I have two datasets A and B with a common column severity. I need to identify which row of B has the severity closest to the severity of the each row of A. It is called the nearest neighbor problem. I am  reading in both A and B and then for each row of A search for the nearest neighbor in B using a do loop. However, I am not sure how to create index for B so that a do loop can be done.    Please NOTE : There are no common variables  and the subjects are in different studies. The purpose of the analysis is to figure out who in B is the closest subject to each one in A. .  In terms of disease severity.   The final output should be below:-- please HELP AND URGENT :- Severity_A subject_A Severity_B subject_B 1.2 1 1.5 6 1.2 1 2.6 7 1.2 1 4.5 8 1.2 1 5.1 9 1.2 1 5.7 10 2.3 2 1.5 6 2.3 2 2.6 7 2.3 2 4.5 8 2.3 2 5.1 9 2.3 2 5.7 10 … …       Any help will be greatly appreciated. Thank you very much! ... View more