Question

I have a data frame where each row is an observation concerning a pupil. One of the vectors in the data frame is an id for the school. I have obtained a new vector with counts for each school as follows:

tbsch <- table(dt$school)

Now I want to add the relevant count value to each row in dt. I have done it using for() looping through each row in dt and making a new vector containing the relevant count and finally using cbind() to add it to dt, but I think this is very inefficient. Is there a smart/easy way to do that ?

Answer 1

using jmsigner’s data you could do:

dt$count <- ave(dt$school, dt$school,  FUN = length)