Question

How to count frequencies occurring in two columns ?
Sample datas :

> sample <- dput(df)
structure(list(Nom_xp = c("A1FAA", "A1FAJ", "A1FBB", "A1FJA", 
"A1FJR", "A1FRJ"), GB05.x = c(100L, 98L, NA, 100L, 102L, 98L), 
    GB05.1.x = c(100L, 106L, NA, 100L, 102L, 98L), GB18.x = c(175L, 
    173L, 177L, 177L, 173L, 177L), GB18.1.x = c(177L, 175L, 177L, 
    177L, 177L, 177L)), .Names = c("Nom_xp", "GB05.x", "GB05.1.x", 
"GB18.x", "GB18.1.x"), row.names = c(NA, 6L), class = "data.frame")

Counting frequencies :

apply(sample[,2:5],2,table)

Now, how to combine the count by prefix of columns, or by every two columns ? The expected output, for the four first columns would be a list:

$GB05
98 100 102 106
3  4   2   1
$GB18
173 175 177
2   2   8

One way to get the count for the first two columns :

  table(c(apply(sample[,2:3],2,rbind)))
98 100 102 106 
  3   4   2   1

But how to apply this to a whole data.frame ?

Answer 1

If you want to apply table to your whole data frame, you can use :

table(unlist(sample[,-1]))

Which gives :

 98 100 102 106 173 175 177 
  3   4   2   1   2   2   8 

If you want to group by column name prefix, for example the 4th first characters, you can do something like this :

cols <- names(sample)[-1]
groups <- unique(substr(cols,0,4))
sapply(groups, function(name) table(unlist(sample[,grepl(paste0("^",name),names(sample))])))

Which gives :

$GB05

 98 100 102 106 
  3   4   2   1 

$GB18

173 175 177 
  2   2   8