I have a dataframe like so:

```
sport   contract start contract end visits spends purchases
basket   2013-10-01     2014-10-01   12      14      23
basket   2014-02-12     2015-03-03   23      11      7
football 2015-02-12     2016-03-03   23      11      7
basket   2016-07-17     2013-09-09   12       7      13

```

I would like to conditionally replace the columns [4:6] with NAs, based on the variables "sport" and "contract start".
So for instance:

```
i1 <- which(df$sport =="basket" & df$contract_start>="2014-01-01")

```

will index all the rows in which my conditions are met.
Is there an easy piece of code to add to the above, that will replace df[4:6] with NAs given the above conditions?
I would like to end up with something like that:

```
sport   contract start contract end visits spends purchases
basket   2013-10-01     2014-10-01   12      14      23
basket   2014-02-12     2015-03-03   NA      NA      NA
football 2015-02-12     2016-03-03   23      11      7
basket   2016-07-17     2013-09-09   NA      NA      NA

```

Thanks!
A.