Question

I have a large dataframe (~ 600K rows) with a string-value column (link)

doc_id,link
1,http://example.com
1,http://example.com
2,http://test1.net
2,http://test2.net
2,http://test5.net
3,http://test1.net
3,http://example.com
4,http://test5.net

and I would like to count the number of times a certain string value occurs in the frame. The result should look like this:

link, count
http://example.com, 3
http://test1.net, 2
http://test2.net, 1
http://test5.net, 2

Is there an efficient way to do this in R? Converting the frame into a matrix doesn’t work because of the frame size. Currently I am using the plyr package, but this is too slow.

Answer 1

The table function counts occurrences – and it’s very fast compared to ddply. So, something like this perhaps:

# some sample data
set.seed(42)
df <- data.frame(doc_id=1:10, link=sample(letters[1:3], 10, replace=TRUE))

cnt <- as.data.frame(table(df$link))
# Assign appropriate names (optional)
names(cnt) <- c("link", "count")
cnt

Which gives the following output:

  link count
1    a     2
2    b     3
3    c     5