Looping through dataframes in R

I am trying to loop through all the rows of a column in a DataFrame. I read in the csv using data.table. I am new to R and was wondering what way i would go about doing something like this:

for i in row_2_of_dataframe: 
 if i == 0: 
  #Do something to that value
 else: 
  #Leave it the way it is 

Any help would be great.