRAN (R 4.0.2)
colorspace 2.0-0 2020-11-11 [1] CRAN (R 4.0.2)
crayon 1.4.1 2021-02-08 [1] CRAN (R 4.0.2)
curl 4.3 2019-12-02 [1] CRAN (R 4.0.1)
desc 1.3.0 2021-03-05 [1] CRAN (R 4.0.2)
digest 0.6.27 2020-10-24 [1] CRAN (R 4.0.2)
ellipsis 0.3.1 2020-05-15 [1] CRAN (R 4.0.2)
fansi 0.4.2 2021-01-15 [1] CRAN (R 4.0.2)
farver 2.1.0 2021-02-28 [1] CRAN (R 4.0.2)
ggplot2 * 3.3.3 2020-12-30 [1] CRAN (R 4.0.2)
glue 1.4.2 2020-08-27 [1] CRAN (R 4.0.2)
gridExtra 2.3 2017-09-09 [1] CRAN (R 4.0.2)
gtable 0.3.0 2019-03-25 [1] CRAN (R 4.0.2)
inline 0.3.17 2020-12-01 [1] CRAN (R 4.0.2)
isoband 0.2.4 2021-03-03 [1] CRAN (R 4.0.2)
jsonlite 1.7.2 2020-12-09 [1] CRAN (R 4.0.2)
labeling 0.4.2 2020-10-20 [1] CRAN (R 4.0.2)
lattice 0.20-41 2020-04-02 [1] CRAN (R 4.0.3)
lifecycle 1.0.0 2021-02-15 [1] CRAN (R 4.0.3)
loo * 2.4.1 2020-12-09 [1] CRAN (R 4.0.2)
magrittr 2.0.1 2020-11-17 [1] CRAN (R 4.0.2)
MASS 7.3-53.1 2021-02-12 [1] CRAN (R 4.0.2)
Matrix * 1.3-2 2021-01-06 [1] CRAN (R 4.0.2)
matrixStats 0.58.0 2021-01-29 [1] CRAN (R 4.0.2)
mgcv 1.8-35 2021-04-18 [1] CRAN (R 4.0.3)
munsell 0.5.0 2018-06-12 [1] CRAN (R 4.0.2)
nlme 3.1-152 2021-02-04 [1] CRAN (R 4.0.3)
pillar 1.6.0 2021-04-13 [1] CRAN (R 4.0.3)
pkgbuild 1.2.0 2020-12-15 [1] CRAN (R 4.0.2)
pkgconfig 2.0.3 2019-09-22 [1] CRAN (R 4.0.2)
prettyunits 1.1.1 2020-01-24 [1] CRAN (R 4.0.2)
processx 3.5.1 2021-04-04 [1] CRAN (R 4.0.2)
ps 1.6.0 2021-02-28 [1] CRAN (R 4.0.2)
R6 2.5.0 2020-10-28 [1] CRAN (R 4.0.2)
RColorBrewer 1.1-2 2014-12-07 [1] CRAN (R 4.0.2)
Rcpp 1.0.6 2021-01-15 [1] CRAN (R 4.0.2)
RcppEigen 0.3.3.9.1 2020-12-17 [1] CRAN (R 4.0.2)
RcppParallel 5.1.2 2021-04-15 [1] CRAN (R 4.0.3)
rlang 0.4.10 2020-12-30 [1] CRAN (R 4.0.2)
rprojroot 2.0.2 2020-11-15 [1] CRAN (R 4.0.2)
rstan * 2.21.2 2020-07-27 [1] CRAN (R 4.0.2)
scales 1.1.1 2020-05-11 [1] CRAN (R 4.0.2)
StanHeaders * 2.21.0-7 2020-12-17 [1] CRAN (R 4.0.2)
tibble * 3.1.1 2021-04-18 [1] CRAN (R 4.0.3)
utf8 1.2.1 2021-03-12 [1] CRAN (R 4.0.2)
V8 3.4.1 2021-04-23 [1] CRAN (R 4.0.2)
vctrs 0.3.7 2021-03-29 [1] CRAN (R 4.0.2)
viridisLite 0.4.0 2021-04-13 [1] CRAN (R 4.0.3)
withr 2.4.2 2021-04-18 [1] CRAN (R 4.0.3)
[1] /Library/Frameworks/R.framework/Versions/4.0/Resources/library
```