Uninformative error when calling posterior_predict with ndraws greater than the number of samples

[andrjohns]

From this post on the forum, calling posterior_predict and requesting ndraws greater than the number of post-warmup draws in the model gives an error from the sample.int function (reprex below).

It would probably be better to have an input validation step for the ndraws value:

Reprex:

library(brms)
#> Loading required package: Rcpp
#> Loading 'brms' package (version 2.17.0). Useful instructions
#> can be found by typing help('brms'). A more detailed introduction
#> to the package is available through vignette('brms_overview').
#> 
#> Attaching package: 'brms'
#> The following object is masked from 'package:stats':
#> 
#>     ar
fit <- brm(time ~ age, data = kidney,
           iter = 100, chains = 1,
           backend = "cmdstanr")
#> Start sampling
#> Running MCMC with 1 chain...
#> 
#> Chain 1 WARNING: There aren't enough warmup iterations to fit the 
#> Chain 1          three stages of adaptation as currently configured. 
#> Chain 1          Reducing each adaptation stage to 15%/75%/10% of 
#> Chain 1          the given number of warmup iterations: 
#> Chain 1            init_buffer = 7 
#> Chain 1            adapt_window = 38 
#> Chain 1            term_buffer = 5 
#> Chain 1 Iteration:  1 / 100 [  1%]  (Warmup) 
#> Chain 1 Iteration: 51 / 100 [ 51%]  (Sampling) 
#> Chain 1 Iteration: 100 / 100 [100%]  (Sampling) 
#> Chain 1 finished in 0.1 seconds.

pp <- posterior_predict(fit, ndraws = 200)
#> Error in sample.int(length(x), size, replace, prob): cannot take a sample larger than the population when 'replace = FALSE'

^{Created on 2022-06-28 by the reprex package (v2.0.1)}

[paul-buerkner]

Thank you. Fixed now.

[andrjohns]

Speedy as always, thanks Paul!