[MUSIC] We've learned that all elements in an array must be of the same type. Such arrays are said to be homogenous. What if we want to group together heterogeneous data somehow? That is, data of multiple types. Well MATLAB, like many other languages, provides ways to do that. The easiest way is to use a struct. A struct is different from an array in three ways. One, structs have fields, not elements. Two, we access the fields of a struct by names, not indices. And three, fields within the same struct can have different types. What's especially cool is that the field of struct can be another struct, and that ability of a struct to have another struct inside it makes structs very versatile. The syntax for creating or accessing the field of a struct relies on the period, or dot. If we type r.ssn equals 12345678, MATLAB will create a new variable named r, make it a struct, create a field called ssn for that struct, and assign it the value 12345678. So the syntax is simply to put a dot between the variable name and the name of the field. Easy. Now if we check the data type of the variable r with the class function, we get struct, as expected. But if we check the data type of the ssn field, we get double, which is also what we should expect, since we assigned a number to that field. Okay, so we've created a brand new struct. Now we can add a new field to that struct, simply by typing r.name equals quote, Homer Simpson, end quote. MATLAB will modify our struct by adding a new field called name, and it will assign the string, Homer Simpson, as the value of that field. Now here comes the cool part I mentioned. Typing r.address.street equals single quote, 742 Evergreen Terrace, single quote, creates another field for r called, address. And because of the dot after address, this new field is itself a struct, with its own field, which is called street. And then it assigns a string, 742 Evergreen Terrace, to the field street of the field address of the variable r. As you can see MATLAB doesn't print the inside of this new struct in the command window directly. Instead, it just signals that the field address of the struct r is another struct. As I said, allowing a field to be a struct makes structs really versatile. But there is more versatility here than that, because structs can hold arrays, and arrays can hold structs. In other words, the field of a struct can be an array. And it goes the other way too, the element of an array can be a struct. But in that direction, we encounter the homogeneity rule, all array elements must be the same type. So if they're structs, they must all have the same fields. We'll see some examples shortly, but first, you'll not be surprised when I tell you that MATLAB has some useful built-in functions that deal with structs. For example, isfield will tell you whether a struct has a certain field. You just give it the struct, and the name of the field you're looking for, and it'll return true or false. And setfield can add new fields to a struct in a dynamic manner, meaning that you don't have to know the name of the field when you're writing your program. The user can specify it while your program is running, for example. Another handy function is called rmfield, which means remove field. It can produce a struct that is a duplicate of another struct, except that a specified, specified field is missing. You give it the struct, and the name of the field you want to remove, and it returns a copy with that field removed. Finally, if you want to create a struct with a set of names in one shot, you can simply call the function name struct, and give it the field names you want as arguments. Okay. We've learned about structs and now we're ready to see them working in MATLAB. Information about a bank account is pretty heterogeneous. An account has an owner, which is probably going to be a string, an account number, which will be a number, and a balance which will be a number, etcetera. Let's create a struct that represents a bank account. The value that we've assigned to the variable, account, here, is a struct, class will tell us that. And its first two fields are doubles. Its third field is a struct, as we can see right here, called, owner. An owner has a couple of fields, which we can look at by typing the variable name dot and the name of the field owner, to see what's inside it, and it has two fields. When we see these with this colon here, we know that we're looking at a struct. And they happen to be both of the same type, as we can see. These quotes are included by MATLAB, when it gives the value of a struct field that happens to be a char. If we want to turn a count into an array of structs, it's easy enough to do, we just give it another element. And we did that by assigning something to one of its fields. As you can see, MATLAB prints out only the field names, once we have a struct array. Also, the new struct, well the second element of the array, magically, has all the same fields that the first element already had, not just the account number we created. This has to happen, to keep the types of both elements the same. Remember that homogenous rule? If some elements had one set of fields, and others had another set, well homogeneity'd be out the window. So what values are in these fields that showed up auto-magically? Let's have a look. Well they're initialized to the empty array. Let's assign values to more fields of our new struct. If we want to see the entire struct, we can do this. But if we want to see the value of a field of a substruct, we have to type in the entire hierarchical name. The array name, the index in parentheses, the dot, the field name, another dot, and the field name of the substruct. Well let me just do one. And I'll do the other one. While the structs must have the same field names, the values stored in those fields don't have to match. They can have different types, and or different sizes. For example, the length of the strings in owner.names are different for elements one and two, let's prove that. One's 9, one's 11. And substructs don't even have to have the same set of fields. There, we've added a sub struct to owner, giving it the value 23, for element one. So what about element two? Does a new field, called age ,show up under it's owner, with, maybe, a empty matrix, or something in it. No, reference to nonexistent field, age, it doesn't even have that field. The homogeneity rule for arrays is only skin deep, it extends only to the first level of the fields. Every element of the array account has to have the fields, number, balance, and owner, but different things can be inside them. Okay, that's a look at how fields of structs are accessed, and the homogeneity rule for an array of structs. Now let's try some of the functions that work with structs, some that we've mentioned before. First let's clear this cluttered screen, and look at the owner field of each element of account. Putting that 1 colon 2 in there makes it show each one of these elements, and we see there's quite a few things in 1 and not much in 2. Well let's look at isfield. So what we see here, is that isfield takes this argument, which is account 2, the second one, dot owner, it looks at the field of account 2, called owner, and checks to see if there's a field in there called, age. Well there's not, so it returns faults or zero. Then we do the same thing with element one. Well if we go inside of owner for element 1, we find there is a field called age, and so we get true. As we said, isfield returns true or false, depending on whether the given structure has a field, with a given name. And then there's rmfield, which you may remember, removes the field from a struct. Let's remove age from account1.owner. There, age field is gone, along with it's value, which was 23. You give the thing you want to remove the field from, then you give the name of the field that you want to remove. Now let's just check to make sure that it's gone. Wait. Why is age still there? We just removed it. Well we made one of those rookie errors. The function rmfield doesn't change its argument. This was its argument, account1.owner. That's not the way MATLAB works. It returns a changed version of its argument, that's what we saw here. So when we checked out the argument that we gave it, its unchanged, its still the same. If we want to change it, we have to assign something to it. That's easy to do, we do it like this. First of all, I'll click my up arrow and get that command back. And over here, I type account1.owner, and the result from rmfield will be assigned back to it. So let's hit Return, and see what happens. Now let's check, age is gone. Okay, we got one last function, that works on structs, to look at ,and its name is struct. You'll remember when we started all these struct examples, we needed to have some structs to work with, and you do that by giving a variable some fields. We started out by giving account the field named, number. And then we gave it balance, and so forth. We kind of did it piecemeal, one field at a time. But you don't have to do it that way. You can do it all in one fell swoop, with a builtin function, whose name is struct, like this. Man, I've got my typing in high gear here. I am rea, I am smoking it, 'kay? Whoa. Remember we're professionals here, we don't recommend you try typing like this at home. But anyway, the inputs to the struct function come in pairs, organized as field and value. So here is the field called, area. Here's the value called, cs, and you can see it showed up here. Here's a field called, number, and 103, which is a double, is the value for that field, title, there you go. This method of setting values for fields by processing a list of field value, field value, field value pairs, is not unique to MATLAB. It first appeared in a language called Lisp, which means list processing, shortly after Fortran was invented in the late 50s. Lisp, and derivatives of it, are still used today for writing programs that have artificial intelligence. We're still using this method of giving values to fields. Good ideas never die. And with that bit of programming language history, we conclude our coverage of structs. [MUSIC] [APPLAUSE]
