a dataframe, array or vector. For seq_along_rows, and
seq_along_cols sequence along the first and last dimensions,
respectively. Atomic vectors are treated as 1 dimensional
arrays (i.e., seq_along_rows is equivalent to seq_along when x is an
atomic vector or list).
which_dim
a scalar integer or character string, specifying which
dimension to generate a sequence for. Negative numbers count from the back.
Value
a vector of integers 1:nrow(x), safe for use in for loops and
vectorized equivalents.