so this code is starting to shape up. It has a lot of functionality. Here we can store variables, we can add people, but it has a bit of an issue. If we have this contract and we want to say, Oh, what was Kelly's favorite number? Let's say we have four people on the array. Let's have Pat seven, John eight, Mariah ten, and Chelsea twenty-two, or two hundred thirty-two. And we asked the question, Ah, what was Chelsea's favorite number? Well, the way we would do it is we'd have to actually loop through this list of people to find Chelsea. So, we'd say, Okay, what's at zero? Ah, okay, that's Pat, that's not Chelsea. What's at two? Ah, that's John. What's at three? Ah, that's Mariah. Excuse me, what's at three? Ah, that's Chelsea, favorite number two hundred thirty-two. Okay, perfect, I found Chelsea. But obviously, if we had a thousand people in this array, that would be a very tedious process, and that would take a long time for us to find the person that we wanted. So, the question then is

is there a better data structure than a list or an array to use that makes it much easier to access and find the information about the people that we want, and the answer to this is using a mapping. You can think of a mapping as a dictionary; it's a set of keys, with each key returning a special set of information about that key. So, it's similar to a dictionary. If you look up the word the, the word the will have a whole bunch of texts associated only with the word the. So, let's create a mapping type, and you'll see how it works in practice. So, we're going to create a mapping like this with this mapping keyword, and we'll say the key is going to be a string, and it'll point to a uint256. So, this is going to be our type, obviously. What comes after the type? Our visibility. So, let's just go ahead and do public for this as well. And I like to be very explicit with my naming, so I'm going to say name to number, signifying that this map maps

someone's name to their favorite number, and now with this, we have essentially a dictionary where every single name or every single string is going to map to one number. So, for example, if we looked up Chelsea, we'd automatically get returned the 232 that we're looking for instead of having to iterate through this list. So, let's add some capabilities to our add person function here so that we can update our mapping. So, we have this list of people.push, which adds someone to the array. Let's also update this so it will also add somebody to our mapping. To do that, we'll say name to favorite number, and we put these little brackets in here to specify what the key is, and we'll say underscore name, and we'll assign that to underscore favorite number. What this line is doing is now it's saying, All right, in our mapping up here in our name to favorite number mapping, anytime you look for the person's name, you'll automatically get their favorite number back. And now you have a much quicker way to access people's favorite

numbers just by knowing their name. So let's go ahead, let's compile this, let's deploy this, let's delete our old one, we'll deploy this. Now we have this new name to favorite number blue button. And so let's go through the same problem. So let's say we have Pat 7, John 16, Mariah 32, Chelsea 232. Now same thing, list of people 0 returns Pat, listed people 1 John, so on and so forth. Or we could just go down here and let's say let's look up Chelsea and we automatically get 232. We get Chelsea's favorite number back automatically. Same thing if we look up Pat, we get 7. If we look up John, we get Mariah. And if we look up cheesecake, we get nothing back in a mapping. The default value for all the keys is 0. So if you look up a key or a word that we haven't added in the mapping yet, it defaults to the default value of whatever that type is. Since ours is a string to you at 256, you would

256 default type is zero. If we look for a key that we haven't added, we get zero back.

Lesson 2 Part 8 | Mappings - Solidity & Foundry Full Course

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