Count occurrences of value in a set of variables in R (per row)

Question

Let's say I have a data frame with 10 numeric variables V1-V10 (columns) and multiple rows (cases).

What I would like R to do is: For each case, give me the number of occurrences of a certain value in a set of variables.

For example the number of occurrences of the numeric value 99 in that single row for V2, V3, V6, which obviously has a minimum of 0 (none of the three have the value 99) and a maximum of 3 (all of the three have the value 99).

I am really looking for an equivalent to the SPSS function COUNT: "COUNT creates a numeric variable that, for each case, counts the occurrences of the same value (or list of values) across a list of variables."

I thought about table() and library plyr's count(), but I cannot really figure it out. Vectorized computation preferred. Thanks a lot!

Answer 1

I think that there ought to be a simpler way to do this, but the best way that I can think of to get a table of counts is to loop (implicitly using sapply) over the unique values in the dataframe.

#Some example data
df <- data.frame(a=c(1,1,2,2,3,9),b=c(1,2,3,2,3,1))
df
#  a b
#1 1 1
#2 1 2
#3 2 3
#4 2 2
#5 3 3
#6 9 1

levels=unique(do.call(c,df)) #all unique values in df
out <- sapply(levels,function(x)rowSums(df==x)) #count occurrences of x in each row
colnames(out) <- levels
out
#     1 2 3 9
#[1,] 2 0 0 0
#[2,] 1 1 0 0
#[3,] 0 1 1 0
#[4,] 0 2 0 0
#[5,] 0 0 2 0
#[6,] 1 0 0 1