Keyed alignment and join cardinality

The idea

Two tables can describe the same entities in different row orders. Combining their columns by position silently creates incorrect records unless alignment is guaranteed. Match returns the first position of each requested key in a lookup vector, allowing the result to follow the request order. An absent key yields NA by default and needs a stated missing-key policy.

First-match behavior is not the same as a relational join. Duplicate lookup keys make match choose only the first occurrence, while an all-match join may multiply rows. Validate lookup uniqueness when the domain promises one value per key. Test reordered keys, duplicates, and missing keys; checking only a perfectly aligned example cannot establish that the join answers the intended question.

ids <- c("b","a","x")
lookup <- c("a","b")
values <- c(10,20)
cat(values[match(ids,lookup)], sep=",")
cat("\n")

20,10,NA

Match reorders values into request order and preserves the missing request as NA.