For anyone interested in data analysis in R or Python (specifically Wickham's dplyr or McKinney's pandas), I can't recommend reading Wickham's paper on the split-apply-combine methodology enough. It really helped me understand working without looping.

The paper's examples might be slightly dated as they use dplyr's predecessor, plyr. I imagine they're pretty similar though.

http://vita.had.co.nz/papers/plyr.html