Example of a non-trivial function to apply over all columns of a table.

gc.r

complete_table = function (data, labels) {
    tbl = table(data)
    label_pos = match(labels, names(tbl))
    tbl = tbl[label_pos]
    names(tbl) = labels
    tbl[is.na(tbl)] = 0
    `class<-`(tbl, 'table')
}

# GC = (C + G) / (A + C + G + T)
gc = function (gene) {
    codon_components = lapply(strsplit(names(gene), ''),
                              complete_table, labels = c('A', 'C', 'G', 'T'))
    codon_components = Map(`*`, codon_components, gene)
    base_freqs = Reduce(`+`, codon_components)
    with(as.list(base_freqs), (C + G) / (A + C + G + T))
}

# Usage:

gc(c(AAA = 4, AAC = 2, AAG = 0, AAT = 0, ACA = 3, …, TTT = 6))

# Or, in the case of  my actual table:

genes = transform(genes, GC = apply(genes, 1, gc))