updated section

Author: venom1204

vignettes/datatable-joins.Rmd

@@ -721,7 +721,7 @@ Products[ProductPriceHistory,
          `:=`(price = last(i.price), last_updated = last(i.date)),
          by = .EACHI]
 ```
-- `by = .EACHI` groups by i's rows (1 group per Products row).
+- `by = .EACHI` groups by i's rows (1 group per ProductPriceHistory row).
 - `last()` returns last value including `NA`:
 
 ```{r}
@@ -762,16 +762,20 @@ Dynamically updating multiple columns from `ProductPriceHistory`:
 ```{r}
 update_cols <- intersect(c("price", "category", "stock"), names(ProductPriceHistory))
 
+```
 for (col in update_cols) {
-  Products[ProductPriceHistory, on = .(id = product_id), (col) := get(paste0("i.", col))]}
+  Products[ProductPriceHistory,
+           on = .(id = product_id),
+           (col) := i[[col]],
+           env = list(col = col)]}
 ```
 - Ensures multiple columns are updated efficiently in a loop.
 
 **Summary**
 - `last(x)` vs `tail(x,1)`: Both return last element, but `tail()` returns list for lists.
 - `:=` always modifies `x`, never `i`. For right joins, update `i` directly via `i[, ... := x[.SD]]`.
 - `.EACHI` is crucial for per-row operations; simple joins use first match.
-
+- Note: Older functions like `mapvalues()` from the deprecated `plyr` package were previously used for recoding values. It is recommended to use data.table’s native update-join methods for efficient and future-proof code.
 ***
 
 ## Reference