`WHERE(x)`

produces a variate listing the units of `x`

that are logically true, i.e. non-zero (synonym `WHICH`

). If all the units are false, it produces a variate of length one, containing a missing value. If `x`

is restricted or qualified (by `$[]`

), `WHERE`

looks only in the defined subset of units for true values. However, the unit numbers in the result give their positions in the full vector `x`

, not the subset.

# WHERE

Updated on December 4, 2017