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For a practical use of set intersection, 

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we'll use our previous example – 
the patients in a drug trial. 

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I've created a new Python file for this 
example, which I've called trial_patients.py. 

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It's quite possible that there will be more than 
one trial being conducted, at the same time. 

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Giving different experimental drugs, to the 
same person, probably isn't a good idea. 

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In terms of the trials, it would 
be difficult to know which drug 

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was responsible for any observed effects.
It's also not good for the patient's health. 

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So we want to identify any patients who might 
have been included in more than one trial. 

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Set intersection is ideal for that – it 
can tell us who's in more than one set. 

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The intersection of 2 sets is the set of 
elements that are present in both sets. 

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That's what we want here.
Back in IntelliJ, 

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I'll create two sets, for different trials: 

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We're using patients in these sets, but the 
sets could contain anything that we're studying. 

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You might have a set of animals known to prey 
upon a certain species, and a set of animals 

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observed in an area where that species lives.
The intersection of those sets will tell 

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you what's likely to be eating the 
species that you're investigating. 

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Okay, let's check if anyone's 
been included in both trials: 

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Line 4 creates the intersection 
of our two sets. Run the program 

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and intersection produces a set 
containing Charley and Georgia. 

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You'd almost certainly spotted that already – 
there are only 4 people in each set after all. 

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We're showing how things work here, and 
giving you the confidence to rely on the results. 

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If there were hundreds, or thousands, 

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of people in each trial, you'd find 
it hard to spot people in both sets. 

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Okay, that's set intersection.
We could also have used the intersection 

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operator, which is the ampersand.
I won't use that example again, 

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because you can easily work out how to create 
the check_set intersection, using the operator. 

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What I will do, is look at something that might 
not be obvious, when you use the set operators. 

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When we looked at the union and union update 
operators, we used two sets in our examples. 

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If you have several sets, the methods 
are often more convenient – and we saw 

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a demonstration of that, when we created the 
union of all the adverse_interactions sets. 

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But that doesn't mean you can't use the 
operators, when you have several sets. 

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To demonstrate that, I'll use the 
example I mentioned, in the slides at 

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the start of this discussion of sets.
I'll comment out our existing code: 

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then I'll create the sets of 
animals that we had in the slides: 

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Ok, that's our farm and wild animals sets. 
Let's also have a set of animals that can 

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be ridden – that was the example I 
mentioned, in the earlier slides. 

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We've now got a third set, of 
some animals that can be ridden. 

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I'll use the intersection operator, to 
create the intersection of all three sets: 

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I'll run the program 

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and the intersection of these three 
sets is a set containing only horse. 

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Horse is the only animal that 
appears in all three sets. 

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I'll finish by showing the same 
thing, using the intersection method. 

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We can pass more than one set to 
the methods, as we've already seen: 

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I'll run the program:
Once again, we get a set 

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containing the only animals that appear in 
all three sets. That's horse, in this case. 

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I wont give an example of using the intersection 
update method, or the operator. Instead, 

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we'll use it in the exercise that follows.
Have a go at the exercise, and review the 

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last few videos if you need to. 
I'll see you, in the next video.