How to debug R program?

Hi everyone:
      I wrote a R program which has loops. When I run the program, it 
crashed. I would like to identify in which loop the pragram crashed, how 
can I debug ?  I'm new to R, could somebody please give me a general idea 
about debugging in R.(I'm a C/C++ programmer and have general knowledge 
about program debugging.)
     Thank you!

Best,

Feng