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Inverse Which

Source: R/unwhich.R
unwhich.Rd

Reconstructs the TRUE positions from the index vector returned by which(), producing a logical vector of length n. Note that this is not a perfect inverse: which() discards NA and FALSE positions, so the original vector cannot be fully recovered.

Usage

unwhich(
  idx,
  n = if (length(idx) > 0L && !anyNA(idx) && all(idx > 0L)) max(idx) else 0L,
  useNames = TRUE
)

Arguments

idx

a vector of non-zero whole-number indices. Positive values mark TRUE positions; negative values mark FALSE positions (all others become TRUE). As in base R, positive and negative indices must not be mixed. Duplicate indices are allowed and result in a single TRUE (or FALSE) at that position.

n

a single non-negative whole number giving the length of the result. For positive idx, defaults to max(idx); for negative or empty idx, defaults to 0L. Must not be less than max(abs(idx)).

useNames

logical. If TRUE (default) and idx has names, those names are attached to the corresponding TRUE positions of the result; all other positions receive an empty string. If FALSE or idx is unnamed, the result has no names. Ignored for negative indices.

Value

a logical vector of length n.

Details

Negative indices follow standard R semantics: unwhich(-2, 5) returns a vector with TRUE everywhere except position 2. Positive and negative indices must not be mixed.

Note

The positive-index construction (rv[indices] <- TRUE with name propagation) is based on code by Nick Sabbe; negative-index handling and input validation are original additions.

References

Sabbe, N. (2012). Inverse of which. https://stackoverflow.com/questions/7659833/inverse-of-which

See also

which()

Other vector.utils: nz()

Examples

ll <- c(TRUE, FALSE, TRUE, NA, FALSE, FALSE, TRUE)
names(ll) <- letters[seq_along(ll)]
i <- which(ll)

# reconstruct TRUE positions (names preserved on TRUE positions)
unwhich(i, length(ll))
#>     a           c                       g 
#>  TRUE FALSE  TRUE FALSE FALSE FALSE  TRUE 

# without names
unwhich(i, length(ll), useNames = FALSE)
#> [1]  TRUE FALSE  TRUE FALSE FALSE FALSE  TRUE

# negative index: TRUE everywhere except position 2
unwhich(-2, 5)
#> [1]  TRUE FALSE  TRUE  TRUE  TRUE

# empty index -> all-FALSE vector
unwhich(integer(0), n = 5L)
#> [1] FALSE FALSE FALSE FALSE FALSE