Trying to predict in R

I created a data set using a random row generator:

training_data <- fulldata[sample(nrow(fulldata),100,]

I am under the impression that I can create a second data set of the rest of the data ... rest_data <- fulldata[-training_data] is the code I jotted down in my notes but I am getting

"Error in '[.default'(fulldata, -training_data) :

What part of my code is incorrect?