comparing fields within a record

Hi,

I hope this isn't a really simple question, I've been struggling  
with it
for a while.

I'm looking for a way to get a function to go through a data frame  
line by
line, compare fields, and produce a result, kind of a transform and  
an if
statement combined (I tried to put them together and it didn't work).

So, consider data Sales:

House number     Inspected    Sold
1                           9/2/2011     10/10/2011
2                          9/4/2011      10/20/2011
3                          10/31/2011    8/28/2011
4                           8/3/2011     11/1/2011

I want to find all the records which were inspected after they were  
sold.
Ideally, this code would create a fourth field that would be a  
logical.

I tried

Sales<-transform(Sales, Checked=if(Sales$Inspected <= Sales$Sold)  
"OK")