index_for_recode() identifies the positions of elements in x that match
any of the values specified in the from vector. This function is primarily
used to facilitate the recoding of values by pinpointing which elements in
x correspond to the from values and thus need to be replaced or updated.
index_for_recode(x, from)An integer vector of the same length as x, containing the indices
of the matched values from the from vector. If an element in x does not
match any value in from, the corresponding position in the output will be
NA. This index information is critical for subsequent recoding operations.
A vector of values in which to search for matches.
A vector of values to match against the elements in x.