如何在函数内的数据框中添加列

I have a data frame, and I want to do some calculation with existing columns and create new column in my data set which is a combination of existing... I can do this easily outside function... but if I wrap the code witin function, the changes I made (inside functions) are not visible outside function... i.e. the new column doesn't exist...I have a data frame, and I want to do some calc