And sometimes you just need one of something. When it comes to Swift collection types, if you want one of something, you want a set.
If arrays are ordered lists and if dictionaries are unordered lists with unique keys, what are sets? Good question. According to the Big Nerd Ranch Guide Swift Programming, a "set is an unordered collection of distinct instances."
What does that mean? That means that sets are like arrays in that sets have a single value. That means sets are like dictionaries in that they are both unordered lists. And like a dictionary whose keys must be unique, a set's values must be unique. In our post on dictionaries, we showed how multiple Rockies' outfielders could play left-field. In a set, though, one only Rockies outfielder can play left-field.
Why are sets important?
Sets are important when uniqueness is your highest priority because sets do the best job of sorting out unique items and keeping out duplicates.
When should I use sets?
So the big question is, when is a good time to use a set as opposed to an array or a dictionary? Another good question. Sets put a priority on uniqueness. In arrays, you can have multiple values that are the same (a team roster full of people named Javy). In dictionaries, you can also have multiple values that are the same (four left-fielders for the Rockies), but the keys must be unique (the names of the Rockies' left-fielders have to be different). With sets, uniqueness is paramount. It's kind of like retired numbers (yes, I know the Yankees have two 8s and two 42s, but play along). Let's look at the San Francisco Giants' retired numbers.
Alright, how'd we get here? First, created a variable, giantsRetiredNumbers. We then informed the computer that we would assign a Set of Integers (Set<Int>()) to giantsRetiredNumbers. Starting on line 6, we populated our set with integers that represent retired numbers from the San Francisco Giants. We've got Monte Irvin (20), Willie Mays (24), Juan Marichal (27), and Willie McCovey (44) among others.
What happens if the Giants hire a new clubhouse manager, a young kid who's never heard of McCovey, and tries to give old #44 to a September callup? Let's see:
We may have ordered #44 to be inserted into the giantsRetiredNumbers set a second time (line 16), but as we can see on the right side of line 16, there's still only one #44! Interesting that there is no error here, but maybe Xcode is getting chill about how it rejects stuff. One can hope.
What can I do with sets?
We can loop through sets like this:
For each retired number in the giantsRetiredNumbers set we printed each one which gives appears at the bottom of the playground console immediately above.
Sets also play nicely together. Remember Venn diagrams? NO?!? OK, here's one based on the old Simon & Garfunkle song.
Venn diagrams, like the one above, have two circles that represent distinct groups. In this case, the groups are "people who are breaking my heart" and "people who are shaking my confidence daily." The intersection of these two circles, that is, the one thing these two groups have in common, is Cecilia.
The union of these two circles is people who are breaking my heart AND people who are shaking my confidence daily.
Sets also have a method that checks whether or not there are duplicates between them. Let's walk through some examples of these.
First, let's check to see if Barry Bonds has his number retired with the Giants.
Just checking! No retired number for Bonds with the Giants. And that's how to check if there is a specific item in a set. First, we created a constant, barryBonds. Then we told the computer what set we wanted to check, the giantsRetiredNumber set. Then we ran the ".contains" method on the giantsRetiredNumbers set. We then had to pass an argument in the parentheses so the computer knew what value to check for, in this case, 25.
Let's say we wanted to create a super team of retired Giants and retired Pirates. What an outfield! Mays, Clemente, McCovey! (Stargell can play first). How would we do that? Let's take a look.
How'd we do that? First, we created our Pittsburgh Pirate super team, a set filled with the unique retired numbers of guys like Roberto Clemente, Willie Stargell, and Honus Wagner (line 17). Then we created a superTeam constant. To that superTeam constant, we told the computer to take the giantsRetiredNumbers set and to create a union with the piratesRetiredNumbers set.
Also notice that while the Giants had a 4, 11, 20, and a 42 like the Pirates, in the superTeam set there is only one 4, 11, 20, and one 42. Uniqueness: that's what sets do.
What if we want to see what retired numbers the Giants and Pirates have in common? How do we do that? Let's give it a go.
And the winners are 4, 11, 20, and 42 (line 19)! Let's walk through how we did that. First, we created a new constant, superTeamIntersect that we'll assign the numbers that the two teams have both retired. Then we picked one of the teams, in this case, piratesRetiredNumbers and asked the computer what numbers it and the giantsRetiredNumbers set have in common through the ".intersect" method.
Now what if you want to make sure that two sets do not have the same items in common? In this case, you are not looking for the exact items in common or not in common, just a "True, the two sets have nothing in common" or "False, the two sets do have something in common." Let's give it a shot.
False! And why is it false? Because as we learned in the intersect section above the two teams have both retired 4, 11, 20, and 42! So it is false that the two teams retired numbers are disjoint (in other words, it is true that they are not disjoint because they do in fact have four numbers in common).
Challenge: Use sets to create a super team between the retired numbers of the Cubs and the Tigers. What is their union? Where do they intersect? Tweet me what you find (@randallmardus).
On Deck: Enums, Structures, and Classes!
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