R：过滤器（函数句柄，数据）在 R 中是什么意思

Question

The variable getgenes has a column named "GENE". This column has redundant strings. The purpose is to get the strings that appeared g(g can be 2,3,4...etc) times.

x <- Filter(function(elem) length(which(getgenes$GENE == elem)) == g, getgenes$GENE)

I searched Filter function in R but cannot find a match that use functon as 1st argument.

What does the Filter means here?

Answer 1

From the help page, "Filter extracts the elements of a vector for which a predicate (logical) function gives true." If you look at the source, you can see that

x <- Filter(function(elem) length(which(getgenes$GENE == elem)) == g, getgenes$GENE)

is equivalent to:

x <- getgenes$GENE[which(as.logical(unlist(lapply(getgenes$GENE,
                                                  function(elem) length(which(getgenes$GENE == elem)) == g))))]

However, this seems a roundabout way of arriving at the answer. Under some mild assumptions about the structure of getgenes and g, I believe the expression can be simplified to:

x <- getgenes$GENE[getgenes$GENE %in% names(which(table(getgenes$GENE) == g))]

if you are willing to forgo duplicates, it can be simplified even further.