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# Why do we convert this to a set?

self.locations.append(f'{column}{num}')

available_locations = set(self.locations) - set(used_locations)

My understanding is that locations is a string ex."D4". Why do we convert it to a set instead of just doing a direct comparison using strings?

Hi Christopher! In the set_cards method, you're looping through each card option and assigning it a location (A2, A4, B2, etc.)

You don't want two cards with the same location. As you loop through, it's essential that you assign each card a location that hasn't been used already.

Using sets makes this easy to do in one line of code.

``` available_locations = set(self.locations) - set(used_locations)
```

In each iteration of the loop, available_locations equals the difference of 2 sets: self.locations and self.used_locations.

Remember that in set terminology, calculating the "difference" between 2 sets returns everything BUT the intersection of the two sets (any elements present in both sets make up the "intersection.")

When we "use" a location, it will be in the set of used_locations on the next iteration of the loop. It will also be in the set of self.locations. Thus, it won't be in the set of available_locations- because, remember, available_locations calculates the difference between the self.locations set and the used_locations set.

I hope this helps! Let me know if that wasn't clear.