R：将列表中每个列表的元素求和，然后在数据框中返回结果

Question

I have a list of lists in R; each list has the results of a Grep command indicating the position where the search string was found. The command

> matches<-lapply(ListOfFiles, function(x)
+ grep("SearchString",readLines(x),ignore.case = T))

produces

//I copy the results that the function actually yields for the sake of the example

> matches<-list(c(11L, 13L), c(9L, 12L, 14L, 15L, 16L, 19L, 20L, 22L, 25L
+ ), c(5L, 8L, 11L), c(10L, 11L, 13L, 14L, 16L), c(5L, 7L, 9L), 
+ integer(0))

> matches
[[1]]
[1] 11 13

[[2]]
[1]  9 12 14 15 16 19 20 22 25

[[3]]
[1]  5  8 11

[[4]]
[1] 10 11 13 14 16

[[5]]
[1] 5 7 9

[[6]]
integer(0)

I need to transform this to a simple data frame of 6 rows and 1 column, with each "cell" having the sum of each of the 6 lists of matches .

If at all possible, please try to explain the syntax I should employ; I'm new to R and sometimes I find examples difficult to follow if several things are nested at once.

Answer 1

Actually just figured it out on my own. The answer is:

data.frame(unlist(lapply(matches, function(x) sum(x))))

the first part yiels a list of lists with one element each, the summation of the elements of each list

> lapply(matches, function(x) sum(x))
[[1]]
[1] 24

[[2]]
[1] 152

[[3]]
[1] 24

[[4]]
[1] 64

[[5]]
[1] 21

[[6]]
[1] 0

the second part generates a vector from that. Evidently it is a recursive function:

> unlist(lapply(matches, function(x) sum(x)))
[1]  24 152  24  64  21   0

Finally it is transformed into a dataframe using the data.frame() function.