I am a beginner in R and am having some trouble performing operations on my data. I have a dataframe whose columns contain three types of data, "applcn_ic", "admssn_ic", and "enrlt_ic", for each year for 12 years. (All values are integers, and I have other unrelated columns in the dataframe as well.) So I have columns "applcn_ic2001" through "applcn_ic2013", "admssn_ic2001" through "admssn_ic2013", and "enrlt_ic2001" through "enrlt_ic2013". For each type of data, I would like to find the difference between consecutive years and save that as a new variable (e.g. save "applcn_ic2005" minus "applcn_ic2004" as "applcn_diff_2005"). How do I do that?