Question

I have a simple dataframe with two columns:

df <- data.frame(x = c(1,1,2,2,3), 
                 y = c(rep(1:2,2),1), 
                 target = c('a','a','a','b','a'))

I would like to compare the strings in the target column (find out whether they are equal or not, i.e., TRUE or FALSE) within every level of x (same number for x).
First I would like to compare lines 1 and 2, then 3 and 4 …
My problem is that I am missing some comparisons, for example, line 5 has only one case instead of two – so it should turn out to be FALSE.
Variable y indicates the first and second case within x.

I played around with ddply doing something like:

ddply(df, .(x), summarise,
        ifelse(as.character(df[df$y == '1',]$target), 
               as.character(df[df$y == '2',]$target),0,1))

which is ugly …
and does not work …

Any insights how I could achieve this comparison?

Thanks

Answer 1

ddply(df, .(x), function(d) NROW(d) == 2 & d$target[1] == d$target[2])

This assumes you want the value to be TRUE only if there are exactly 2 rows with that ‘x’ value. If it is possible for there to be 3 or more, and you want it to be TRUE if all target values are identical, you could do:

ddply(df, .(x), function(d) NROW(d) > 1 & length(unique(d$target)) == 1)