File Comparisons for csv files in C++

I have two csv files in which I need to compare the 5th field in one file (I'll call this the 'KeyFile') and compare it to the 4th field in another file (called the 'PatientFile'). Each record in the PatientFile must be compared to this KeyFile. (A nested while loop should do the trick!) The output would be a new comma delimited file that contains both the contents of the KeyFile and the PatientFile. I will manipulate this file further in excel for insertion into a data base. I would like this program to be alterable easily so that I can change which fields are compared. So I might want the 3rd field in the KeyFile to be compared to the 4th field in the PatientFile, etc. I was a programmer in an ancient life but just don't have time to work out the bugs. I can probably alter this program if it is so written.