step_smote breaks with some over_ratios

[rowanjh]

The problem

step_smote throws an error when a class is marginally below the target upsampling threshold. This only occurs in rare cases when smote tries to synthesize a fraction of a sample (n < 1).

The issue occurs in themis:::smote_impl. When samples_needed contains a number smaller than 1, themis:::smote_data returns an empty matrix, subsequently causing an error at the line out_df[var] <- data[[names(samples_needed)[i]]][[var]][1]

One quick solution might be to floor the ratio_target when checking which classes need to be upsampled in themis:::smote_impl, as below. This would ignore the variables fractionally below the target threshold.
which_upsample <- which(table(df[[var]]) < ratio_target) # orig
which_upsample <- which(table(df[[var]]) < floor(ratio_target)) # possible solution

Reproducible example

library(recipes)
library(themis)
set.seed(123)

dat <- data.frame(
    outcome = c(rep("X", 101), rep("Z", 50)),
    X1 = rnorm(151))

rec_49 <- recipe(outcome ~ ., data = head(dat)) |>
    step_smote(outcome, over_ratio = 0.49, seed = 231)
rec_50 <- recipe(outcome ~ ., data = head(dat)) |>
    step_smote(outcome, over_ratio = 0.5, seed = 231)
rec_51 <- recipe(outcome ~ ., data = head(dat)) |>
    step_smote(outcome, over_ratio = 0.51, seed = 231)

prep(rec_49, dat) # works
prep(rec_51, dat) # works
prep(rec_50, dat) # does not work
#> Error in `step_smote()`:
#> Caused by error in `[<-.data.frame`:
#> ! replacement has 1 row, data has 0

#> Backtrace:
#>      x
#>   1. +-recipes::prep(rec_50, dat)
#>   2. +-recipes:::prep.recipe(rec_50, dat)
#>   3. | +-recipes:::recipes_error_context(...)
#>   4. | | +-base::withCallingHandlers(...)
#>   5. | | \-base::force(expr)
#>   6. | +-recipes::bake(x$steps[[i]], new_data = training)
#>   7. | \-themis:::bake.step_smote(x$steps[[i]], new_data = training)
#>   8. |   +-withr::with_seed(...)
#>   9. |   | \-withr::with_preserve_seed(...)
#>  10. |   \-themis:::smote_impl(...)
#>  11. |     +-base::`[<-`(`*tmp*`, var, value = `<fct>`)
#>  12. |     \-base::`[<-.data.frame`(`*tmp*`, var, value = `<fct>`)
#>  13. |       \-base::stop(...)
#>  14. \-base::.handleSimpleError(...)
#>  15.   \-recipes (local) h(simpleError(msg, call))
#>  16.     \-recipes:::stop_recipes_step(call = call(step_name), parent = cnd)
#>  17.       \-recipes:::stop_recipes(...)
#>  18.         \-rlang::abort(...)

^{Created on 2023-02-08 with reprex v2.0.2}

Session info

sessioninfo::session_info()
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